Question

The wrong statement in the following is (1) Sulphonation of benzenc takes place only with hot concentrated sulphuric acid. (2) In the nitration mixture concentrated sulphuric acid is uscd for the formation of nitronium ion. (3) Bccause of unsaturation benzene casily undergoes addition rcactions. (4) Benzene burns with a sooty flame.

Short answer

Statement 3 is incorrect.

Step-by-step solution

Identify the Statements

There are four statements given: 1. Sulphonation of benzene takes place only with hot concentrated sulfuric acid. 2. In the nitration mixture, concentrated sulfuric acid is used for the formation of a nitronium ion. 3. Because of unsaturation, benzene easily undergoes addition reactions. 4. Benzene burns with a sooty flame.

Evaluate Each Statement

We need to evaluate each of the four statements to identify the one that is incorrect.

Check Statement 1

Statement 1 is true. Sulphonation of benzene occurs when benzene is heated with concentrated sulfuric acid or oleum.

Check Statement 2

Statement 2 is true. In the nitration of benzene, concentrated sulfuric acid is used to generate the nitronium ion \(NO_2^+\) which is the actual electrophile in the reaction.

Check Statement 3

Statement 3 is incorrect. Benzene does not easily undergo addition reactions due to its aromatic stability; it typically undergoes substitution reactions to preserve the aromatic ring.

Check Statement 4

Statement 4 is true. Benzene burns with a sooty flame due to its high carbon content.

Identify the Wrong Statement

Based on the evaluation, statement 3 is the wrong statement.

Key concepts

Sulphonation

Sulphonation is a type of substitution reaction that involves introducing a sulfonyl functional group \(\text{SO}_3\text{H}\) into a benzene ring. The process requires concentrated sulfuric acid, often at high temperatures. When benzene is heated with concentrated \( \text{H}_2\text{SO}_4 \) or oleum, a sulfonation reaction occurs, forming benzene sulfonic acid. This reaction happens because benzene is an electron-rich molecule and easily reacts with the highly electrophilic sulfur trioxide (SO\textsubscript{3}) generated in the reaction.

Nitration

Nitration is another substitution reaction where a nitro group \(\text{NO}_2\) is introduced into the benzene ring. In a nitration mixture, concentrated sulfuric acid ( \( \text{H}_2\text{SO}_4 \)) acts as a catalyst. It forms the highly electrophilic nitronium ion ( \( \text{NO}_2^+ \)) from nitric acid ( \(\text{HNO}_3\) ). This ion quickly reacts with benzene, replacing one of its hydrogen atoms and forming nitrobenzene. This reaction highlights the importance of sulfuric acid not just as an acid, but as a dehydrating agent necessary to form \(\text{NO}_2^+\).

Aromatic Stability

Aromatic stability is a special stability feature of benzene and similar aromatic compounds. Benzene, with its six π-electrons delocalized over six carbon atoms, is exceptionally stable. This delocalization forms a

Substitution Reactions

Substitution reactions in benzene are favored over addition reactions because they preserve the aromatic ring. In a substitution reaction, one hydrogen atom in the benzene is replaced by another atom or group without disrupting the π-electron system. This means the unique aromatic stability of the benzene ring is maintained. Benzene undergoes electrophilic aromatic substitution reactions where it attracts positively charged species (electrophiles) like \( \text{NO}_2^+ \) and \( \text{SO}_3 \). These reactions are the hallmark of benzene's chemical behavior, contrasting sharply with alkenes, which readily undergo addition reactions due to their less stable π-bonds.