SYNTHESIS OF TANNING MATTERS
1. AROMATIC SULPHONIC ACIDS
In organic chemistry distinction is made between sulphonic acids of the
aliphatic and the aromatic series, the characteristic group of these
acids being the so-called _sulphonic acid group_, HSO_3.
When sulphides or mercaptans in glacial acetic acid solution are heated
with permanganate, the resulting sulphonic acid compounds exhibit great
similarity to compounds containing free carboxyl groups. The sulphonic
acid group may also be directly introduced either by concentrated, or by
fuming sulphuric acid, or by elimination of halogen by the action of
sodium or silver sulphite on the halogen derivatives of the aliphatic
compounds. Saturated hydrocarbons do not react with sulphur trioxide,
but unsaturated hydrocarbons are readily attacked by SO_3. Similarly,
halogenated compounds and alcohols react with concentrated or fuming
sulphuric acid forming sulphonic and hydrosulphonic acids respectively.
The aromatic compounds form, as a rule, sulphonic acids with much
greater facility. Benzene, for instance, is easily converted into the
_m_-disulphonic acid by gently heating with fuming sulphuric acid;
stronger heating converts the _m_- into the _p_-disulphonic acid, and at
190° C. the trisulphonic acid is formed. Toluene treated with fuming
sulphuric acid first yields _o_- and _p_-sulphonic acids, finally _o_-
and _p_-disulphonic acids, ethylbenzene at the boiling point
_p_-ethylbenzene-sulphonic acid. Of the three isomeric xylenes _o_- and
_m_-xylene dissolve in concentrated, _p_-xylene in fuming sulphuric acid
only.
The action of sulphuric acid on naphthalene is stronger even than on
benzene. Equal parts of naphthalene and sulphuric acid heated to 100°
C. yield 80 per cent. [Greek: a] and
20 per cent. [Greek: b]-monosulphonic acid. At 160°-170°C. 25 per cent
[Greek: a]- and 75 per cent. [Greek: b]-sulphonic acid is formed, and at
higher temperatures [Greek: b]-monosulphonic acid only. If, on the other
hand, 8 parts of naphthalene are heated with 3 parts of concentrated
sulphuric acid to 180° C., two different naphthyldisulphonic acids are
obtained.
Complete solution of the substance in sulphuric acid is, generally
speaking, a criterion of complete sulphonation. A completely sulphonated
compound should remain clear on dilution with water, or, in case
precipitation occurs, the precipitate should be completely soluble in
alkali or ammonia. It is necessary to submit the product to this test,
since many organic substances are soluble in concentrated sulphuric acid
without undergoing any alteration in composition.
Phosphoruspentoxide or potassium sulphate considerably increase the
sulphonating property exhibited by fuming sulphuric acid.
The separation of the sulphonic acids from sulphuric acid is effected by
salting out the former with common salt, or by removing the sulphuric
acid with calcium, barium, or lead salts, provided that the sulphonic
acid salts of these metals are soluble in water.
The sulphonic acid, in its chemically pure state, is best obtained from
its crystalline barium salts, which are decomposed with the equivalent
of sulphuric acid; another way is to decompose the calcium salts of the
sulphonic acids with oxalic acid. The sulphonic acids are frequently
hygroscopic and are easily soluble in water; the majority of their
barium and lead salts are also soluble in water. The sulphonic acids are
insoluble in ether. The halogens do not easily react with sulphonic
acids, but when they do they usually replace the sulphonic acid
group. In order to prepare the halogen substitution products, therefore,
use is made of sulphonic chlorides. The latter are obtained by the
action of chlorosulphonic acid on aromatic hydrocarbons; a simpler
method, however, is to treat the dry alkali sulphonates with phosphorus
pentachloride--
C_6H_5SO_3Na + PCl_5 = C_6H_5SO_2.Cl + NaCl + POCl_3
Derivatives of sulphonic chlorides are sulphonamides, which are easily
prepared from the former by grinding with ammonium carbonate--
C_6H_5SO_2.Cl + (NH_4)_2CO_3 = C_6H_5.SO_2.NH_2 + NH_4Cl + CO_2 + H_2O
Sulphonic chlorides react with alkaline sulphides to form
thiosulphonic acids--
C_6H_5SO_2.Cl + K_2S = C_6H_5SO_2.SK + KCl
Sulphonic chlorides, dissolved in ether, yield sulphinic acids on
reduction with zinc dust or metallic sodium--
C_6H_5SO_2.Cl + H_2 = C_6H_5SO_2.H + HCl
In the sulphonic acid compounds it is assumed that the sulphur is
hexavalent, and it is hence possible to consider the sulphones to be
esters of sulphinic acid.
==O
R--S==O
--H
The sulphones are mostly solid bodies, which soften prior to melting
when heated. They are very stable towards chemical reagents; for
instance, saponification of a mono-sulphone very rarely yields sulphinic
acid.
If a hydroxyl is substituted for a hydrogen atom in the aromatic
hydrocarbons, the action of sulphuric acid is greatly facilitated; thus,
by merely mixing phenol with sulphuric acid, the sulphonic acid is at
once formed, whereby, in the cold, _o_-phenolsulphonic acid prevails
which on heating for some time to 100°-110° C. is completely converted
into _p_-phenolsulphonic acid. In the absence of free sulphuric acid the
conversion of _o_- into _p_-phenolsulphonic acid is brought about by
heating the aqueous solution. Phenol-2,4-disulphonic acid is prepared
from _o_- or _p_-phenolsulphonic acid, whereas phenol-2,4,6-trisulphonic
acid is prepared directly from phenol by heating with concentrated
sulphuric acid in presence of phosphorus pentoxide. Phenolsulphonic
acids are also obtained by fusing benzenedisulphonic acid with alkali.
Cresol is not so easily sulphonated as is phenol; _o_-cresol when heated
eight to ten hours at 90° C. with one and one-half times its weight
of concentrated sulphuric acid, yields _o_-cresol-_p_-sulphonic acid.
The phenolsulphonic acids are strong, rather stable acids; their
alcoholic hydroxyl-hydrogen atom may, similarly to that of the phenols,
be substituted by a metal or an alkyl radical.
From [Greek: a]- and [Greek: b]-naphthol a number of sulphonic acids may
easily be prepared; viz., mono-, di-, and trisulphonic acids. Nearly
all these acids are important as basic materials in the dyestuff
industry, especially 2,6-[Greek: b]-naphtholmonosulphonic acid (S-acid),
2,3,6-[Greek: b]-naphtholdisulphonic acid (R-acid) and 2,6,8-[Greek:
b]-naphtholdisulphonic acid (G-acid).
2. Condensation of Phenols
Phenolsulphonic acids exhibit pronounced tendencies to condensation, for
which purpose A. v. Baeyer (1872) employed aldehydes. The reaction is
rather violent, and yields, in addition to well-defined crystalline
substances, amorphous bodies resembling rosins. In addition to
formaldehyde, paraformaldehyde, trioxymethylene, methylal,
hexamethylene-tetramine, and other substances containing a reactive
methylene group, as well as acetaldehyde, benzaldehyde and other
aldehydes may be employed to induce reaction.
A number of these condensation products are derivatives of diphenylamine
or hydroxybenzyl alcohols. When the latter are heated, either by
themselves or in presence of acids, anhydrides and polymerisation
products are formed producing hard, brittle, fusible substances,
insoluble in water but fairly soluble in organic solvents. The same
substances are formed when phenols are condensed with formaldehyde,
especially in the presence of acid contact substances and excess of
phenol by sufficiently long heating at certain temperatures. The
substances referred to are termed "Novolak": similar to these are the
so-called "Resols," insoluble and non-fusible substances, very resistant
to chemical and physical action. Another member of the series is the
so-called "Bakelite" or "Resitol," which does not fuse but softens when
heated and swells in organic solvents. The ultimate product of this
class of substances is "Resit" which is obtained when concentrated
hydrochloric acid is allowed to act upon a mixture of phenol and
formaldehyde; the temperature rises spontaneously, and a hard, porous,
insoluble mass of great resistance is obtained. By heating resols,
resitols are formed which, on further heating, are finally converted
into resits. [Footnote: _Ber.,_ 1892, 25, 3213.]
Of all these products, bakelite (resitol) has found the greatest
industrial application; in its purest form, this substance is a nearly
colourless or light yellow body of sp. gr. 1.25 and, being a poor
conductor of heat and electricity, constitutes an excellent insulating
material; it is exceedingly resistant towards most chemical reagents
even in concentrated forms of the latter. Its pronounced refractivity,
and the ease with which it may be worked, makes bakelite a favourite
substitute for amber (Ger. Pat, 286, 568). Similarly, the resols which
can be easily moulded are used either as such or mixed with sand,
pulverised cork, asbestos or wood, and the moulded substances then
converted into the more highly resistant bakelite by heating.
The constitution of these bodies no doubt depends largely on their
method of preparation; Baekeland [Footnote: _Chem. Ztg.,_ 1913, 73,
733.] considers resit a polymerised hydroxybenzylmethylene glycol
anhydride; Raschig, a diphenylmethane derivative (e.g.,
dihydroxydiphenylmethane alcohol); Wohl [Footnote: _Ber.,_ 1912, 45,
2046.] considers them polymerisation products of methylene derivatives
of tautomeric phenol.
CH===CH
H_2C:C{ }CO
CH===CH
[Note: Lower Right CH has double bond to CO]
This group possesses the characteristic property of being capable of
converting animal hide into leather when suitably dissolved. The author
has dissolved a number of these water-insoluble condensation products in
alkali and alcohol and was able to demonstrate their tanning effects on
pelt; bakelite is easily soluble in alkali; a faintly alkaline solution
partially precipitates gelatine, and completely so when the alkali is
neutralised. This latter solution gives a dirty brown precipitate with
iron salts.
These condensation products gained extraordinary importance for the
tanning trade when Stiasny [Footnote: Ger. Pat, 262,558; Austr. Pat,
58,405.] succeeded in preparing them in water-soluble form when they are
enabled to directly exert their tannoid properties. This may be done by
acting upon two molecules of concentrated phenolsulphonic acid with one
molecule of formaldehyde, the temperature thereby not exceeding 35°C. By
condensation, however, considerable heat is liberated, and hence the
rise in temperature can only be limited by adding the diluted
formaldehyde drop by drop, whilst stirring and cooling, to the
phenolsulphonic acid. The original letters patent is worded as follows:
10 kilos each of crude phenol and sulphuric acid (66° Bé.) are heated
with stirring for two hours at 105°-106°C., cooled to about 35°C., and
463 kilos 30 per cent. formaldehyde added during three hours, the
temperature thereby not exceeding 35°C.; the stirring is continued for a
couple of hours after the final addition of formaldehyde. This yields
about 24 kilos of the crude condensation product. On a commercial scale,
however, cresol (cresylic acid) is substituted for phenol. There are
three isomers of cresol, viz., _o_-, _m_-, and _p_-cresol, and it was
naturally of interest to investigate whether one or the other of the
isomers exerted any particular influence on the properties of the final
product. It was found, however, that condensation products from the
three isomers were distinguishable from one another neither in physical
nor in tannoid properties. It is hence possible to employ crude cresol,
which contains varying quantities of the _o_-, _m_-, and _p_-compounds,
in the manufacture of these tanning matters. [Footnote: Gen Pat,
291,457.]
The tar obtained from the Rochling coal-gas generator contains
considerable quantities of phenols (B.P.=200°-250°C.), and the author
has protected the use of these for the production of synthetic tannins
by Ger. Pat, 262,558. A deep brown viscous mass is obtained which, when
partly neutralised, yields similar results to those given by the product
above referred to.
It may be anticipated that by analogy from the chemical reactions taking
place in the condensation of phenols on the one hand and cresolsulphonic
acid on the other, that all other homologues of phenol, its polyvalent
derivatives, substitution products and acids, would yield similar
condensation products.
The particular position occupied by the aromatic hydroxy compounds in
the chemistry of substance possessing tannoid character is not only
evidenced by the natural classification of the tannins, tannin
derivatives, and decomposition products so far isolated and
investigated, but also by other chemical behaviour shown by these
substances. Meunier and Seyewetz [Footnote:_Collegium_, 1908, 315,
195.], for example, were able to show that phenol, _p_-aminophenol,
chlorophenol, trinitrophenol, catechol, resorcinol, hydroquinone,
monochlorohydroquinone, orcinol, pyrogallol, and gallotannic acid
precipitate gelatine from its aqueous solution, that is, to a certain
extent possess tanning properties.
The author has extended this series somewhat and obtained the following
results:--
Relative Behaviour Towards
Substances Gelatine. Hide Powder. Pelt.
Tribromophenol Slight ppte. Tans Surface tannage
[Footnote: In alcoholic solution]
_o_-Nitrophenol No ppte. " "
Br-_o_-Nitrophenol Slight ppte. " "
Tribromopyrogallic Ppte. " "
acid
Bromophloroglucinol " " No tannage
Galloflavine Slight ppte. " "
Bromosalicylic acid " " "
Bromo-[Greek: b] " " Tans
-naphthol
[Footnote: In alcoholic solution]
Rosolic acid " " "
[Footnote: In alcoholic solution]
Gallic acid No ppte. No tannage No tannage
By the condensation of their sulphonic acids, it may be demonstrated
experimentally how the tannoid properties of nearly every member of the
series are intensified. Investigattion in this direction, however, has
not been systematically undertaken, for which reason the author
determined to examine this subject; but the enormous number of samples
required, obtainable only with great difficulty during the war, made it
impossible to conclude completely the researches in this field. What
little has so far been done relatively to this subject should, when
collected, indicate the way to be pursued in this wide field of
investigation. What follows will hence comprise the conversion of a few
of the most important members of this series of substances into their
methylene-condensation products with a brief discussion of the
qualitative and tannoid reactions of the latter.
The didepside of phenolsulphonic acid is obtained by condensing
carbomethoxyphenolsulphonic chloride with sodium phenolsulphonate in the
presence of the calculated amount of caustic soda. A product of the
composition
CH_3.0.COO.C_6H_4SO_2.0.C_6H_4.SO_3Na
is first obtained, which on saponification with soda yields the
pure didepside--
HO.C_6H_4.SO_2.C_6H_4.SO_3.Na
By acidifying the concentrated solution the didepside is obtained as a
white crystalline substance; a solution of which precipitates gelatine
without, however, exhibiting any tanning effect upon animal hide. If, on
the other hand, the above ester is converted into the chloride
CH_3O.COO.C_4H_4SO_2.O.C_6H_4.SO_2Cl
by treatment with PCl_5, and the chloride thus obtained further
condensed with sodium phenolsulphonate, saponified, and the solution
acidified, the pure tridepside
HO.C_6H_4.SO_2.O.C_6H_4.SO_2.O.C_6H_4.SO_3Na
is precipitated as white crystalline needles which not only precipitate
gelatine, but are capable of converting animal hide into
leather.[Footnote: _Chem. Ztg._, 1919, 43, 318.]
Of the class of hydroxy-cymenes _thymol_,
C_6H_3.CH_3.C_3H_7OH,
was converted into the water-soluble sulphonic acid by warming with
concentrated sulphuric acid at 50° C., the sulphonic acid being
subsequently easily condensed with formaldehyde by slightly heating the
mixture. The condensation product thus obtained is a viscous brown mass
which is easily soluble in water, precipitates gelatine completely,
gives a bluish-black coloration with iron salts, and gives a precipitate
with aniline hydrochloride. To investigate its tannoid properties, the
mixture was brought to the acidity 1 gm = 10 c.c. N/10 NaOH and a piece
of bated calf skin was then introduced into a solution measuring about
2° Bé. After eighteen hours the pelt was nearly tanned through, and a
further twenty-four hours completed the tanning process, after which a
light fat-liquor was given. The dried leather was brownish-grey in
colour, possessed soft and full feel and good tensile strength.
On account of their importance, the three dihydroxybenzenes were
examined with a view to test their suitability for conversion into
tannoid substances.
_o_-Dihydroxybenzene, catechol, yields a sulphonic acid easily soluble
in water, which on the careful addition of formaldehyde assumes a blue
colour. The compound thus obtained may be heated to 100° C., without
depositing insolubles. A further addition of formaldehyde, however,
results in the formation of a considerable quantity of insolubles whilst
the liquid assumes a brown coloration. If, on the other hand, the
sulphonic acid is diluted with twice its volume of water, formaldehyde
added and the mixture heated on the water bath, the liquid immediately
turns brown, the formaldehyde is completely fixed, and a condensation
product soluble in water results. The latter gives a brownish-black
coloration with ferric chloride, completely precipitates gelatine, but
gives no opalescence with aniline hydrochloride. Tanning experiments
with the partly neutralised (1 gm.= 10 c.c. N/10 NaOH) substance
resulted in both grain and flesh being tanned with a black colour,
whereas the interior of the pelt was pickled (white colour). After a
further forty-eight hours, however, the black colour penetrated the
pelt, and tannage was complete. The washed and lightly fat-liquored
leather was soft, of full feel and good tensile strength, and was
greyish coloured throughout.
With regard to the black colour possessed by leathers tanned with
synthetic tannins the following should be noted. When sulphonating and
especially when condensing substances, black dyestuffs or very finely
divided carbon in the colloidal state are often formed. Such a substance
does not deposit the black particles, even when filtered through kaolin,
and hence convert pelt into leather possessing black colour on the
surface. The hide in this case acts as a perfect filtration medium,
whereby the surface layers retaining the coloured particles assume their
colour; thus only the pure tanning matter enters into the interior,
which then, according to the composition of the former, imparts a colour
varying from white to light brown to the inner layers.
_m_-Dihydroxybenzene, resorcinol, is also easily sulphonated by
concentrated sulphuric acid, the brownish-coloured sulphonic acid being
easily soluble in water. If the sulphonic acid is diluted with three
times its volume of water, cooled down, a few drops of formaldehyde
added and the mixture heated on the water bath to completely fix the
formaldehyde, and this process repeated till no more formaldehyde is
taken up, a brown water-soluble condensation product results, the
aqueous solution of which precipitates gelatine completely, aniline
hydrochloride only partly and which gives a deep blue colour with ferric
chloride.
A piece of calf skin immersed in a solution of the partly neutralised
(as above) product was tanned through in twenty-four hours; when lightly
fat-liquored, the resulting leather possessed a yellowish-green colour
and good tensile strength, and was soft and full.
_p_-Dihydroxybenzene, hydroquinone, was converted into the water-soluble
sulphonic acid by heating it with concentrated sulphuric acid at 100°
C.; the sulphonic acid, mixed with formaldehyde at ordinary temperature,
immediately solidifies to a white mass, which is soluble in water and
which had completely fixed the formaldehyde. If, however, this mass is
heated for some time to 100°C, it assumes a light brown coloration and
its solubility in water is diminished. A slight excess of formaldehyde
and the application of heat result in dark violet insoluble condensation
products. The aqueous solution precipitates gelatine, gives a deep blue
colour with ferric chloride, but gives no precipitate with aniline
hydrochloride; on the other hand, addition of potassium nitrite produces
the yellow colour characteristic of hydroquinone.
The product effects a slower tannage (seven days) than the former
product, when a brown, soft, but rather empty leather of good tensile
strength is obtained.
Of the _trihydroxybenzenes_ pyrogallol and phloroglucinol only were
included in these investigations.
When pyrogallol is sulphonated with concentrated sulphuric acid a
violet-coloured sulphonic acid, soluble in water, is obtained, which,
when treated with formaldehyde first in the cold and then when heated,
yields a solid deep red-coloured mass, which precipitates gelatine but
not aniline hydrochloride, and gives a blackish-brown colour with ferric
chloride. The partly neutralised substance in aqueous solution tans pelt
in twenty-four hours with black colour on the surface only, the
intermediary layer being pickled (white colour) only, but the
black-coloured tanning matter ultimately penetrates the pelt, which
tanned through in seven days. The resultant leather is coloured black
throughout, is full, soft, and possesses good tensile strength.
Sulphonation of phloroglucinol succeeds at higher temperatures only, the
sulphonic acid being a solid which is scarcely soluble in water, the
latter then assuming a wine-red colour. The condensation
product--prepared as described for resorcinol, but requiring higher
temperature--is a brick-red powder, insoluble in water.
The same end-product also seems to be obtained by simply heating the
sulphonic acid at a higher temperature; this also induces condensation
with the formation of a reddish-brown mass insoluble in water. It is, of
course, impossible to attempt any tanning experiments with this product
in aqueous solution; attempts at dissolving the condensation product in
alcohol proved barren of result, since only traces of impurities
accompanying the substance dissolved, imparting a light reddish-brown
colour to the solution. In highly concentrated alcohol, however, the
condensation product is somewhat soluble, yielding a reddish-brown
solution. A piece of pelt introduced into the alcoholic solution was
surface tanned only after forty-eight hours, leaving the remainder of
the pelt pickled; extending the experiment over a further four days
produced no change in the pelt. The latter was therefore rinsed with
water, lightly fat-liquored and dried, when a soft but empty leather of
grey colour and good tensile strength was obtained. It appears,
therefore, to be a case of pseudo-tannage, where an infinitesimal amount
of synthetic tannin produces a tanning effect without, however, a true
tannage being effected.
The Elberfelder Farbenfabriken have protected the use of the
condensation products of di- and polyhydroxybenzenes by Ger. Pat.,
282,313; owing to the high cost of the latter substances, however, it is
doubtful whether synthetic tannins prepared from these materials would
not be too expensive for any other than pharmaceutical purposes.
Before leaving the phenols, mention must be made of the quinones, the
use of which for tanning purposes was first protected by Ger. Pat.,
206,957 (30th April 1907). According to this patent, only 400 gm. of
quinone are required for the conversion into leather of 400 kilos pelt,
drum tannage being preferable. During the process the leather first
assumes a reddish colour, changing through violet to brown; its
resistance to water, acids, and alkalies is said to be considerably
greater than that exhibited by all other kinds of leather.
The chemistry of the quinone tannage has been investigated, and an
explanation given by Thuau [Footnote: _Collegium_, 1909, 363, 211.]
assumes a reaction between the quinone and the amino groups of the hide
protein with the formation of hydroquinone--
+-O OH
| | |
2R.NH_2 + 2C_8H_4 | = C_6H_4 + C_6H_4(O.NH.R)_2
| | |
+-O OH
(Pelt.) (Leather.)
Fahrion has shown that, during the tanning process, the quinone loses
its active oxygen, and this can only be brought about by the amino group
of the hide protein, the amino group only being capable of effecting
reduction of the quinone. An analogy is here offered by
dianilinoquinone. A spent quinone liquor contains considerable amounts
of hydroquinone. The tannage may also be effected by exposing pelt
saturated with hydroquinone to oxidation by the air. The pelt, which is
unaltered by the hydroquinone bath, on being removed from the latter,
and in the presence of alkali, assumes a red colour at first, which
changes into violet, blue, and finally brown, the pelt being thereby
converted into a quinone-tanned leather.
It may be noted that quinone only effects pseudo-tannage; quinone mixed
with water deposits, in time, a black amorphous substance practically
insoluble in water. This substance is easily adsorbed by hide powder,
but is not capable of converting the latter into that insoluble form
into which it is converted by the natural tannins.
Amongst polyhydric alcohols, the behaviour of the methyl ester of
catechol, _guaiacol_ was investigated. The sulphonic acid was prepared
by heating guaiacol with concentrated sulphuric acid, the resulting
water-soluble product possessing a light, brownish-green colour. On
condensing the sulphonic acid with formaldehyde, the same precautions
were observed as in the case of resorcinol, but complete fixation of the
formaldehyde could only be obtained by finally heating the product for a
short time over a free flame, at about 105° C. Condensation was
indicated by the brownish appearance of the liquid. No insoluble
products were formed. The condensation product easily dissolves in
water, the solution assuming a rich brown colour and exhibiting the
following reactions: gelatine is completely precipitated, aniline
hydrochloride produces opalescence, and ferric chloride a deep brown
coloration.
Tannage, with the partly neutralised product, was rapid, the pelt being
nearly tanned through in twenty-four hours, excepting a small white
streak in the middle; after a further twenty-four hours this streak had
vanished, and the completely tanned, dark grey-coloured leather, after
washing, fat-liquoring, and drying, was soft, full, and of good tensile
strength, very similar to the leather yielded by the
catechol-condensation product.
Of the nitro-compounds, trinitrophenol, C_6H_2(NO_2)_3OH (picric acid),
was investigated. If a concentrated solution of picric acid is brought
into contact with pelt it will penetrate the latter completely in a few
days; it is, however, difficult to fat-liquor the resultant leather,
since the fat is absorbed only with difficulty. If a pelt treated in
this way be dried, a soft but rather flat leather results, the colour of
which easily rubs off, the leather also tasting intensely bitter. These
disagreeable qualities prevent a general use of this material for
tanning purposes; in spite of them, however, picric acid, in admixture
with boracic acid, salicylic acid, and glycerol, is used in the
production of the so-called transparent leather. The latter is very
flexible and possesses great tensile strength, but loses the latter
quality when exposed to heat, and, when stored, also loses its
flexibility. By simply washing with water, the leather is reconverted
into pelt.
When picric acid is treated with hot sulphuric acid and formaldehyde
gradually added, a dark coloured water-soluble condensation product is
formed which strongly precipitates gelatine. Exposed to the action of
bromine, the condensation product yields a mass which is insoluble in
water.
Experience has taught that the amino bodies--the basic N-derivatives of
the phenols--do not yield substances possessing tannoid properties on
condensation. On account of their importance, however, a few have been
included in this series of investigations.
Aminobenzene, C_6H_5NH_2, aniline, treated with sulphuric acid, yields
the water-soluble aniline sulphate, which, on cautious addition of
formaldehyde, yields a reddish-coloured gel, insoluble in water, in
addition to a small volume of a reddish-yellow liquid. The latter
precipitates gelatine, but is not capable of converting pelt into
leather. The insoluble gel is likewise insoluble in alcohol, so that
tanning experiments with this substance are excluded.
Dimethylaniline, C_6H_5N(CH_3)_2, when treated with sulphuric acid, yields a
product soluble in water which neither reacts with nor fixes
formaldehyde. Hence the substance does not precipitate gelatine.
If, on the other hand, nitrosodimethylaniline,
NO
|
C_6H_4
|
(CH_3)_2
is sulphonated, and the water-soluble sulphonation product heated with
formaldehyde for some time, the product remains soluble in water and
precipitates gelatine. No tanning effect could, however, be detected.
Arylsulphaminoarylsulphonic acids and arylsulphoxyarylsulphonic acids
precipitate gelatine but are devoid of tannoid character. The latter is
acquired by compounds belonging to this class containing two or more
sulphamino groups, or when they, in addition to one sulphamino group,
contain a sulphoxy group and another sulphonic group. According to
Ger. Pat., 297,187 (Society oc Chemical Industry, Basle), such compounds
are obtained when, for instance, sodium sulphanilide in alkaline
solution acts upon nitrotoluenesulphochloride, and the resulting
nitrotoluenesulphamino compound is subsequently reduced with acetic acid
and iron. The resulting aminotoluenesulphaminobenzenesulphonic acid is
finally treated with p-toluenesulphonic chloride till the latter
disappears. A compound of the composition
-----NH-----SO_2-----
^ ^ ^
| | | | | |
| | | |---NH---| |
V V V
SO_2Na CH_2
is thereby obtained, which, when acidified, is readily capable
of being used for tanning purposes.
The intermediary product of the aminotoluenesulphaminobenzenesulphonic
acid obtained by this process may again be employed for the purpose of
reacting with one-half molecule soda and 1 molecule
nitrotoluenesulphonic chloride. The following compound is obtained--
---NH---SO_2--- ---NH---SO_2---
^ ^ ^ CH_3 ^
| | | | | | | |
| | | | | | | |
v v ---NH---SO_2--- v v
SO_3Na CH_3 CH_3
If _p_-toluenesulphaminobenzenesulphonic chloride is condensed
with sodium sulphanilide, a compound,
---SO_2---NH--- NaSO_3
^ ^ ^
| | | | | |
| | | | | |
v v ---SO_2---NH--- v
SO_3Na
is obtained which, when acidified, exhibits tannoid properties.
On condensing sodium phenolsulphonate with nitrotoluenesulphonic
chloride, reducing the condensation product and condensing the latter
with _p_-toluenesulphonic chloride, a compound similar to the above is
obtained--
---O---SO_2---
^ ^ ^ CH_3
| | | | | |
| | | | | |
v v ---NH---SO_2--- v
NaSO_3 CH_3
Again, a similar product is obtained when
_p_-toluenesulphaminobenzenesulphonic chloride or its homologues or
isomers are condensed with sodium-_o_-cresylsulphonate--
---SO_2---NH--- SO_3Na
^ ^ ^
| | | | | |
| | | | CH_3| |
v v ---SO_2---O--- v
CH_3
The chloride of this compound may again be condensed, for instance, with
sodium aminotoluenesulphaminobenzene-sulphonate, and yields the
compound--
---NH---SO_2---
^ ^ ^ ---NH---SO_2--- ^
| | | | | | | |
| | | | | | | |
v v ---NH---SO_2--- v v CH
NaSO_3 CH_3
The three latter compounds, when dissolved in water and the solution
acidified, exert tanning action.
It is also possible to employ mixtures of arylsulphaminobenzylsulphonic
acids in acidified aqueous solution for tanning purposes. According to
Ger. Pat., 297,188, such mixtures are obtained by nitrating
benzylchloride and heating with an equimolecular amount of sodium
sulphite; the sodium nitrobenzylsulphonate thus obtained is reduced to
aminobenzylsulphonic acid with iron and acetic acid, and finally
condensed with the calculated amount of _p_-toluenesulphonic chloride. A
mixture _o_- and _p_-toluenesulphaminobenzylsulphonic acid [Footnote 1:
Cf. also Ger. Pat, 319,713 and 320,613.] thus results.
Amongst _aromatic alcohols_ the dihydric alcohols show characteristic
behaviour; the latter combine with sulphonic acids with the elimination
of water, condensation taking place without formaldehyde, and the
resulting products being soluble in water and possessing tannoid
properties. [Footnote 2: Ger. Pat., 300,567, of 20th September 1917.]
In addition to phenolic mono- and disulphonic acids (and higher
sulphonation compounds), the homologues, cresols, xylenols, and
naphthols enter into reaction. The two components condense with great
ease, liberating heat; dilute solutions (of the components) are heated
to about 100° C., the process being complete in a few minutes. The
products obtained are exceedingly pure and are easily
crystallisable. Employing 1, respectively 2, molecules of sulphonic
acid, the reactions take place according to:--
OH CH_2.OH OH OH
}C_6H_4 + HO.C_6H_3{ = H_2O + }C_6H_3-CH_2-C_6H_3{
HSO_3 CH_2.OH HSO_3 CH_2.OH
OH
OH CH_2.OH CH_2.C_6H_3{
}C_6H_4 + HO.C_6H_3{ = 2(H_2O) + HO.C_6H_3{ HSO_3
HSO_3 CH_2.OH | OH
CH_2.C_6H_3{
HSO_3
OH CH_2.OH OH OH
}C_6H_3.CH_3 + HO.C_6H_3{ = H_2O + }C_6H_2.CH_3.CH_2.C_6H_3{
HSO_3 CH_2.OH HSO_3 CH_2.OH
OH
OH CH_2.OH CH_2.C_6H_2.CH_3{
}C_6H_3.CH_3 + HO.C_6H_3{ =2(H_2O) + HO.C_6H_3{ HSO_3
HSO_3 CH_2.OH | OH
CH_2.C_6H_2.CH_3{
HSO_3
OH CH_2.OH OH OH
}(C_10)H_6 + HO.C_6H_3{ = H_2O + }(C_10)H_5.CH_2.C_6H_3{
HSO_3 CH_2.OH HSO_3 CH_2.OH
OH
OH CH_2.OH CH_2.(C_10)H_5{
}(C_10)H_6 + HO.C_6H_3{ =2(H_2O) + HO.C_6H_3{ HSO_3
HSO_3 CH_2.OH | OH
CH_2.(C_10)H_5{
HSO_3
The condensation products above enumerated were tested with regard to
their tanning power, both non-neutralised and partly neutralised (1:10,
1:20, and 1:30 c.c. N/10 NaOH) samples being examined. In all cases
rapid tannage was observed yielding firm and soft leathers of light
brown colour and varying degrees of swollenness.
Relatively to their reactions, all the products strongly precipitate
gelatine, whereas only the condensation products of phenol, cresol, and
xylenol derivatives give a characteristic coloration with iron salts.
The tannin contents of the non-neutralised condensation products lie
between 72-80 per cent.--figures which clearly indicate the purity and
efficiency of these substances.
Notable amongst _aromatic acids_ is salicylic acid, C_6H_4.OH.COOH,
which at higher temperatures is easily sulphonated with concentrated
sulphuric acid; the sulphonation product represents a white solid, which
easily dissolves in water forming a clear liquid. The sulphonic acid,
when mixed with about one-third of its weight of water and heated to
about 120° C., is easily condensed with formaldehyde. Towards the end
of the reaction, considerable frothing sets in, but in spite of the high
temperature required by this reaction no insoluble bakelites are
formed. A reddish-brown fluid is obtained easily soluble in water, to
which it imparts a brown colour. An aqueous solution of the product
completely precipitates gelatine, gives a strong opalescence with
aniline hydrochloride and a deep violet coloration with ferric chloride.
Neutralised as usual, the product, in a 3° Bé solution, converts pelt
within three days into a white, full leather of good tensile strength.
This process has been patented by the Deutsch-Koloniale Gerb und
Farbstoff Gesellschaft (German-Colonial Tanning and Colour Extracts
Ltd.) in Karlsruhe, the letters patent also including the ring
homologues of salicylic acid. Similar results are obtained when
cresotinic acid (hydroxy-toluic acid), OH.C_6H_3.CH_3.COOH, is employed
as base.
If the phenyl ester of salicylic acid, _Salol_,
HO.C_6H_4.CO.O.C_6H_5
is sulphonated, a product is obtained which is easily soluble in water,
but which is identified as a mixture of the sulphonation products of
salicylic acid and phenol, the salol being dissociated on
sulphonation. The temperature must not exceed 80° C. by condensation
with formaldehyde, or insoluble bakelite will be formed from the phenol;
the aldehyde must also be added gradually. An aqueous solution of the
partly neutralised condensation product has a pronounced tanning effect
on pelt, and converts the latter into leather in one to two days; the
leather being very similar to that produced by the salicylic acid
condensation product. The qualitative reactions of the product in
aqueous solution are the same as those given by the salicylic acid
condensation product.
Salicylic acid may, however, also be condensed with formaldehyde without
first being sulphonated; in this case, a little hydrochloric acid should
be present. A product slightly soluble in water is obtained, which may
be looked upon as being methylenedisalicylic acid. In alkaline solution
it is easily soluble,
[Footnote 1: Its solubility in alcohol and alkalies renders this product
an effective and cheap substitute for shellac.--_Transl._]
the liquid possessing an intensely bitter taste. The sodium salt gives a
deep violet coloration with ferric chloride, a slight precipitate with
gelatine, and slight opalescence with aniline hydrochloride. In contact
with pelt, however, it exhibits no tanning effect, but when dissolved in
alcohol, a pickling effect may be observed.
[Footnote 2: A similar reaction is observable in the case of the sodium
salts of METHYLENEDISALICYLIC acid brommated or iodised, which form a
clear solution varying from red to reddish-brown.]
The attempt at preparing a condensation product from
sodium-_m_-hydroxybenzoate by means of formaldehyde and bisulphite is
worthy of attention. A dark brown, viscous liquid is obtained which is
perfectly soluble in water, and the aqueous solution of which gives
opalescence with gelatine, a precipitate with aniline hydrochloride, and
a bluish-black coloration with ferric chloride. Its behaviour towards
pelt is very similar to that of phenolsulphonic acid, and it yields a
similar leather.
A very similar condensation product was obtained by condensing
sodium-_p_-hydroxybenzoate with formaldehyde and subsequent sulphonation
with sulphuric acid. From a practical standpoint, however, these
substances cannot be employed, since their tanning action is only
effective in acid solutions of such concentration of acid as would
gelatinise the pelt.3
If, on the other hand, non-condensed methane derivatives of phenol,
_e.g._, hydroxyphenylmethanesulphonic acid, are partly neutralised and a
solution of the product thus obtained used for tanning experiments, no
tanning action is observable. The acidified solution does not
precipitate gelatine, and gives a dark brown coloration only with ferric
chloride.
GALLIC ACID, C_6H_2(OH)_3COOH, when heated with sulphuric acid, is
easily converted into the insoluble rufigallic acid, which is also
insoluble in alcohol. If, however, gallic acid is heated with an excess
of sulphuric acid, the product cooled and treated with formaldehyde, a
deep brown condensation product is obtained which is soluble in alcohol,
and in this state is capable of converting pelt into a substance similar
to leather which, though rather hard, possesses good tensile
strength. This water-insoluble condensation product is also soluble in
alkalies, the solution exhibiting properties similar to that described
above. Gallic acid, therefore, is not a suitable base for the production
of synthetic tannins soluble in water.
Phthalic acid also is difficult to sulphonate: the sulphonated compound
treated with formaldehyde gives only water-insoluble condensation
products.
3. Condensation Of Naphthalene Derivatives
The simplest method of condensing [Greek: b]-naphthalene-sulphonic acid
is to heat it at 135° C. at a pressure of 20 mm. for several
hours.[Footnote: Austr. Pat., 61,061, of 10th September 1913.] The
resulting product is a cheesy mass which reacts strongly acid. By
reducing the acidity of the substance to 1 gm. = 10 c.c. N/1O NaOH, a
grey, cheesy mass results, which easily dissolves in water, the solution
being coloured a light yellow-brown and precipitating gelatine aniline
hydrochloride; no coloration, however, appears on adding ferric
chloride.
The condensation of [Greek: b]-naphthalenesulphonic acid, however,
proceeds with much greater energy in the presence of formaldehyde. In
practice, for instance, 10 kilos of naphthalene is heated with the same
weight of concentrated sulphuric acid (66° Bé), when the mixture is
converted into [Greek: b]-naphthalenesulphonic acid by heating for
several hours at 150°-160° C; the sulphonation completed, the sulphonic
acid is cooled to about 85° C., and 4 kilos of formaldehyde (30 per
cent, by weight) slowly added; finally, the product is stirred at the
temperature mentioned till all formaldehyde has combined.[Footnote:
Austr. Pat., 69,194, of 25th June 1915; Ger. Pat, 290,965.]
Tanning experiments with this product yielded, in a short time, a nearly
white coloured leather (see later).
In addition to formaldehyde, there are other substances which induce
condensation of naphthalenesulphonic acid; if, for instance, sulphur
chloride is allowed to act upon [Greek: b]-naphthalenesulphonic acid, a
light brown solid of pronounced acidic character is obtained; if the
latter is partly neutralised with caustic soda, a greyish-brown solid
results, which dissolves in water with a light brown colour, the
solution precipitating gelatine and aniline hydrochloride, but giving no
coloration with ferric chloride.[Footnote: Austr. Pat., 96,194.]
Tanning experiments with this product in aqueous solution gave a light
brown, rather soft leather, and this, in addition to the qualitative
reactions of the substance, prove that this method of condensation
hardly alters the character of the product from a tanning point of
view. The brown coloration imparted to the leather tanned with this
condensation product owes its existence to coloured intermediary
products.
Attempts at condensing chloronaphthalenesulphonic acid and
nitronaphthalenesulphonic acid resulted in soluble condensation products
which gave some of the reactions given by the tannins (precipitation of
gelatine and aniline hydrochloride), but which were incapable of tanning
pelt, a light tannage being effected on the surface only.
[Greek: a]-Naphthol dissolved in hot concentrated sulphuric acid and
heated for some time on the water bath, yields the light brown,
water-soluble [Greek: a]-naphtholsulphonic acid. A dilute solution of
the latter, when treated with formaldehyde in the cold, undergoes no
change; on heating the mixture on the water bath a brown precipitate is
thrown down. If gelatine solution is added to the opaque liquid, a
yellow flocculent precipitate separates. If caustic soda is added to the
opaque liquid containing the condensation product described above, a
clear solution results from which no deposit separates on the addition
of acetic acid. Gelatine is precipitated by this solution.
The concentrated hot a-naphtholsulphonic acid, upon addition of
sufficient formaldehyde, effervesces strongly and yields a dark brown
condensation product insoluble in water, but soluble in caustic soda. If
acetic acid is added in excess to the alkaline solution, the resultant
solution strongly precipitates gelatine.
A suspension in water of the insoluble condensation product does not
precipitate gelatine.
b-Naphthol, dissolved in hot concentrated sulphuric acid and heated for
some time, yields the light brown, viscous b-naphtholsulphonic acid. A
dilute solution of the latter, mixed with formaldehyde, remains clear;
when heated on the water bath, however, it assumes a dark,
reddish-yellow colour, and remains soluble in water and precipitates
gelatine strongly. This condensation product, on adding excess of
caustic soda, assumes a deep blue coloration, the alkaline solution
giving no precipitate with gelatine; on adding acetic acid the solution
turns brown, remains clear, and now precipitates gelatine.
The concentrated b-naphtholsulphonic acid heated with formaldehyde on
the water bath yields as condensation product a dark, reddish-yellow
mass, soluble in water, which precipitates gelatine. A dilute solution,
when allowed to act upon pelt, gave in a few days a light brown leather,
the properties of which are very similar to those possessed by vegetable
tanned leathers.
The use of naphtholsulphonic and aminonaphtholsulphonic acids for the
manufacture of synthetic tannins is protected by Ger. Pats., 293,640,
293,693, 293,042, and 303,640. [Footnote: _Cf._ Austr. Pat., 70,162.]
It is a remarkable fact that non-condensed methane derivatives of
naphthol, _e.g._, b-naphthol-a-methanesulphonic acid, dissolved in water
and partly neutralised, are devoid of tanning character when allowed to
act upon pelt. Neither does this substance precipitate gelatine, but it
does give a deep blue coloration with ferric chloride.
The condensation product of b-naphthol above referred to precipitates
gelatine and aniline hydrochloride and gives a brown coloration with
ferric chloride.
Thionaphtholsulphonic acid, when acted upon by formaldehyde, yields a
condensation product of the following constitution:--
HSO_4 ^ ^ SH SH ^ ^ HSO_4
| | | | | |
| | |_____CH_2_____| | |
v v v v
This is a light yellow powder which, dissolved in water, yields an
opaque solution; the latter only exhibits any tanning properties when it
is not neutralised and even slightly acidified and then precipitates
gelatine, aniline hydrochloride and barium chloride; dissolved in
alkali, it forms a clear, yellow solution devoid of tannoid
properties. Leather tanned with the acidified solution is very similar
to those tanned with the phenolsulphonic acid condensation products; its
colour, however, is more pronouncedly yellow.
b-Naphthol condensed with hydrochloric acid and formaldehyde yields a
methylenedinaphthol, which is insoluble in water; the sodium salt,
however, easily dissolves. The same condensation, however, takes place
in alkaline solution with direct formation of the sodium salt. The
condensation product gives a slight precipitate with gelatine, and a
bluish-grey precipitate with ferric chloride; acids re-precipitate the
insoluble methylene compound. Towards pelt it exhibits tanning
properties, whereby the insoluble product referred to above is
deposited, and soft, full, and white leather is obtained, possessing,
however, but little tensile strength.
4. Condensation of the Anthracene Group
Anthracene heated with excess sulphuric acid yields the water-soluble
anthracenesulphonic acid; the latter, when heated with formaldehyde,
yields water-soluble, reddish-brown condensation products, which remain
soluble on prolonged heating with formaldehyde. The aqueous solution of
the condensation product shows no particular reactions; it gives a
flocculent precipitate with gelatine and a green precipitate with copper
sulphate, soluble with blue colour in excess of the reagent.
The partly neutralised solution tans pelt--to which it imparts a brown
colour--in eight days, but on the surface only; the inner layers are
merely pseudo-tanned (white colour). When dried, pelt thus treated
yields a full and soft leather with brown grain and flesh possessing but
little tensile strength. Hence, this condensation product exerts a
pickling rather than a tanning effect.
Anthraquinone heated with sulphuric acid and treated with formaldehyde
in the usual manner, yields a substance which, when mixed with water,
forms an opaque, milky solution. This is not altered by excess of
caustic soda. The aqueous solution precipitates gelatine and aniline
hydrochloride; all other tannin reagents give no reaction.
The partly neutralised solution of the condensation product exerts, in
the main, a pickling action on pelt; only the surface of which is
tanned, with brown colour, the remainder being merely pickled (white
colour). During "tannage," bakelite is formed in the liquid, and
practically all solubles originally present are deposited. The tannage
completed, a light brown, fairly soft and full leather, possessing
little tensile strength, results; this leather can be washed only with
great difficulty and approaches more the character of a pickled pelt.
1-Hydroxyanthraquinone, 1,5-dichloroanthraquinone,
l,5-diaminoanthraquinone, 1-methylaminoanthraquinone,
1-benzoylamino, 6-chloranthraquinone, 1-_m_-toluidoanthraquinone, when
treated with sulphuric acid and formaldehyde, all yield condensation
products which are but little soluble in water, and which do not at all
precipitate gelatine. Tanning experiments with these condensation
products in alcoholic solution yielded empty leathers of pronounced
pickle character.
If, however, 1-methylamino-4-bromanthraquinone is condensed with
sulphuric acid and formaldehyde, a condensation product is obtained
which is but slightly soluble in water, but which precipitates gelatine.
When phenanthrequinone is heated with excess of sulphuric acid for some
time, a water-soluble, reddish-yellow coloured condensation product
results. The latter, when treated with formaldehyde in the cold and then
finally heated, gradually fixes the formaldehyde and forms a substance
soluble in water. If the heating, however, is prolonged, insoluble
bakelites are formed, which are neither soluble in alkali nor in
alcohol.
An aqueous solution of these condensation products gives no reactions
with the usual tannin reagents, though it completely precipitates
gelatine. When acting upon pelt, the partly neutralised dilute solution
of the condensation product pickles the former, and after a few days the
pelt is converted into a light brown, full, and rather soft leather
possessing good tensile strength.
When the condensation product is acted upon by bromine in hot aqueous
solution, an additive compound is formed and the resulting product is
soluble in water. The aqueous solution of the brominated product gives
no special reactions with the usual tannin reagents, but precipitates
gelatine completely. Its tanning action upon pelt is much slower than
that of the original condensation product; the surface of the pelt only
is tanned with brown colour, the inner pelt being only pickled (light
brown colour). When dried, a hard and empty leather of good tensile
strength is obtained, possessing mainly the properties of a pickled
pelt.
CO OH
^ ^ ^
QUINIZARENE, | | | | , treated with sulphuric acid
| | | |
v v v
CO OH
and formaldehyde, yields a condensation product which is but little
soluble in water and which does not precipitate gelatine.
QUINOLINE, when sulphonated and condensed with formaldehyde, yields a
dark coloured condensation product, completely soluble in water; the
solution does not precipitate gelatine.
OXYQUINOLINE exhibits similar behaviour.
On the other hand, the use of _retene_ (methylisopropylphenanthrene),
CH_3 ^ ___________ ^
| | | |
| |___CH:CH___| |
C_3H_7 v v
for the production of synthetic tannins, is protected by
Ger. Pat., 290,965 [Footnote 1: _Cf_ Austr. Pat., 69,194]
5. Di- and Triphenylmethane Groups
If DIPHENYLMETHANE, (C_6H_5)_2CH_2, is heated with excess sulphuric
acid, a dark blue mass, easily soluble in water, is obtained. The
product gently heated with formaldehyde yields a brown, water-soluble
condensation product; once condensation is complete, the product will
stand stronger heat. If, on the other hand, more formaldehyde is added,
brown, water-insoluble bakelites are formed. The water-soluble
condensation product precipitates gelatine, but not aniline
hydrochloride. Dissolved in water, it possesses tannoid properties: the
pelt is, however, tanned on the surface only, the intermediary layers
being merely pickled; after four days in the solution, the pelt after
drying was found to be converted into a greyish-brown, badly coloured
leather, which was empty, hard, and possessed but little tensile
strength.
CARBAZOLE (dibenzopyrrole),
^ _____ ^
| | | |
| |__ __| |
v v v
N_3
on the other hand, was found a suitable base for the commercial
production of synthetic tannins; its use is protected by Ger. Pat,
290,965.
TRIPHENYLMETHANE, (C_6H_5)_3CH, heated with excess sulphuric acid,
yields a nearly black mass which, when condensed with formaldehyde in
the cold, and subsequently heated, yields a mass which is soluble in
water. With gelatine and aniline hydrochloride it exhibits reactions
similar to those given by the diphenylmethane condensation products; its
tanning properties also are similar to those of the latter. The
resultant leather is black, but is soft and full and possesses good
tensile strength.
Baeyer's observation, [Footnote: _Ber_., 1872, 5, 280, 1096.] that
pyrogallol on condensation with formaldehyde yields an amorphous body
soluble in water, which precipitates gelatine and is very similar to
tannin, was confirmed by Caro [Footnote: _Ibid_., 1892, 25, 947.] and
Kahl. [Footnote: _Ibid_., 1898, 31, 114.] These investigators found
that by the condensation of phenols and hydroxybenzoic acids with
formaldehyde, diphenylmethane derivatives were formed; pyrogallol yields
hexahydroxydiphenylmethane--
C_6H_2(OH)_3
CH_2{
C_6H_2(OH)_3
Nierenstein [Footnote: _Collegium_, 1905, 221.] repeated these
experiments, and found that in addition to the insoluble
diphenylmethanes, water-soluble bodies were formed, which latter
precipitate gelatine. The condensation product yielded by gallic acid
was identified as hexahydroxyaurinecarboxylic acid--
_C_6H(OH)_3COOH
C{-C_6H(OH)_3COOH
| }C_6H(OH)_2COOH
O
which is formed in addition to hexahydroxydiphenylmethane-dicarboxylic
acid--
C_6H(OH)_3COOH
CH_2{
C_6H(OH)_3COOH
Baeyer's experiment with pyrogallol probaly also yields, according to
Nierenstein, another compound of the following constitution--
C_6H_2(OH)_3
C{-C_6H_2(OH)_3
|_}C_6H_2(OH)_2
O
Nierenstein considers these bodies confirmation of his hypothesis of the
existence of a "tannophor,"--CO--, in the tannins.
This supposition was adopted by Stiasny [Footnote: _Gerber_, 1905, 233.]
and Kauschke [Footnote: _Collegium_, 1906, 362.] and the latter points
out that these easily soluable substances exhibit tanning
properties. Nierenstein [Footnote: _Ibid_., 1906 424.] was further able
to show that by all processes of condensation between phenols (or
hydroxybenzoic acids) and formaldehyde, compounds of the character of
hydroxyaurine (or hydroxyaurinecarboxylic acid) were formed in addition
to the insoluble hydroxydiphenylmethanes (or
hydroxydiphenylmethanecarboxylic acids), the former possessing the
characteristic tannophor group and hence precipitating gelatine, _i.e._,
exerting tanning action. If the formation of leather is viewed in the
light of Schiff's base, [Footnote: _Ibid_., 1905, 159.] one may
consider the constitution of a hexahydroxyaurinecarboxylic acid leather
as follows:--
_C_6H_2(OH)_3.COOH
C{-C_6H_2(OH)_3.COOH
|_}C_6H_2(OH)_2.COOH
R-N
In the preparation of these and similar condensation products,
Nierenstein and Webster [Footnote: _Ber_., 1908, 41, 80.] observed a
peculiar steric effect of the carboxyl group. Each 2.5 gm. of the phenol
or the acid in question were dissolved in 30 c.c. of water, the solution
brought to boil and 5 c.c. formaldehyde (20 per cent.) and 2.5
c.c. hydrochloric acid added drop by drop; the precipitate formed was
filtered off after twenty-four hours, dried at 110° C. to constant
weight, extracted (in a Gooch crucible) freely with water, and the
residue again dried at 110° C. till constant. The following values were
obtained:--
Total Insol. Aq. Sol. Aq. Oxy-
Precipitate Diphenylmethane aurinecarboxylic
in Grammes. Derivatives Acid.
Per Cent. Per Cent.
Phloroglucinol 2.4002 100 ...
Hydroquinone 2.3716 100 ...
" 2.0542 100 ...
Pyrogallol 2.5150 100 ...
" 2.7940 100 ...
Pyrocatechol 2.9805 100 ...
" 2.9574 100 ...
Resorcinol 2.9954 100 ...
" 2.9725 100 ...
Gallic acid 2.0706 78.84 21.16
" 1.2240 83.18 16.82
" 1.1405 59.94 41.06
[Greek: b]-Resorcylic acid 2.1040 51.08 48.92
" " 2.2008 47.12 52.88
Protocatechuic acid ... ... ...
" " ... ... ...
Vanillic acid ... ... ...
Tannin 2.0599 ... Nearly all sol.
Digallic acid 2.1042 80.16 19.84
Leucodigallic acid 2.0041 1.94 98.06
With the introduction of the carboxylic group the tendency of
condensation to diphenylmethane derivatives is lessened; by
protocatechuic acid the tendency is nil. Nierenstein considers this
reaction analogous to the formation of cork, to the genetic relation of
which with the diphenylmethane formation Drabble and Nierenstein have
referred in an earlier publication. [Footnote: _Biochemical Jour._.,
1907, 2, 96.] It is hence possible that the plants may employ
formaldehyde as a methylation medium, and produce these insoluble
condensation products for the purpose of ridding themselves of the
poisonous phenols and aromatic hydroxy acids (and tannins), in addition
to oxidising processes whereby
phlobaphenes, ellagic acid, etc., are formed.
The reaction between phenols and aldehydes has been further studied by
Michael, [Footnote: _Amer.Jour_., 5, 338; 9, 130.] who prepared a
condensation product from phenol and resorcinol with benzaldehyde, and
Russanow, [Footnote: _Ber_.9 1889, 22, 1944.] who also employed
benzaldehyde and phenol. Lipp [Footnote: Diss., Bern., 1905.]
investigated the action of benzaldehyde and piperonal on phenols,
anisoles, cresols, cresylic ether, resorcinol, and the ether of the
latter and phenol, and showed that when free phenols are condensed with
benzaldehyde the hydroxyls occupy the same position as by the
interaction between benzaldehyde and the corresponding phenolic
ethers. The resulting dihydroxytriphenylmethane derivatives form
beautiful crystals, which on oxidation are converted into benzaurines,
the constitution of the latter probably being--
O= ^=_____ ^ OH
| | | |
| |== __| |
=v v v
C
|
C_6H_5
In alkalies, the hydroxylated triphenylmethanes dissolve without
imparting any colour to the solution; by concentrated sulphuric acid
they are taken up with intense coloration.
If the hydroxyls occupy the ortho-position to methyl, they may form
xanthenes by splitting off water--
O
^ ^ ^
| | | |
| | | |
CH_3 v v v CH_3
CH
|
C_6H_5
In the benzene series this reaction is difficult to establish, and has
to be induced by distilling the particular dihydroxy-diphenylmethane at
ordinary pressure. In the naphthalene series, on the other hand, the
ring closes up by, for instance, the condensation of [Greek: b]-naphthol
with benzaldehyde or paraldehyde, and yields the following compounds:--
C_10H_6 C_10H_6
C_6H_5-CH{ }O CH_3-CH{ }O
C_10H_6 C_10H_6
These xanthenes are white, silk-glossy needles, which are soluble in
water and in alkalies. In concentrated sulphuric acid, they are taken up
with beautiful fluorescence.
6. Summary
From the qualitative reactions of the different condensation products
described it may be seen that their tannoid properties are not dependent
on whether they precipitate gelatine or are adsorbed by hide powder or
not. Hydroxynaphthylmethanesulphonic acid, for instance, precipitates
gelatine but does convert pelt into leather; on the other hand, sodium
dicresylmethanesulphonate does not precipitate gelatine, and neither
does it tan pelt; nevertheless it is adsorbed by hide powder as "tanning
matter". The author discovered that _o_-nitrophenol does not precipitate
gelatine, but has some tanning action on both hide powder and pelt.
Relatively to the possibilities of forming condensation products
possessing tannoid properties, the following may be stated:--
All mono- and polyhydric phenols may be converted into true tanning
matters by either condensing them as such, or after their conversion
into the corresponding sulphonic acids, by substances capable of
eliminating the elements of water. It makes no difference to the final
product whether the condensation is the first step followed by
sulphonation and consequent solubilisation of the intermediary insoluble
product, or whether, vice versa, the sulphonic acid is subjected to
condensation. Alkaline solution of phenols may also be condensed, the
reaction products, when condensed, constituting tanning matters soluble
in water.
Among the substitution products of the phenols, the thio-, chloro-,
bromo-, nitro-, and aminophenols as a rule yield tanning matters similar
in character.
The quinones are as such--_i.e._, without being condensed--substances
possessing tannoid properties.
The aromatic dihydric alcohols are easily condensed with the different
sulphonic acids and yield valuable tanning matters.
Of aromatic acids all those which yield water-soluble sulphonation
products seem suitable for the industrial production of tanning
matters. If the acids themselves do not yield water-soluble sulphonation
products, the alkali salts of the latter may be condensed with
formaldehyde, and the resulting products then constitute tanning matters
provided their solutions can be neutralised or faintly acidified without
the solute being thrown out of solution in insoluble form.
The diphenyl derivatives of the above groups often possess tannoid
properties.
The same holds good of those compounds with condensed nuclei
(naphthalene, anthracene, etc.), and all their derivatives which satisfy
the above conditions.
The choice of condensing agent is, as a rule, of little
significance. Elimination of the elements of water by the mere
application of heat succeeds in few cases only, since the high
temperature required to induce reaction in many cases causes
decomposition of the substances. This difficulty is overcome by heating
_in vacuo_. Condensation with formaldehyde always succeeds, with
acetaldehyde and benzaldehyde only partly.
The action on hide powder, pelt, and gelatine by these characteristic
substances is tabulated below:--
Relative Behaviour towards
Substance. Gelatine. Hide Powder Pelt
Formaldehyde ... ... Tanning
Phenol Ppte. ... ...
Chlorophenol " ... ...
Surface
Tribromophenol Slight ppte Tanning tanning
_o_ Nitrophenol No ppte " "
Bromonitrophenol Slight ppte " "
Trinitrophenol Ppte " Tanning
Bromotrinitrophenol Slight ppte " "
_p_ Aminophenol Ppte ... ...
_m_ Dihydroxybenzene " ... ...
Orcinol " ... ...
_p_ Dihydroxybenzene " Tanning Tanning
Monochloro _p_
dihydroxybenzene " ... ...
_o_ Dihydroxybenzene " ... ...
Pyrogallic acid " ... ...
Surface
Tribromopyrogallic acid " Tanning Tanning
Gallic acid No ppte Not tanning Not tanning
Bromophloroglucinol Ppte Tanning "
Gallotannic acid " " Tanning
Galloflavine Slight ppte " Not tanning
Quinone " " Tanning
Bromosalicylic acid " " Not tanning
Dinaphthylmethanedisulphonic acid Ppte " Tanning
Diphenylmethanedisulphonic acid " " "
Dicresylmethanedisulphonic acid " " "
Sodium
dicresylmethanedisulphonate acid No ppte " Not tanning
Dixylylmethanedisulphonic acid Ppte " Tanning
Naphtholdisulphonic acid " Not tanning Not tanning
Methylenedinaphthol " Tanning Tanning
Hydroxyphenylmethanesulphonic " " "
acid Not tanning
Hydroxynaphthylmethanesulphonic Slight ppte " "
acid
Diaminonaphthylmethanedisulphonic Ppte Tanning Not tanning
acid
Dihydroxynaphthylmethanedisulphonic
acid " " "
Dichloronaphthylmethanedisulphonic
acid " " Surface tanning
Dinitronaphthylmethanedisulphonic
acid " " "
Dithionaphthylmethanedisulphonic
acid " " Tanning
Bromo _[Greek: b]_ naphthol [1] Slight ppte " "
Rosolic acid_ [1] Ppte " "
[Footnote 1: In alcoholic solution.]