1911 Encyclopædia Britannica/Amidines
AMIDINES, in organic chemistry, the name given to compounds of general formula R·C : (NH)·NH2, which may be considered as derived from the acid-amides by replacement of oxygen by the divalent imino (= NH) group. They may be prepared by the action of ammonia or amines on imide chorides, or on thiamides (O. Wallach, A. Bernthsen); by the action of ammonium chloride or hydrochlorides of amines on nitriles; by condensing amines and amides in presence of phosphorus trichloride; by the action of hydrochloric acid on acid-amides (O. Wallach, Ber., 1882, 15, p. 208); and by the action of ammonia or amines on imino-ethers (A. Pinner, Ber., 1883, 16, p. 1647; 1884, 17, p. 179). They are monacid bases, which are not very stable; they readily take up the elements of water (when boiled with acids or alkalies), yielding amides and ammonia. On dry distillation they yield nitriles and ammonia. When warmed with sulphuretted hydrogen they yield thiamides, R·C : (NH)·NHR + H2S = R·C(NH2)(SH)NHR = R·CSNH2 + NH2·R or RCS·NHR + NH3. With β-ketonic esters, HO(CH3)C : CH·CO2R, they yield oxypyrimidines (A. Pinner, Ber., 1890, 23, p. 3820).
Formamidine, HC : (NH)NH2, is only known in the form of its salts, the hydrochloride being obtained by the action of ammonia on the hydrochloride of formimido-ethyl ether (A. Pinner, Ber., 1883, 16, p. 357). Acetamidine, CH3C : (NH)·NH2, is alkaline in reaction, and readily splits up into acetic acid and ammonia when warmed with acids. Its hydrochloride melts at 163° C., and crystallizes from alcohol in colourless deliquescent prisms. Acetic anhydride converts the base into an acetamino-dimethyl pyrimidine, acetic acid and acetamide being also formed.
Benzamidine, C6H5·C : (NH)NH2, forms colourless crystals which melt at 75-80° C. When warmed it breaks down into ammonia and cyanphenine (s-triphenyl triazine). It condenses with acetic anhydride to form a methyldiphenyl triazine, acetamide being also formed; with acetyl-acetone to form dimethylphenyl pyrimidine (A. Pinner, Ber., 1893, 26, p. 2125); and with trimethylene bromide to form a phenyl tetrahydropyrimidine (Pinner). H. v. Pechmann (Ber., 1895, 28, p. 2362) has shown that amidines of the type R·C : (NY)·NHZ sometimes react as if they possessed the constitution R·C : (NZ)·NHY; but this only appears to occur when Y and Z are groups which function in the same way. If Y and Z are groups which behave very differently, then there is apparently no tautomerism and a definite formula can be given to the compound.
The formulae of the ringed compounds mentioned above are here shown: