Problem 50 - A Complete Resource Book in Chemistry for JEE Main

Question

On sulphonation of \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}\) (a) m-chlorobenzenesulphonic acid is formed (b) benzene sulphonic acid is formed (c) o-chlorobenzene sulphonic acid is formed (d) o- and p-chlorobenzene sulphonic acid is formed

Step-by-Step Solution

Answer

(d) o- and p-chlorobenzene sulphonic acid is formed.

Step 1: Understanding Sulfonation Reaction

Sulphonation is an electrophilic aromatic substitution reaction where a sulphonyl group, typically in the form of \( ext{-SO}_3H\), is substituted in the benzene ring. This substitution is influenced by the presence of substituents on the aromatic ring.

Step 2: Analyzing Chloro Group's Directing Effect

The chloro group (\( ext{-Cl}\)) on the benzene ring is an ortho-para directing group. It is electron-withdrawing through induction but also has lone pairs to donate through resonance. Therefore, it directs incoming groups to the ortho and para positions relative to itself.

Step 3: Predicting Product Composition

Given that chlorobenzene (\( ext{C}_6 ext{H}_5 ext{Cl}\)) is sulphonated, the sulphonyl group will be directed to position at the ortho and para positions of the chloro group due to its directive effect. Hence, both \( ext{o-chlorobenzene sulphonic acid}\) and \( ext{p-chlorobenzene sulphonic acid}\) will be formed.

Key Concepts

Electrophilic Aromatic Substitution

When we talk about electrophilic aromatic substitution, we're diving into a really fundamental reaction type in organic chemistry. This is the kind of reaction where an electrophile replaces a hydrogen atom that's attached to an aromatic system, like benzene. Now, why benzene? Well, benzene rings are stable and have a rich body of electrons above and below their plane. This electron cloud attracts electrophiles, which are positively charged species or molecules that crave electrons.
The sulfonation of benzene is a classical example of a reaction where an electrophile, in this case, the sulfonyl group (represented as \(\text{-SO}_3H\)), targets the benzene ring. The challenge is that this isn't just a free-for-all - the position that the incoming group takes depends a lot on what's already attached to the benzene ring. That explains why some positions are more reactive than others. This leads us into the exciting world of directing effects.

Chloro Group Directing Effects

Now, let's get into the nitty-gritty about the chloro group's directing effects. The chloro group, denoted as \(\text{-Cl}\), on a benzene ring can be quite the character. It's peculiar due to its dual nature.
On one hand, it's electron-withdrawing because chlorine is electronegative. This means it pulls electron density towards itself through something we call induction. But, plot twist: chlorine has lone pairs of electrons (those unshared electrons) that it can push towards the benzene ring through resonance.
Because of these contrasting actions, we end up with a chloro group situation where it's a bit of a tug-of-war. The resonance effect usually wins because it's stronger, causing the chloro group to be ortho-para directing. This means if something is going to be added to our benzene ring, it's more likely to plop down at the ortho or para positions relative to that sneaky chlorine. And this effect is super crucial in predicting where new groups attach during reactions.

Ortho-Para Directing Groups

Ortho-para directing groups are fascinating players in the aromatic substitution game. They guide new groups to specific "favored" spots on the benzene ring, and it's all thanks to electronic influences.
The ortho position is right next to the substituent already on the ring, while the para position is directly opposite. These spots become more attractive for incoming electrophiles when the directing group is present.
With our case study, chlorobenzene, the \(\text{-Cl}\) group helps push substitution to these positions because of its resonance operation, despite its electron-withdrawing character.
In practical terms, when we want to add a sulfonyl group, we're most likely to see it end up sticking to the ring at these ortho and para positions due to the dominant directing effect of chlorine. So when you think ortho-para directing, think of pathways being lit up for newcomers to integrate into the benzene community.