like –CH group, there is no steric hindrance to H-bonding. In case the alkyl group is bigger than CH group, there will be steric hinderance to H-bonding. Therefore, the change of nature of the alkyl group, e.g., from –CH to –C H results in change of the order of basic strength. Thus, there is a subtle interplay of the inductive effect, solvation effect and steric hinderance of the alkyl group which decides the basic strength of alkyl amines in the aqueous state.
The order of basic strength in case of methyl substituted amines and ethyl substituted amines in aqueous solution is as follows: (C H ) NH > (C H ) N > C H NH > NH (CH ) NH > CH NH > (CH ) N > NH (b) Arylamines versus ammonia p K b value of aniline is quite high. Why is it so? It is because in aniline or other arylamines, the -NH group is attached directly to the benzene ring. It results in the unshared electron pair on nitrogen atom to be in conjugation with the benzene ring and thus making it less available for protonation.
If you write different resonating structures of aniline, you will find that aniline is a resonance hybrid of the following five structures. On the other hand, anilinium ion obtained by accepting a proton can have only two resonating structures (kekule). We know that greater the number of resonating structures, greater is the stability. Thus you can infer that aniline (five resonating structures) is more stable than anilinium ion.
Hence, the proton acceptability or the basic nature of aniline or other aromatic amines would be less than that of ammonia. In case of substituted aniline, it is observed that electron releasing groups like –OCH , –CH increase basic strength whereas electron withdrawing groups like –NO , –SO H, –COOH, –X decrease it. . Alkylation Amines undergo alkylation on reaction with alkyl halides