Effect of nitro group basicity order of o,m and p nitroanilines?

Question

Would someone please help me with this? The order of basicity of o,m and p nitroanilines is m>o>p. My book states that ortho isomer has -I effect, -M effect as well as ortho effect, and meta isomer has only -I effect while para isomer has -I and -M effect. These effects destabilize the conjugate acid.

My actual doubt is why exactly the nitro group has a negative mesomeric effect on ortho and para isomer, when actually it is a meta deactivator. Shouldn't it be having no effect at ortho and para positions, and destabilize the conjugate acid at meta position? I'm confused.

Answer 1

First we have to understand what affects basic nature of a compound.

1. Tendency to donate lone pair
2. Electronic effects— Inductive, resonance etc
3. Factors stabilizing conjugate acid (or acidity of conjugate acid
4. Nature of solvent

Tendency to donate lone pair is affected by the acidity of conjugate acid and electronic effects.

Grab a pen and paper and draw conjugate acids of

• $$2-nitroaniline$$
• $$3-nitroanliline$$
• $$4-nitroaniline$$

And you know that electron withdrawing inductive (–I) effect increases acidity and electron withdrawing resonance (–M or –R) also increases acidity.

In conjugate acid of

• $$2-nitroaniline$$ You will find there is only –I inductive effect and there is no –M effect of $-NO_2$ group because of SIR (In SIR - STERIC INHIBITION OF RESONANCE - the nitro $-NO_2$ group shows steric hindrance to $-NH_3^+$ group and the p–orbital of Nitrogen (in $-NH_3^+$ group) moves out of plane of p–orbital of Carbon (in benzene ring) which inhibits resonance or does not allow resonance to happen). So in conjugate acid of $2-nitroanliline$ there is only –I effect which increases acidity.
• $$3-nitroanliline$$ You will find that there is no –M effect obviously!! But there is –I effect of nitro ($-NO_2$) group which increase the acidity of conjugate acid of $3-nitroanliline$ but it is less than what we have seen in $2-nitroanliline$ (due to distance factors). Therefore conjugate acid of $2-nitroanliline$ is more acidic than conjugate acid of $3-nitroanliline$
• $$4-nitroanliline$$ You will find there are both –M and –I effect which increase acidity of conjugate acid of $4-nitroanliline$. So conjugate acid of $4-nitroanliline$ is more acidic than conjugate acid of $3-nitroanliline$ or $2-nitroanliline$

Conclusion Order of acidity of conjugate acids $$conj.\ acid\ of\ 3-nitroanliline<conj.\ acid\ of\ 2-nitroanliline<conj.\ acid\ of\ 4-nitroanliline$$

Since the order of basicity of 'orginal' compounds is just opposite of order of acidity of conjugate acids, so answer to your question is as follows $$3-nitroanliline>2-nitroanliline>4-nitroanliline$$ Or m>o>p

Answer 2

First thing we have to know about a nitro group (NO2) is this " NO2 is a meta director". But, NH2 is a Ortho, Para director. Secondly, the basicity is the tendency of donating a pair of electron. How spontaneously a compound donates a pair of electron that will determine it's basicity.

Among the o,p&m nitro aniline, the meta nitro anilines is the most unstable isomer. The para isomer is more stable than Ortho isomer and hence the p-nitro aniline has no need to donate a pair of electron. The m-nitro aniline is unstable and so it can donate a pair of electron easily than the other two. Thus, the order of basicity of the nitro substituted anilines follows m > o > p.