The Mesomeric Effect And Aromatic Amines

The Mesomeric Effect And 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.

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) ?

Let’s think it through.

If inductive effects were solely responsible, we’d expect the basicity of nitrogen to decrease as it gets closer to the electron-withdrawing nitro group.

Thus we’d expect the para isomer to be the most basic, since it’s furthest away from nitrogen, and the ortho isomer to 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.

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.

The ortho- has the nitro group closer to the amine than the para– , which explains why it is less basic. (“inductive effects”, as we mentioned above).

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