Diacetyls

One disadvantage of using acetic anhydride is that with primary amines, traces of the diacctyl compound, RN(COCH3)2, niay be formed the chances of this secondary acetylation are, however, usually remote, and recrystallisation from an aqueous solvent will generally hydrolyse the diacetyl derivative rapidly back to the mono-acetyl compound. 

The above diamines can alternatively be identified as their diacetyl derivatives. Heat under reflux 0 5 g. of the diamine with 3 ml. of acetic acid—NaOH solution will cause the separation of the diacetyl derivative. Recrystallise from ethanol. M.ps. of the diacetyl derivatives of 0-, m-, and />-phenylene diamine are 185 , 191 , and 304 respectively (p. 551). 

If the heating be prolonged and excess of acetic anhydride is employed, variable amounts of the diacetyl derivative are formed ... 

The production of diacetyl derivatives is facilitated by the presence of substituents in the ortho position thus an excellent yield of diacetyl-o-toluidine results when o-toluidine is heated with an excess of the reagent ... 

In general, however, the diacetyl derivatives are unstable in the presence of water, undergoing hydrolysis to the mono-acetyl compound, so that when they (or a mixture of mono- and di-acetyl derivatives) are crystallised from an aqueous solvent, e.g., dilute alcohol, only the mono-acetyl derivative is obtained. A further disadvantage of the use of acetic anhydride in the absence of a solvent is that all the impm-ities in the amine are generally present in the reaction product. Heavily substituted amines, t.g., 2 4 6-tribromoaniline, react extremely slowly with acetic anhydride, but in the presence of a few drops of concentrated sulphuric acid as catalyst acetylation occurs rapidly, for example ... 

Boil a mixture of 10 g. (10 ml.) of o-toluidine and 38 g. (35 ml.) of acetic anhydride in a 75 or 100 ml. Claisen flask fitted with a reflux condenser (Fig. Ill, 28, 1, but with trap replaced by a calcium chloride or cotton wool guard tube) for 1 hour. Arrange the flask for distillation under reduced pressure (compare Fig. II, 20, 1) and distil acetic acid and the excess of acetic anhydride pass over first, followed by the diacetyl derivative at 152-153°/20 mm, some mono-acetyl-o-toluidine (1-2 g.) remains in the flask. The yield of diacetyl-o-toluidine is 14-15 g, it is a colourless, somewhat unstable hquid, which slowly sohdifies to yield crystals, m.p. 18°, To prepare the (mono-) acetyl-o-toluidine, warm a mixture of 5 g. 

When pyridine is treated with zinc dust and acetic anhydride, a type of reductive coupling occurs and the product is diacetyltetrahydrodipyridyl (I) this undergoes a curious change on heating yielding pyridine and a new diacetyl compound, 1 4 diacetyl 1 4-dihydropyridine (II). The latter is reduced by zinc and acetic acid to 4-ethylpyridine (III). 

Methyl ethyl ketone is allowed to react with n-butyl or iso-amyl nitrite in the presence of hydrochloric acid to yield diacetyl monoxime (nitrosomethyl ethyl ketone) ... 

The diacetyl monoxime condenses readily with hydroxylamine hydrochloride or sulphate with the formation of dimethylglyoxime (diacetyl dioxime) ... 

N,N -Diacetylethylenediamine-N,N -dithioacetamide (42) reacts with oj-bromoacetophenone and its p-substituted derivatives to give the expected N,N -diacetyl-N,N -bis(4-phenyl-2-thiazolylmethylethylene diamine) (43) (Scheme 21) with R = H, Me, Cl, Br, MeO, yields ranged from 65 to 80% (482). 

NAPHTHALENE DERIVATIVES] (Vol 16) a-Napthol-creatine phosphate-diacetyl... 

CgHgO, diacetyl [431-03-8], and benzaldehyde [100-52-7], C H O, are aroma chemicals used by the food industry (53). 

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