What will be the correct order of basic strength of the following substituted anilines?

p-Methylaniline > m-Methylaniline > m-Nitroaniline > p-Nitroaniline

The correct answer is option 4. p-Methylaniline > m-Methylaniline > m-Nitroaniline > p-Nitroaniline.

The basicity of aniline and its derivatives is determined by the availability of the lone pair of electrons on the nitrogen atom for protonation. Electron-donating groups increase basicity by increasing the electron density on the nitrogen, while electron-withdrawing groups decrease basicity by pulling electron density away from the nitrogen.

Substituent Effects

Nitro Group \((−NO_2)\):

Electron-withdrawing: The nitro group is strongly electron-withdrawing due to its -M (mesomeric or resonance) and -I (inductive) effects.

Effect on Basicity:

Para Position (p-Nitroaniline): The nitro group at the para position withdraws electron density through resonance, significantly reducing the availability of the nitrogen lone pair for protonation.

Meta Position (m-Nitroaniline): At the meta position, the nitro group cannot participate in resonance directly with the nitrogen's lone pair but still exerts an inductive electron-withdrawing effect, reducing basicity, though less significantly than the para position.

Methyl Group \((−CH_3)\):

Electron-donating: The methyl group donates electrons through +I (inductive) effects and hyperconjugation.

Effect on Basicity:

Para Position (p-Methylaniline): The methyl group at the para position increases electron density on the nitrogen through inductive effects and hyperconjugation, enhancing basicity.

Meta Position (m-Methylaniline): The methyl group at the meta position increases electron density primarily through inductive effects, enhancing basicity but to a lesser extent than the para position.

Comparing the Basicity: Given these effects, we can compare the basicity of the substituted anilines:

p-Methylaniline: The methyl group in the para position strongly donates electron density through both inductive effects and hyperconjugation. This increases the electron density on the nitrogen, making p-Methylaniline the most basic among the four compounds.

m-Methylaniline: The methyl group in the meta position also donates electron density but primarily through inductive effects. This increases the electron density on the nitrogen, though less effectively than in the para position, making m-Methylaniline less basic than p-Methylaniline but more basic than the nitro-substituted anilines.

m-Nitroaniline: The nitro group in the meta position withdraws electron density through inductive effects. This decreases the electron density on the nitrogen, reducing its basicity but less so than when the nitro group is in the para position.

p-Nitroaniline: The nitro group in the para position strongly withdraws electron density through resonance and inductive effects. This significantly decreases the electron density on the nitrogen, making p-Nitroaniline the least basic among the four compounds.

Conclusion: Therefore, the correct order of basic strength from the strongest to the weakest is: p-Methylaniline > m-Methylaniline > m-Nitroaniline > p-Nitroaniline.