Question

On sulphonation of \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{Cl}\) (a) m-chlorobenzenesulphonic acid is formed (b) benzene sulphonic acid is formed (c) o-chlorobenzene sulphonic acid is formed (d) 0 - and \(\mathrm{p}\)-chlorobenzene sulphonic acid is formed

Short answer

(d) 0- and \\(\mathrm{p}\\)-chlorobenzene sulphonic acid is formed.

Step-by-step solution

Understand the reactant

The given compound is trichlorobenzene, specifically \(\mathrm{C}_6\mathrm{H}_3\mathrm{Cl}_3\). This indicates that there are three chlorine atoms on the benzene ring. In this particular problem, we should consider if one of the chlorine positions can lead to the formation of a sulphonated product.

Establish the reaction type

The reaction involved is sulphonation, which typically involves the substitution of a hydrogen atom on an aromatic ring with a sulfonic acid group (\(\mathrm{-SO_3H}\)). However, given that all hydrogens found directly adjacent to the chlorine atoms are replaced, we must determine how the substitution will be oriented among the chlorine atoms.

Determine the directive influence of \\(\mathrm{Cl}\\) groups

Chlorine is an ortho-para directing group due to its ability to donate lone pairs into the benzene ring, activating these positions for electrophilic aromatic substitution. Therefore, when sulphonation occurs, the \(\mathrm{-SO_3H}\) group will likely attach to the ortho- or para- positions relative to any chlorine atoms already on the benzene ring.

Analyze possible product formations

Since there are three chlorine atoms present, we can only expect sulphonation to readily occur at their ortho or para positions. This leads us to conclude that only the ortho (o)- and para (p)- chlorobenzene sulphonic acids will form, as these positions are activated by the chlorine atoms.

Select the correct answer

After considering the directive influence of chlorine and the reaction conditions, it becomes evident that option (d) is correct: both o- and p-chlorobenzene sulphonic acids are formed as products.

Key concepts

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution is a common reaction type in aromatic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. In our context of sulphonation, the electrophile is a sulfonic acid group \(\mathrm{-SO_3H}\). The benzene ring, characterized by its delocalized electrons, acts as a nucleophile, attracting electrophiles. When the reaction occurs, the stability of the aromatic ring remains, though it undergoes a temporary loss of aromaticity during the process. This brief period is followed by the reformation of the aromatic system, maintaining its planar structure. While various substituents can be on the benzene ring, they strongly influence where the new groups, like \(\mathrm{-SO_3H}\), will substitute. The sulphonation reaction, being both specific and predictable, helps chemists effectively manipulate the composition of aromatic compounds.

Ortho-Para Directing Groups

Ortho-para directing groups significantly impact where an incoming electrophile will add to a benzene ring. These groups have lone pairs that they can donate to the benzene ring, creating regions of higher electron density at both the ortho and para positions. In the case of chlorine, it acts as an ortho-para directing group. It is unique because, while it is electron-withdrawing from an inductive effect standpoint, it can donate electrons through resonance due to its lone pairs.When a reaction such as sulphonation occurs, the \(\mathrm{-SO_3H}\) group will preferentially substitute at these activated ortho or para sites, provided they are accessible. This increases the likelihood of products being formed specifically at these positions, as seen with the sulphonation of \(\mathrm{C}_6\mathrm{H}_3\mathrm{Cl}_3\), resulting in both ortho and para chlorobenzene sulphonic acids.

Chlorine Substitution Effects

The substitution effects of chlorine on aromatic rings can be quite nuanced. Despite its typical classification as an electron-withdrawing group, chlorine is an example of an ortho-para director due to its lone pair of electrons. These lone pairs can resonate into the benzene ring, temporarily enhancing the electron density at the ortho and para positions relative to the chlorine. In this way, any subsequent electrophilic substitution, such as the sulphonation of the benzene ring, is more likely to occur at these locations. Understanding this dual behavior helps explain the formation of specific products during aromatic reactions. When analyzed in the context of halogens and sulphonation, it reveals why, among multiple chlorines on a ring, sulphonation happens at the ortho and para positions, leading to the formation of ortho- and para-chlorobenzene sulphonic acid as the main products.