Metcalf, (loc. cit.), gives a method for the preparation of the
sulphonic acids of para toluidine, and states that both the possible
acids are formed, with the meta position (referred to the methyl
group) in good yield. Schultz and Julius, (“Farbestoff Tabellen”,
1894 Edition, Trans, by F. C. Green), and Nevile and Winther, (loc.
cit.), say that the sulphonation of para toluidine gives a mixture of
the sulphonic acids, with the ortho (referred to the methyl group)
sulphonic acid in a much greater yield. As the sulphonation of para
nitro toluene gives (Dissertation, R. S. Norris, BPL. 5976.109) a
ninety-five per cent yield of the ortho sulphonic acid, it would seem
as though the amino compound should give a large yield of the acid
(sulphonic acid group ortho to the methyl group,) and such was found to
be the case.
I followed Metcalf’s directions as follows: in an apparatus fitted with
an automatic stirrer and a thermometer I placed 200 grams (110cc.) of
twenty per cent fuming sulphuric acid. I then slowly added with rapid
stirring 100grams of powdered para toluidine which was obtained from
the Newport Chemical Works. The addition took thirteen minutes. The
initial temperature of the solution was 18°C. The maximum was obtained
when about half the toluidine had been added and was 148°C. The final
temperature was 130°C. After addition was complete I stirred for five
minutes, and then slowly heated the mixture to 180°C. and kept it
there for an hour. Sulphonation was then complete. I then poured it
into an equal volume of cold water (220cc.) in an evaporating dish and
allowed it to stand over night. The next day the dish seemed filled
with grayish crystals; these were filtered off. The solution was
allowed to stand for four weeks with no results. (Metcalf says that the
disulphonic acid separated in three weeks from his solution.)
I dissolved the crude acid obtained as above in ten liters of water
(Metcalf’s directions) and precipitated the excess of sulphuric acid
with barium hydroxide, and then filtered the solution. It was a clear
brownish color. I then evaporated to 100cc. A smell of para toluidine
was always present in the vapor indicating a probable hydrolysis of the
acid. (Nevile and Winther (loc. cit.) say that this sulphonic acid can
be hydrolyzed by heating with water), A test after sulphonation and
before evaporation showed a sample to be completely soluble in water,
and no cloudiness was produced when a sample was dissolved in sodium
hydroxide, as is the case if a solution of para toluidine sulphate is
dissolved in the same reagent. When the volume had reached 100cc. a
light brown crust had formed on top of the liquid and on cooling and
filtering two products were obtained; a light brown powdery substance
in larger amount (47.5 grams.), and a small amount of hard square brown
crystals. On evaporating the filtrate nearly to dryness more material
separated which did not look like either of the above substances; it
weighed twenty grams. This has not been investigated as yet.
As the brownish powdery substance was mixed with the flat crystals
produced above, a method had to be found to separate them and purify
each. I screened them as much as possible with good results. I
removed the last of the fine powder by dissolving it off with hot
water. Metcalf said that they could be separated by their difference
in specific gravity in fifty per cent alcohol but this method was
unsuccessful. I boiled up the aqueous solution of the powdery substance
with animal charcoal, filtered and evaporated to crystallization.
Square, flat, white crystals were obtained identical in all but color
with those mentioned above. They are soluble with difficulty in cold
water, readily in hot, and insoluble in alcohol. In these properties
and in appearance they compare to the ortho sulphonic acid of para
toluidine as made and described by Parks, (Dissertation, 1892).
Metcalf and Nevile and Winther say that the meta sulphonic acid of
para toluidine forms needles, but I obtained nothing that could in any
way be called needles. This sulphonic acid had no melting point, but
charred and decomposed. A test for elements showed nitrogen and sulphur
to be present. The aqueous solution had a strong acid reaction to
litmus.
In order to see if this might be the desired acid I tried to prepare
an acid chloride with phosphorus pentachloride in the usual way, and
then intended to convert it into the amide and get a melting point
for identification. I used equal weights of the acid and phosphorus
pentachloride. I ground them together at ordinary temperature in
a mortar: no result. I heated it on a water bath: no result. I
heated it on a gauze over a flame: no result except decomposition and
carbonization. However I poured water into the mixture and decomposed
the excess of phosphorus pentachloride, and filtered. A gummy brownish
mass remained which was insoluble in all reagents tried, (methyl
and ethyl alcohol, benzine, petroleum ether, (40-60), water, carbon
tetrachloride, dilute hydrochloric acid, acetone, and acetic acid.)
Ethyl ether extracted a very small amount, just enough to color it
yellow, and leave a yellow color on porcelain on evaporation, but
not enough to make a study of the properties. On shaking up with
concentrated ammonia the mass became very finely divided, but no
solution appeared to take place; on filtering and evaporating to
dryness there was no residue.
The material mentioned above as being the final product from the
sulphonation of para toluidine, obtained in a yield of twenty grams or
ten per cent yield and then uninvestigated, has since been identified
as the acid I was looking for, or para toluidine meta sulphonic acid. A
small amount was cooled and diazotized in the usual manner; the diazo
compound seemed to be insoluble in the amount of water used, for a
white crystalline needle-like precipitate appeared. This was decomposed
by boiling with dilute nitric acid (Nevile and Winther, loc. cit.),
and on cooling small tufts of yellow needles separated. These were
filtered off and dried, then recrystallized from alcohol. The best
portions of these were then recrystallized from ether and the melting
point determined. They melted at 79.5-79.8 (uncorr.). Nevile and
Winther gave 79-80, showing that they did not purify their compound.
The product is dinitro para cresol, or 3-nitro, 4-hydroxy, 5-nitro
toluene.