Question

Phenol, when it first reacts with concentrated sulphuric acid and then with concentrated nitric acid, gives (a) \(2,4,6\)-trinitrobenzene (b) o-nitrophenol (c) p-nitrophenol (d) Nitrobenzene

Short answer

(c) p-nitrophenol

Step-by-step solution

Identify the Reaction Type

The problem involves the reaction of phenol with sulphuric acid followed by nitric acid. This sequence of reactions is a type of electrophilic aromatic substitution, where an aromatic compound undergoes substitution by electrophiles.

First Reaction with Sulphuric Acid

When phenol reacts with concentrated sulphuric acid, it undergoes sulfonation mostly on the para position to form para-hydroxybenzenesulfonic acid. However, a minor amount may react at the ortho position also, forming ortho-hydroxybenzenesulfonic acid.

Second Reaction with Nitric Acid

The sulfonated phenol (from step 2) is then nitrated by reacting with concentrated nitric acid. The nitro group substitutes the hydrogen atoms at the ortho and para positions relative to the hydroxy group. Thus, ortho-nitrophenol and para-nitrophenol are formed, with para-nitrophenol typically being the major product.

Determine the Final Product

Reviewing the products from the nitration step, the para-nitrophenol is the major product formed, matching the choice given in the options.

Key concepts

Phenol Nitration

The nitration of phenol is an essential part of understanding electrophilic aromatic substitution. This reaction involves the introduction of a nitro group ( O_2^+ ) into the aromatic ring of phenol. Phenol's aromatic ring has electrons delocalized across it, which are very attractive to electrophiles. This makes phenol a prime candidate for such substitutions. When phenol is treated with concentrated nitric acid, the introduction of nitro groups usually targets the ortho and para positions relative to the hydroxy group due to their enhanced electron density.

However, if phenol is first reacted with concentrated sulfuric acid before nitration, the reaction pathway can be influenced. Sulfuric acid acts as a blocking agent, directing the nitration away from the positions it would typically affect first.

• Phenol is first treated with concentrated sulfuric acid to introduce sulfonate groups.
• This sulfonation process mainly occurs at the para position but can occur at the ortho position as well.
• Subsequent nitration with nitric acid leads to a preferential substitution at the para position because after the sulfonate group leaves during nitration, para nitrophenol is the more stable product due to favorable steric and resonance effects.

This makes para-nitrophenol the major product of this reaction sequence.

Sulfonation Reaction

Sulfonation is a classic example of electrophilic aromatic substitution where a sulfonic acid group is introduced into the aromatic ring. When phenol is treated with concentrated sulfuric acid, a sulfonation reaction occurs. This reaction is crucial for understanding how directing effects impact the final product in multi-step reactions like this one.

In the presence of sulfuric acid, phenol can yield two products: ortho-hydroxybenzenesulfonic acid and para-hydroxybenzenesulfonic acid.

• The para product tends to be more favorable due to steric hindrance being lower at this position than at the ortho position.
• The reaction can reversibly convert between these two sulfonated products if the conditions allow, making para substitution a common outcome.

Once sulfonation has taken place, the sulfonic acid leaves creating a favorable site for the incoming nitro group in the subsequent nitration step.

Ortho and Para Positions

In electrophilic aromatic substitution reactions involving phenol, the positioning of substituents on the benzene ring can be predicted and controlled. The hydroxyl group (-OH), being an activator, directs incoming groups to the ortho and para positions relative to itself due to its electron-donating properties.

When phenol undergoes sulfonation, the greater availability of electrons at the para and ortho positions makes them more likely to react with electrophiles like sulfur trioxide or nitric acid.

• The ortho position is directly adjacent to the substituent already present, which can cause steric hindrance, making reactions here slightly less favorable than at the para position.
• The para position is directly opposite the activating group, allowing for easier access and accommodation of larger groups.

Understanding these positional preferences is vital for predicting the major products of reactions involving phenol. In many cases, as seen with phenol nitration following sulfonation, para-substitution is more thermodynamically favorable, resulting in a higher yield of para-nitrophenol.