PERKIN : THE DETERMINBTIOS OF ACETTIL GROUPS. 107

XV.-The D e t e r m i n a t i o n of A c e t y l Groups.

By ARTHUR GEORGE PERKIN, F.R.S.

FOR the estimation of acetyl groups in the acetyl derivatives of phenolic substances, the indirect method which consists in the deter- mination of the percentage of regenerated phenol given by hydrolysis is, when available, the most advantageous. The employment of sul- phuric acid for this purpose appears to have been first described by Liebermann (Bey . , 1884, 17, 1682), who analysed acetylquercetin by means of a 70 per cent. acid at the temperature of the water-bath. Various modifications of this method have been employed by others, and I have shown that hydrolysis with sulphuric acid in the presence of acetic acid (1 gram of substance, 15 C.C. of acetic acid, and 2 C.C. of sulphuric acid approximately) a t the boiling heat; gives good results with most acetyl compounds (Trans., 1896, 69, 210). The product of the reaction, on cautious dilution with boiling water, as a rule, deposits the phenol in a crystalline condition, and the substance is thus very easily collected and washed. This method has also been employed with a satisfactory result at the ordinary temperature (Trans., 1898, 74, 1034), and again the sulphuric acid can be replaced by hydrochloric acid ( 5 c.c.) when sulphonation is likely t o occur (Trans., 1899, 75, 448). Alkaline hydrolytic agents are sometimes serviceable as regards the indirect method, but in cases where the regenerated phenol is liable to attack by the alkali, strong alcoholic potassium acetate can frequently be employed with advantage.

In many cases, however, the indirect method is not available, and it

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108 PERKIN: THE DETERMINATION OF ACETYL GROUPS.

is then necesaary to determine the acetic acid produced by the hydro- lysis. For this purpose, the decomposition of the acetyl componnd is effected by means of potassium hydroxide, barium hydroxide (Herzig, Monatsh., 1884, 5, 86), magnesia (Schiff, Ber., 1879, 12, 1531), &C.,

and the acetic acid estimated in various ways. The latter reagents have been employed in certain cases where a strong alkali would react on the phenol itself ; for instance, according to Erdmann and Schultz (Ann., 1882, 216, 234), acetylhzmatoxylin evolves formic acid when digested with boiling dilute alkali. F. Wenzel (,Won-cc.tsl~., 1897, 18, 659) prefers to hydrolyse the substance with a slightly diluted sul- phuric acid, sodium phosphate being then added and the acetic acid distilled off under reduced pressure, Such processes, however, involve, as a rule, long and tedious operations, and are not always reliable with small quantities of substance, and it was consequently interesting to determine whether some simpler and more rapid method could not be devised.

It was previously found (Trans., 1899, 75, 433) that the acetyl derivatives of phenolic colouring matters are hydrolysed by boiling alcoholic potassium acetate solution with the production of ethyl acetate and the mono- and di-potassium salts of the substance, and experiments with other 0-acetyl compounds showed that in all cases ethyl acetate was evolved by this method. In order t o study the pro- cess, 0.5 gram of the substance was digested a t the boiling temperature in a small retort with a half-saturated solution of potassium acetate in alcohol for one and a half hours, the volume of the liquid being made up from time to time with fresh quantities of alcohol. The dia- tillate was treated with standard alcoholic potash, heated t o boiling to saponify the ethyl acetate present, and titrated with sulphuric acid to determine the yuantit'y of acetic acid which was thus produced. The earlier experiments carried out in this manner gave satisfactory results, as acetylbutin, C15H,0,(,C,H,0),, yielded 45 -60 per cent. of acetic acid (theory, 45-23 per cent.), and acetylbutein, C15H80,(C21X80)4, 54 per cent. (theory, 54.54 per cent.), but with some other compounds the reaction proceeded but slowly and involved the use of large quantities of alcohol. For instance, penta-acetylcatechin, C,,H,O,( C2H3U)5 (m, p. 129-130°), was not completely hydrolysed after 3 hours' digestion with the acetate (found, acetic acid = 43.35 ; theory requires 60 per cent.), and a somewhat similar result was given by acetylcyanomaclurin, C15H,06(C2H30)5 (found, 35.8 ; theory requires 60 per cent.). Although it appeared quite possihle by a longer treatment, or the employment of a higher temperature, to devise a method by which the use of potassium acetate would be generally applicable, the experiments were not continued, as the following process, in which the acetate is replaced by sulphuric acid, gave satisfactory and rapid results.

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PERKIN : THE DETERMINATION OF ACETYL GROUPS. 109

In carrying out this operation, 0-5 gram of the substance (approx.), 30 C.C. of alcohol, and 2 C.C. of sulphuric acid are placed in a small retort and the mixture gently distilled until about one-third of its bulk remains. Twenty C.C. of fresh alcohol are then added, the opera- tion repeated, and a second, and sometimes a t,hird, quantity of the alcohol run in. The distillate is collected in a Nask containing 20 C.C.

of standard alcoholic potash solution, and a t the end of the operation is connected to a reflux condenser and heated on the water-bath for a few minutes. The product diluted with a little water is titrated with standard sulphuric acid.* It has been usual to distil slowly so that about three-quarters of an hour is thus occupied, and it is of course necessary that the distillation be not carried too far, otherwise an evolution of sulphurous acid mill thus occur. In order that no traces of acid liquid can be carried over by spurting, the tubnlure of the retort is bent from the centre a t a slight angle, and into the neck a small tap funnel is fitted by means of which the fresh alcohol is added during the distillation. The whole operation should not require more than an hour and a quarter, and less than this is usually sufficient. The following acetyl derivatives, specially prepared, have been analysed by this method.

Acetic acid.

Eou11cl. Ace tylalizeriiie ........................ C,,H,O,( C,H,O), 37 '59 Acetylquercetin ..................... C,,H,O7(C2H3O), 58-44 Acetyl~i i ipi i io~al l i i i . . . . . . . . . . . . C,,H,O,(C,H,O), fj2-40

Acetylbutin ......................... C,,H,~O,(C,IT,O), 1 :il:: Acetylcatechiii (111. 1). 129-130"). , C,,H,O,(C,H,O), (j0 21 Acetylcatcchin (rn. 1'. 158-160"). . C,,H,O,( C,H,O), 60*18 Aeetylhesperitiii ...................... C,6H,,0,(C,H,0 ); 12 '5 4

Theory. 37.04 58'59 61-86 45.28 60'00 60'00 -12'06

It was found possible, except in the case of the catechins, to recover the phenolic compounds regenerated by the reaction in a pure con- dition, and it is quite evident that where t'hese substances are insol- uble in cold water an indirect dcterrnination can be simultaneously carried out by this method. The above results require no special com- ment, though it is interesting to observe that acetylhesperitin has given numbers in harmony with those previously obtained by hydro- lysis in the cold (Trans., 1898, 74, 1034). Various other 0-acetyl compounds have been analysed by this method with good result, and these will be referred to in later communications.

It is possible that the sulphuric acid in this process can be replaced by phosphoric acid. I n one experiment with diacetylalizarine an acid of 1.5 sp. gr. gave, during an hour and a half, 1'7% per cent. of acetic

* Norrnal snlphiiyic acid was nsiinlly employed, hut when only small quantities of substance arc available, :I niore dilute acid is preferable.

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11 0 FORSTER AND FIERZ:

acid (theory 37.59) as ethyl acetate, and it seems likely that had the reagent been stronger a more satisfactory result could thus have been obtained.

A cetylc~naiclo-co~a~~ounds.

The method described is also applicable to N-acetyl derivatives, but in this case is not so important, because the detection of amido-groups can be so readily effectpd by the well known “nitri te” process. For these de‘erminations, 4 c.c, of sulphuric acid in 30 C.C. of alcohol have been usually employed, and the solution was concentrated during the operation to at least one-third i ts bulk, for hydrolysis with these com- pounds does not proceed so yeadily as with the 0-acetyl derivatives. The residual liquid is diluted with alcohol and the mixture distilled as before, and this is repeated until the distillate is free from ethyl acetate. The reaction is usually complete in an hour and a half, and the only precaut,ion necewwy is to observe that the concentration does not proceed too far. The following results were obtained with products specially prepared for the purpose :

Acetic acid.

Found. Theory.

Acetanilide ..................... C,H,‘NH*CO ‘CH, { :t :;;} 44’44 Aceto-p-toluiclitlc ............. C7H, ’NH‘CO *CH, 39 *86 40.21 Diacetylbenzidine.. ............ C,,H8( NH’CO’CH,), 44-54 44‘77

CLOTHWORKERS’ Rl4:sEAltcH LABORATORY, THE UNIVEIXITY,

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