Animated Solution for Chemistry - Organic Chemistry: Which of the following compounds will give significant amount of meta-product during mononitration reaction?

Imagine you are tasked with performing a mononitration reaction on various aromatic compounds. The standard reagent for this job is the nitrating mixture, a potent combination of concentrated nitric acid and concentrated sulfuric acid.

This mixture is not just a source of the nitronium ion electrophile; it also creates a fiercely acidic environment. When we look at compounds like phenol, phenyl acetate, and acetanilide, their substituent groups are generally ortho/para directing and do not undergo complete protonation in this medium.

Now, let's turn our attention to aniline. Aniline features an amino group, which is a strong activating group and typically directs incoming electrophiles to the ortho and para positions.

However, there is a catch here. The amino group is also quite basic. When aniline is plunged into the strongly acidic nitrating mixture, an acid-base reaction takes precedence. The basic nitrogen atom eagerly accepts a proton from the sulfuric acid.

This protonation transforms aniline into the anilinium ion. This is a game-changer. The nitrogen atom now bears a full positive charge, making it highly electronegative.

Instead of donating electron density into the benzene ring via resonance, the positively charged nitrogen strongly withdraws electron density through the inductive effect. This makes the anilinium ion a strongly deactivating group.

More importantly, this electron withdrawal is most pronounced at the ortho and para positions, leaving the meta position relatively more electron-rich. Consequently, the anilinium ion directs the incoming nitronium ion to the meta position.

Because a significant portion of aniline exists as the anilinium ion in the acidic nitrating mixture, the reaction yields a surprisingly large amount of the meta-product.

While the para-product is still the major product at about $51\%$, the meta-product is formed in a very significant amount, approximately $47\%$. The ortho-product is formed in a mere $2\%$ yield due to steric hindrance.

This dramatic shift in directing behavior due to the acidic medium is a classic and highly tested concept in organic chemistry!