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Amines Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 16, 2011

Reactions of Quaternary Ammonium Salts

Quaternary ammonium salts are not nucleophilic or basic, since there is no lone pair of electrons on nitrogen as there is in amines. Upon heating in the presence of strong base, quaternary ammonium salts undergo the Hofmann elimination, an E2 elimination in which an alkene and a 3° amine are formed.

Reactions of Quaternary Ammonium Salts

Diazonium Ions

Amines react with nitrous acid (HONO) to form diazonium ions. Alkyl diazonium ions decompose too quickly to be useful, but aryl diazonium ions have rich and highly useful chemistry. The N2 molecule is an extremely good leaving group, so it can be substituted by a number of different nucleophiles. Aryl halides can be prepared by adding CuCl or CuBr to the diazonium ion (the Sandmeyer reaction). The N2 can be replaced with a hydrogen by treatment of a diazonium ion with hypophosphorus acid (H3PO2). Examples of these substitution reactions of aryl diazonium ions are shown below.

Diazonium Ions

Spectral Properties

The N–H stretching and NH bending frequencies occur in the IR spectrum at 3050–3550 cm–1 and 1600–1640 cm–1 respectively. In the N–H stretching region, 1° amines and unsubstituted amides show a pair of peaks for a symmetric and an antisymmetric vibration. In the 1H NMR spectrum, N–H proton signals of amines fall in a wide range (1 – 5 ppm) and are often very broad to due chemical exchange. The signals of N–H protons of amides, appearing at 5 – 8 ppm, are even broader. The mass spectra of amines show an odd mass for parent ions if there is an odd number of nitrogens, and an even mass if there is an even number of nitrogens. For example, Et3N has a molecular weight of 101.

Practice problems for these concepts can be found at:  Amines Practice Problems

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