In the first case, primary amine forms an imine as an intermediate but in the second case, secondary amine forms an imminium ion as an intermediate which is rapidly attacked by hydride ion as shown in Scheme 6 .
Scheme 6
5.11.3 Acylation
Primary and secondary amines react with acid halides to form amides. The acid chloride is more electrophilic and reactive than a ketone or an aldehyde because the electronegative chlorine atom attracts the electron density away from the carbonyl carbon. In the following example, benzoyl chloride reacts with methylamine to give N -methylbenzamide. A base such as pyridine or NaOH is often added to neutralize the HCl produced (Scheme 7).
Scheme 7
In the first step, amine attacks the electrophilic carbonyl group of the acid chloride to form a tetrahedral intermediate. Chloride ion is expelled from the tetrahedral intermediate in second step. Finally loss of a proton gives the amide (Scheme 8). The amide produced in this reaction usually does not undergo further acylation because amides are stabilized by a resonance structure that involves nitrogen's nonbonding electrons and places a positive charge on nitrogen. As a result, amides are much less basic and less nucleophilic than amines.
Scheme 8
5.11.4 Hofmann Elimination
Alkenes can be prepared from amine by elimination reaction. Amines do not undergo elimination, however, because the leaving group would be an amide ion (- NH2) which is a poor leaving group. The exhaustive methylation of amino group converts it to a quaternary ammonium salt, a good leaving group, which can leave as a neutral amine. For example, the exhaustive methylation by methyl iodide gives a quaternary ammonium iodide which can be converted to hydroxide salt by treatment with silver oxide (Scheme 9).
Scheme 9