Amine group in aniline is ortho and para directing. Why does then aniline on nitration gives substantial amount of m-nitroaniline?

To understand why aniline, which has an amino group (-NH2) that is generally ortho and para directing in electrophilic aromatic substitution reactions, produces a significant amount of m-nitroaniline during nitration, we need to delve into the nature of the amino group and the reaction conditions involved.

The Role of the Amino Group

The amino group in aniline is a strong electron-donating group. This characteristic typically enhances the electron density of the aromatic ring, making the ortho and para positions more reactive towards electrophiles, such as the nitronium ion (NO2+), which is the active species in nitration. Therefore, we would expect that nitration of aniline would predominantly yield ortho and para products.

Understanding the Reaction Conditions

However, the nitration reaction is often conducted under conditions that can affect the outcome. In the case of aniline, the reaction is usually performed in the presence of concentrated nitric acid and sulfuric acid. These acidic conditions can lead to protonation of the amino group, converting it into an anilinium ion (-NH3+). This protonation significantly alters the directing effects of the group.

Impact of Protonation

• Protonated Amino Group: When the amino group is protonated, it becomes a -NH3+ group, which is a deactivating group. This change reduces the electron density of the aromatic ring, making it less reactive overall.
• Directing Effects: The -NH3+ group is meta directing rather than ortho or para directing. This means that when the electrophile approaches the ring, it is more likely to attack the meta position due to the reduced electron density at the ortho and para positions.

Resulting Product Distribution

As a result of these factors, even though the amino group is typically ortho and para directing, the conditions of the nitration reaction lead to a significant amount of m-nitroaniline being produced. The presence of the protonated amino group shifts the directing influence from ortho and para to meta, resulting in a notable yield of m-nitroaniline alongside the expected ortho and para products.

Example of Product Distribution

In a typical nitration of aniline, you might observe a product distribution like this:

• m-Nitroaniline: 40%
• p-Nitroaniline: 30%
• o-Nitroaniline: 30%

This distribution illustrates how the reaction conditions can significantly influence the outcome, leading to a substantial formation of m-nitroaniline despite the inherent directing properties of the amino group.

In summary, while aniline is generally ortho and para directing due to the electron-donating nature of the amino group, the acidic conditions during nitration can protonate the amino group, converting it into a meta directing group. This shift in directing influence explains the substantial formation of m-nitroaniline in the reaction. Understanding these nuances is crucial for predicting the outcomes of electrophilic aromatic substitutions in aromatic compounds.