Question

The reaction of amines with mineral acids to form ammonium salts shows that these are basic in nature. Aliphatic amines are stronger bases than ammonia whereas aromatic amines are weaker bases than ammonia. Aliphatic and aromatic primary and secondary amines react with acid chlorides, anhydrides and esters by nucleophilic substitution reaction. The main problem encountered during electrophilic substitution reactions of aromatic amines is that of their high reactivity. Substitution tends to occur at ortho-and para-positions. Hinsberg reagent is used for the identification and distinction between primary, secondary and tertiary amines. Aryldiazonium salts, usually obtained from arylamines, undergo replacement of the diazonium group with a variety of nucleophiles to provide advantageous methods for producing aryl halides, cyanides, phenols and arenes.

30(a)(i). Why \(CH_3 - NH_2\) is a stronger base than \((CH_3)_3N\) in aqueous solution?

Hint:

The basicity order of methylamines in aqueous solution is: \(2^\circ>1^\circ>3^\circ>NH_3\). Always remember that the gas-phase order is different (\(3^\circ>2^\circ>1^\circ\)).

Answer 1

The basicity of amines depends on the availability of the lone pair of electrons on nitrogen for protonation. A stronger base is one that more readily donates its lone pair of electrons to a proton. 
1. In \(CH_3 - NH_2\) (methylamine):
- The nitrogen in methylamine has a lone pair of electrons that is readily available for protonation because the methyl group (\(CH_3\)) is an electron-donating group through its +I (inductive) effect. This increases the electron density on the nitrogen, making it more basic by increasing its tendency to accept a proton (H⁺). 
2. In \((CH_3)_3N\) (trimethylamine):
- The nitrogen in trimethylamine is bonded to three methyl groups. Although methyl groups are electron-donating, the electron density on the nitrogen becomes more dispersed due to the large number of methyl groups, making the nitrogen less able to share its lone pair of electrons. This is because the steric hindrance caused by the three bulky methyl groups reduces the availability of the lone pair for protonation. Therefore, \(CH_3 - NH_2\) is a stronger base than \((CH_3)_3N\) in aqueous solution because the lone pair on nitrogen in methylamine is more available for protonation, while in trimethylamine, the steric hindrance and electron-donating effects of the methyl groups make the nitrogen less basic.