Question

The sulphonation of chlorobenzene gives exclusively (1) an o-product (2) an m-produet (3) a p-product (4) an o-, \(\mathrm{p}\) -disubstituted product

Short answer

(4) an o-, \text{p}-disubstituted product

Step-by-step solution

Identify the Substituent Effect

Chlorobenzene has a chlorine substituent which is electron-withdrawing through induction but electron-donating through resonance. This makes chlorine an ortho-para directing group.

Determine the Directing Influence

Because chlorine directs incoming groups to the ortho and para positions relative to itself, the sulphonation will predominantly occur at these positions.

Understand the Reaction Site Preference in Sulphonation

For sulphonation, both ortho and para positions are possible, but there are generally more para products because of steric hindrance at the ortho positions.

Conclusion Based on Influence

Given the directing influence of chlorine and the steric factors, sulphonation of chlorobenzene will give mainly a para product, but ortho substitution is also observed.

Key concepts

ortho-para directing effect

In the context of aromatic substitution reactions, some substituents can influence where new groups attach to a benzene ring. Chlorine is a good example here. Although it's an electron-withdrawing group through inductive effects, it can actually donate electrons through resonance and stabilize the benzene ring. This dual behavior makes chlorine an 'ortho-para director'. This means that, in a reaction, new groups prefer to add at the ortho (adjacent) or para (opposite) positions relative to the chlorine. As a result, when chlorobenzene undergoes sulphonation, the sulfonic acid group (SO₃H) will predominantly attach to these favored positions.

electron-withdrawing group

Chlorine in chlorobenzene acts as an electron-withdrawing group (EWG) through its inductive effect. This means it pulls electron density away from the benzene ring due to its higher electronegativity. However, chlorine also has lone pairs that can participate in resonance. Through this resonance, chlorine donates electron density back into the ring, which compensates for some of the electron withdrawal. The net effect leads to a decrease in overall electron density, yet enough remains to stabilize any incoming groups at ortho and para positions.

steric hindrance

Steric hindrance is about the physical space and the crowding around the reactive sites on a molecule. In other words, larger groups can block other reactions from happening close by. In the case of chlorobenzene, the ortho positions are close to the chlorine atom, making them more crowded. During sulphonation, this means that it's harder for the sulfonic acid group to fit into these ortho positions without causing strain or overlap with the chlorine. As a result, the para position, which is further away, generally becomes a more favorable site for the sulphonation product since there's less crowding and more room.

reaction site preference

When considering the sulphonation of chlorobenzene, we need to think about where the new sulfonic acid group will prefer to attach. Ortho and para directing effects, along with steric hindrance, influence this decision. Because chlorine directs new groups to the ortho and para positions and due to steric hindrance at the ortho positions, the para position becomes particularly preferred. This results in most of the sulphonation product being in the para form. However, some ortho substitution does still occur, but it's less common than the para substitution.