Question : Amines have a lone pair of electrons on nitrogen atom due to which they behave as Lewis base. Larger the value of Kbor smaller the value of pKbstronger is the base. Amines are more basic than alcohols, ethers, esters, etc. The basic character of aliphatic amines should increase with the increase of alkyl substitution. But it does not occur in a regular manner as a secondary aliphatic amine is unexpectedly more basic than a tertiary amine in solutions. Aromatic amines are weaker bases than ammonia and aliphatic amines. Electron-donating groups such as —CH3, —OCH3, —NH2, etc., increase the basicity while electron-withdrawing substituents such as —NO2, —CN, halogens. etc. decrease the basicity of amines. The effect of these substituents is more at p- than at m-positions.
1. Arrange the following in increasing order of their basic strength:
C2H5NH2, C6H5NH2, NH3, C6H5CH2NH2, (C2H5)2NH
Ans. C6H5NH2 < NH3 < C6H5CH2NH2 < C2H5NH2 < (C2H5)2NH
2. Arrange the following compounds in increasing order of their acidic strength:
Methylamine, dimethylamine, aniline, N-methylaniline
Ans. dimethylamine < methylamine < N-methylaniline < aniline.
3. Rearrange the following in increasing order of their basic strength:
p-toluidine, N, N-dimethyl-p-toluidine, p-nitroaniline, aniline
Ans. p-nitroaniline < aniline < p-toluidine < N, N-dimethyl-p-toluidine.
4. Which is more acidic, aniline or ammonia?
Ans. Due to delocalization of the lone pair of electrons of the N-atom of aniline over the benzene ring, aniline is more acidic than ammonia.
5. (CH3)2NH is more basic than (CH3)3N in an aqueous solution. Give reason.
Ans. The basicity of amine in aqueous solution depends upon the stability of the substituted ammonium cation. Here the combination of three factors, +ve I effect of CH3 groups, hydrogen bonding and steric hindrance favour greater stability for ammonium cation of dimethyl amine than ammonium cation of trimethyl amine. Hence dimethylamine is stronger base than trimethyl amine.