For the same alkyl group, all the three primary, secondary and tertiary amines are more basic than ammonia in aqueous solutions. Their relative basicity, however, depends upon a combination of three factors: +I-effect of alkyl groups, the stability of the ammonium cation (formed after accepting a proton) due to H-bonding with water molecules and the steric effect of the alkyl groups which tend to reduce the extent of H-bonding. All these three factors are favourable for secondary amines thereby making them the strongest bases. Since methyl group has the smallest size, there is no steric hindrance to H-bonding. Consequently, stability of the ammonium cation due to H-bonding with water predominates over the +I-effect of the methyl group thereby making methylamine more basic than trimethylamine. If, however, the alkyl group is bigger than the methyl group, +I-effect of the alkyl group outweighs stability of the ammonium cation due to H-bonding there making tertiary amines more basic than the primary amines. Due to delocalization of lone pair of electrons of the nitrogen atom on the benzene ring, aniline is a weaker base than ammonia. The basic strength of the substituted anilines, however, depends upon the nature of the substituent. Whereas electron-donating groups tend to increase, electron-withdrawing groups tend to decrease the basic strength. The base strengthening effect of the electron-withdrawing groups and base weakening effect of the electron-withdrawing group is, however, more pronounced at p-than at m-position. However, due to ortho-effect, o-substituted anilines regardless of the nature of substituent whether electron-donating or electron-withdrawing.   

What is the order of basicity of 

I. p-Methylaniline                   II. m-Methylaniline 

III. Aniline                             IV. o-Methylaniline

I > II > III > IV

The correct answer is option 1. I > II > III > IV.

Let us delve deeper into the basicity order of p-methylaniline, m-methylaniline, aniline, and o-methylaniline based on their structures and the electronic effects of the methyl substituent:

Aniline (III):

Aniline is a benzene ring with an amino group (-NH₂) attached directly to it. The lone pair of electrons on the amino group can readily accept a proton (H⁺), making aniline a moderately basic compound.

p-Methylaniline (I):

In p-methylaniline, the methyl group (-CH₃) is attached to the benzene ring at the para position relative to the amino group. Methyl groups are electron-donating through both inductive (+I) and hyperconjugation effects. The electron-donating nature of the methyl group increases the electron density on the amino group, making it more basic compared to aniline. Thus, p-methylaniline is more basic than aniline.

m-Methylaniline (II):

In m-methylaniline, the methyl group is attached to the benzene ring at the meta position relative to the amino group. Meta-substituted methyl groups have a weaker electron-donating effect compared to para-substituted methyl groups. The electron-donating effect of the methyl group at the meta position is less effective than at the para position, resulting in m-methylaniline being less basic than p-methylaniline but still more basic than aniline.

o-Methylaniline (IV):

In o-methylaniline, the methyl group is attached to the benzene ring at the ortho position relative to the amino group. Ortho-substituted methyl groups can introduce steric hindrance with the amino group, reducing the availability of the lone pair of electrons on nitrogen for protonation. Additionally, the electron-donating effect of the methyl group at the ortho position is less effective compared to the para position. Therefore, o-methylaniline is expected to be the least basic among the given compounds due to both steric and electronic effects.

Based on these considerations, the order of basicity is: p-Methylaniline (I) > m-Methylaniline (II) > Aniline (III) > o-Methylaniline (IV).

This corresponds to option 1: I > II > III > IV.

This order reflects how the position of the methyl group relative to the amino group affects the basicity of the aniline derivatives.