# 1911 Encyclopædia Britannica/Acid-Amides

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​ACID-AMIDES, chemical compounds which may be considered as derived from ammonia by replacement of its hydrogen with acidyl residues, the substances produced being known as primary, secondary or tertiary amides, according to the number of hydrogen atoms replaced. Of these compounds, the primary amides of the type R·CO·NH$_2$ are the most important. They may be prepared by the dry distillation of the ammonium salts of the acids (A. W. Hofmann, Ber., 1882, 15, p. 977), by the partial hydrolysis of the nitriles, by the action of ammonia or ammonium carbonate on acid chlorides or anhydrides, or by heating the esters (q.v.) with ammonia. They are solid crystalline compounds (formamide excepted) which are at first soluble in water, the solubility, however, decreasing as the carbon content of the molecule increases. They are easily hydrolysed, breaking up into their components when boiled with acids or alkalies. They form compounds with hydrochloric acid when this gas is passed into their ethereal solution; these compounds, however, are very unstable, being readily decomposed by water. On the other hand, they show faintly acid properties since the hydrogen of the amido group can be replaced by metals to give such compounds as mercury acetamide (CH$_3$CONH)$_2$Hg. Nitrous acid decomposes them, with elimination of nitrogen and the formation of the corresponding acid,
RCO·NH$_2$ + ONOH＝R·COOH + N$_2$ + H$_2$O.
When distilled with phosphoric anhydride they yield nitriles. By the action of bromine and alcoholic potash on the amides, they are converted into amines containing one carbon atom less than the original amide, a reaction which possesses great theoretical importance (A. W. Hofmann),
R·CONH$_2$ → R·CONHBr → R·NH$_2$ + K$_2$CO$_3$ + KBr + H$_2$O.
Formamide, H·CONH$_2$, is a liquid readily soluble in water, boiling at about 195° C. with partial decomposition. Acetamide, CH$_3$·CONH$_2$, is a white deliquescent crystalline solid, which melts at 82–83° C. and boils at 222° C. It is usually prepared by distilling ammonium acetate. It is readily soluble in water and alcohol, but insoluble in ether. Benzamide, C$_6$H$_5$.CONH$_2$, crystallizes in leaflets which melt at 130° C. It is prepared by the action of ammonium carbonate on benzoyl chloride. It yields a silver salt which with ethyl iodide forms benzimidoethyl ether, C$_6$H$_5$C: (NH)·OC$_2$H$_5$, a behaviour which points to the silver salt as being derived from the tautomeric imidobenzoic acid, C$_6$H$_5$C: (NH)·OH (J. Tafel, Ber., 1890, 23, p. 104). On the preparation of the substituted amides from the corresponding sodamides see A. W. Titherley (Journ. Chem. Soc., 1901, 59, p. 391). The secondary and tertiary amides of the types (RCO)$_2$NH and (RCO)$_3$N may be prepared by heating the primary amides or the nitriles with acids or acid anhydrides to 200° C. Thiamides of the type R·CSNH$_2$ are known, and result by the addition of sulphuretted hydrogen to the nitriles, or by the action of phosphorus pentasulphide on the acid-amides. They readily decompose on heating, and are easily hydrolysed by alkalies; they possess a somewhat more acid character than the acid-amides.