The Mesomeric Effect And Aromatic Amines

A Puzzle In The Basicity Of Some Disubstituted Aromatic Amines

We’ve seen that several key factors influence the basicity of amines – charge, resonance, inductive effects, pi donor / pi acceptor behavior, hybridization and even the contribution of the nitrogen lone pair towards aromaticity.

In isolation, each of these factors is fairly straightforward to understand. But how do we deal with a situation where multiple variables come into play?

Here’s a common exam question that tests your ability to think through these factors.

Table of Contents

1. Why Is m-Nitroaniline More Basic Than p-Nitroaniline, Even Through The Nitro Group Is Closer To The Amine?
2. Drawing Out The Resonance Forms Of The Nitroanilines Helps Us Understand This Amine Basicity Trend
3. Summary: Basicity Of Disubstituted Aromatic Amines

1. Why Is meta-Nitroaniline More Basic Than para-Nitroaniline, Even Though The NO₂ Group Is Closer To The Amine?

Look at the three amines below: ortho- (1,2), meta- (1,3), and para- (1,4) nitroaniline.

Can you explain why the meta isomer (pK_aH = 2.46) is the most basic, followed by para- (1.0) and ortho- (–0.26) ?

(recall: pKaH = acidity of the conjugate acid)

Let’s think it through.

1. If inductive effects were solely responsible, we’d expect the basicity of the NH₂ to increase with increasing distance to electron-withdrawing nitro group.
2. Thus we’d expect the para isomer to be the most basic, since it has the maximum distance between NH₂  and NO₂, and the ortho isomer should be the least basic since it’s closest to the nitro group.

The ortho- isomer is indeed the least basic compound, which is what this line of thinking would expect.

But that doesn’t explain why the meta isomer is the most basic.

2. Drawing Out The Resonance Forms Of The Nitroanilines Helps Us Understand This Amine Basicity Trend

As is so often the case, we can gain a much better understanding of this situation by drawing out some resonance forms.

The para- and ortho- isomers each have a significant resonance form where the nitrogen lone pair donates into the ring and a pi bond breaks in the nitro group. (Recall that we’ve described this behaviour as “pi-donation”).

The resulting =NH₂+ group is non-basic (no free lone pair).

The meta– isomer lacks this resonance form, which explains why it is the most basic of the three isomers.

Why is ortho- less basic than the para- ? Inductive effects. The ortho- isome has the nitro group closer to the amine than the para– isomer does.

By the way, the effect of resonance upon acidity/basicity is sometimes called the “mesomeric effect“, as distinguished from the “inductive effect”.

3. Summary: Basicity Of Disubstituted Aromatic Amines

A variety of factors contributes to the basicity of amines, but in conjugated pi systems (such as aromatic rings) two factors dominate.

• Strongly electron-withdrawing groups reduce the basicity of nearby amines through the inductive effect. This reduces electron density on the amine. As the distance between the EWG and the amine increases, basicity increases.
• However: when an amine is adjacent to a pi system, it’s also possible for it to act as a pi-donor. When a pi-acceptor is also part of the pi system (e.g. NO₂, CN, carbonyl) the basicity can be considerably attenuated due to important resonance form contributors.
• Pi-donation is particularly important when the amine and pi acceptor are ortho or para to each other. In the case of the nitroanilines, this effect outweighs the resonance effect.
• Pi-donation is also why the NH₂ of amides is considerably less basic than the NH₂ of amines.

Notes