Amines

Aliphatic and Aromatic Amines Amines are derivatives of ammonia (NH3) in which hydrogen atom/s are replaced by the corresponding number of alkyl or aryl group. Based on the number of hydrogen-atom/s replaced by alkyl or aryl groups, amines can be classified as primary (1), secondary (2) and tertiary amines (3). Based on the nature of group linked to the central N-atom, amines are classified as aliphatic amines, aromatic amines and aryl alkyl amines. Ammonolysis of alkyl halides, reduction of nitro compounds, reduction of nitriles, Gabriel phthalimide synthesis, Hoffmann bromamide degradation reaction and reduction of oximes and amides, are used for preparation of the amines. Amines have higher boiling point than corresponding hydrocarbons and other non-polar compounds but lower than that of alcohols. Lower amines are soluble in water because they form hydrogen bonds with water molecules. Aliphatic amines are stronger bases than ammonia due to +I (inductive) effect of alkyl groups. Aromatic amines are weaker bases than ammonia and aliphatic amines due to the electron withdrawing nature of the aryl group. Larger the value of Kb or smaller the value of pKb, stronger is the base. Basicity of an amine in solution depends upon the extent to which NH4+ ion is stabilized. 3 amines are weakest base due to overcrowding at nitrogen atom by alkyl groups. Basic character of an amine depends on the ease of the formation of the cation by accepting a proton from the acid. Higher the stability of the cation, greater will be the basic nature of the amine. In case of substituted aniline, the electron releasing groups increase basicity whereas electron withdrawing groups decreases basicity. In gaseous phase, tertiary amine is most basic. The basicity of aliphatic amines in aqueous solution depends on + I effect, steric factor and solvation of ions. As a result, in aqueous solution, sec. amine is the most basic. Introduction of alkyl group in amine is called alkylation. Primary and secondary aliphatic & aromatic amines undergo acylation through nucleophilic substitution by acid derivatives. Amines undergo Benzoylation (Schotten-Baumann method) and Carbylamine reaction. Different types of amines give different products on reaction with nitrous acid. Hinseberg test is used to distinguish between primary, secondary and tertiary amines. Amines undergo electrophilic substitution reaction, such as Bromination, Nitration and Sulphonation. Cyanides and isocyanides Cyanides are the derivatives of hydrogen cyanides (HCN) in which Hydrogen atom is replaced by alkyl group and they are represented as R–CN whereas isocyanides are the derivatives of hydrogen isocyanides (HNC) in which Hydrogen atom is replaced by alkyl group and are represented by R–NC. In common system, cyanides are named as alkyl cyanides while isocyanides as alkyl isocyanides. The IUPAC name of cyanides is alkanenitrile while that of isocyanides is alkylcarbylamine. Cyanides are prepared by the reaction of alkyl halides with potassium cyanide, by dehydration of amides, by dehydrogenation of primary amines over Copper or Nickel at high temperature. Isocyanides are prepared by heating an alkyl iodide with silver cyanide in aqueous ethanolic solution, by heating a primary amine and chloroform with ethanolic potassium hydroxide solution (Carbylamine reaction). Cyanides and isocyanides undergo hydrolysis and reduction. Acid hydrolysis of cyanides gives a carboxylic acid while that of isocyanides gives a primary amine. Reduction of cyanides gives a primary amine while that of isocyanides gives a secondary amine. Diazonium salt Diazonium salt is an important aromatic compound. Diazonium salt is prepared by treating aniline with nitrous acid and hydrochloric acid at 273-298K. The conversion of primary aromatic amine into diazonium salt is known as diazotisation. Diazonium salt is a reactive compound which undergoes Sandmeyer reaction, Gatterman reaction, replacement reactions etc. Sandmeyer reaction involves the replacement of halide or cyanide ion. The introduction of Cl–, Br– and CN– ion in the benzene ring in the presence of copper powder is called Gatterman reaction. Replacement of diazo group by a variety of nucleophile form aryl halides, cyanides, phenols and arenes. Coupling reaction of aryldiazonium salts with electron rich aromatic compounds such as phenols and amines give intense coloured azo compounds (azo dyes). This is an example of electrophilic substitution reaction. The colour of azo compound is due to the presence of unsaturation center (chromophoric groups) and extended conjugation of the double bond of both the arene rings through –N=N– double bond.

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