William W. Bannister, John R. Pennace, David W. Smith, Charles F. Chelton, and James A. Wareing
Lowell Technological Institute Lowell, Massachusetts 01854 and William A. Curby Lohey Clinic, 211 Summit Avenue Brookline, Massachusetts 02146
Detection and Separation of Alkyl and Aralkyl Amines by Carbarnate Formation
The reaction of aliphatic or arylaliphatic amines with carbon dioxide forming solid carbamates is a well known phenomenon, and is evidenced, for example, by the encrustations found on and in bottles of such amines on exposure to air R R G
Pyridines, anilines, perfluoroalkylamines, and similar m i n e s in which the amino nitrogen if flanked by unsaturated or other electron withdrawing groups are too weakly basic to afford significant carbamate formation. Except for trimethylamine and certain polycyclic N-hridgeheaded amines, in which there is minimal steric hindrance, most trialkyl amines are also essentially unreactive to carbon dioxide. For the more reactive primary or secondary amines, carbamate formation is accompanied by considerable evolution of heat, quite noticeably so even when Dry Ice is used as the source of carbon dioxide. Most primary and symmetrically substituted secondary amines, when dissolved in benzene, hexane, chloroform or other non-hydroxylic solvents, generally afford solid carbamates. Under similar conditions, unsymmetrically substituted secondary amines react with carbon dioxide to form aelatinous sus~ensionsin manv instances, although solid products have been observed id some cyclohexyl and long chain substituted secondary amines. Traces of water areoften required to facilitate gelformations. Carbamates as a class are thermally unstable, and these compounds will also very readily hydrolyze to form the amine (or amine salt) in the presence of equivalent quantities of strong base or acids. The lower molecular weight carbamates (e.g., with eight or fewer carbon atoms) are quite soluble in water; higher molecular weight carhamates tend to be hydrophobic and may form gelatinous sludges on contact with water. (Carbon disulfide reacts similarly with amines, with formation of zwitterionic dithiocarbamates. Due to the toxicity, flammability and expense of carbon disulfide, carbon dioxide is the preferred reactant in the laboratory procedures to be discussed in this communication.) These characteristics of carbamate formation enable the use of carbon dioxide as an entirely non-destructive means of detecting and often differentiating between primary
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/ Journal of Chemical Education
and secondary aliphatic and arylaliphatic amines, and for the separation of such amines from other functionallv suhstituterl compounds or from tertiary or aromatic amines. We have found this to be pmicularly effecti\,e as an adiunct t o the Hinsbere test and other amine classification and separation techniques. In a typical example of the application of this technique, a mixture of 2.0 g of aniline and 2.0 g of cyclohexyl m i n e in 20 ml of hexane is carboxylated by the slow addition of small pieces of Dry Ice. Considerable heat is evolved until an excess of the Dry Ice has been added, whereupon cooling is effected to room temperature or below. The solid N-cvclohexvlcarbamate, which is ~ r e c i p itated quantitatively; can bk separatedby filtratibn followed by trituration of the carbamate with hexane. Alternatively, the mixture of hexane, unreacted aniline, and N-cyclohexylcarbamate can be poured into a separatory funnel containing 50-100 ml of carbonated water (achieved by adding Dry Ice to the water). The carbamate is nearly quantitatively dissolved in the water phase, and the aniline nearly quantitatively remains in the hexane layer. After removal of the water layer, the hexane layer is washed with about 50 ml of carbonated water, and dried over MgSOn. By glpc analysis (using a Penn-Walt 223 on 80/100 Gas Chrom R packing) more than 97% of the cyclohexylamine was shown to have been removed by this technique; it appeared also that 95% or more of the aniline had been retained, unreacted. (For gelatinous carbamate suspensions, such as may be formed from unsymmetrically substituted secondary amines, relatively low-speed centrifuging of the reaction mixture was found to be a particularly effective technique for separation of the gelled carbamate from non-reacted components of the mixture.) Addition of sodium hydroxide solution to the carbamate precipitate or water solution causes immediate hydrolysis, with the cyclohexylamine being extracted into hexane or other organic solvent. Alternatively, the suspension of cyclohexylamine in the sodium hydroxide-sodium carbonate-water solution can be subjected directly to the Hinshere test bv addition of benzenesulfonvl chloride. thus establishing" the primary character of thi amine and affordina- the characterizing - benzenesulfonamide derivative.l 'However, see also Gambill, C. R., Roberts, T. D., and Shechter, H., J. CHEM. EDUC., 49(4), 287 (1912)regarding the reactivity of benzenesulfonylchloride with tertiary amines.
