The cooling of the reaction flask must be very efficient, a 10-15 cm.
blanket of a thorough mixture of ice and salt being used.
If this precaution is not employed, the time for the addition
of the sulfuric acid is greatly increased, provided the temperature
of the reaction mixture is still kept within the limits mentioned.
If a cork is used for the steam distillation of the reaction mixture
of acetone and sulfuric acid, it should be coated well with pitch and wired
into the flask. This is necessary because the vapors of the reaction
mixture attack an ordinary cork very badly, and soften it so much
that it is necessary to rewire it to prevent it from slipping out.
A rubber stopper is satisfactory and may be used in several runs.
The evolution of gas is so vigorous that it is not possible to distil
more than 2 l. of the original reaction mixture at one time
in the apparatus described. The connections on the apparatus,
in which the mesitylene is obtained from the crude reaction mixture,
should be tight, since the fumes evolved during the heating
are very irritating.
The product which distils during the initial heating and the three
minutes of steam distillation is mainly satisfactory material; the rest
of the steam distillation yields only a small amount of pure product.
The two portions of the distillate are, therefore, kept separate,
since the second distillate always contains a considerable amount
of high-boiling product which tends to cause emulsification of
the alkali in the purification. No recovery of acetone is made.
The mechanism of the reaction is undoubtedly as follows:
when the sulfuric acid and acetone are in contact for long periods
of time, several molecules of the acetone condense to form aldol
condensation products. These do not break down into mesitylene until
the temperature is raised in the second part of the experiment.
While the original reaction mixture is standing, the temperature
gradually rises to 40'0 or 50'0 in the course of six to ten hours,
and then gradually cools off again. It is probable that at the end
of this time (when the flask has cooled again) the reaction mixture
could be distilled with nearly as good a yield as is obtained
after standing eighteen to twenty-four hours.
The wide variation in yields which are mentioned in the experimental
part is probably due to a slight change in the grade of the chemicals
which are used in this preparation. 3. Other Methods of Preparation
The cheapest and most convenient method by which mesitylene may
be prepared is by the action of a dehydrating agent upon acetone;
the agent most commonly used is sulfuric acid.[1] It has been shown
also that phosphoric acid will convert acetone to mesitylene.[2] A number
of other methods have also been used for the preparation of mesitylene:
the action of sulfuric acid on methyl acetylene;[3] the action
of sulfuric acid on mesityl oxide and phorone;[4] the action of
aluminium{sic(british)} chloride on methyl chloride and benzene;[5]
the action of mineral acids upon mesitoyl or benzoyl mesitylene;[6]
the action of phosphoric acid upon diaceto-mesitylene;[7] the treatment
of methylene-3-dimethyl-1, 5-cyclo-hexene-1 with bromine and then
with alcoholic potash.[8]
[1] Ann. 141, 131 (1867); 147, 43 (1868); 278, 210 (1893); Bud.
soc. chim. (2) 40, 267 (1883); J. prakt. Chem. (1) 15, 129
(1838); Am. Chem. J. 15, 256 (1893); 20, 807 (1898).
[2] J Chem. Soc. 99, 1251 (1911).
[3] Ber. 8, 17 (1875).
[4] Ber. 7, 1169 (1874); 10, 858 (1877).
[5] Ber. 12, 329 (1879); Ann. chim. phys. (6) 1, 461 (1884).
[6] Ber. 32, 1910 (1899).
[7] Ber. 32, 1563 (1899).
[8] Ber. 43, 3093 (1910).