Read the following passage and answer the questions based on it.

Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two large groups in ketones hinders the attack of nucleophile to carbonyl carbon than in aldehydes. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively than in the former.

A new C-C bond formation is possible in:

(A) Cannizzaro reaction
(B) Friedel-Crafts alkylation
(C) Clemmensen reduction
(D) Riemer-Tiemann reaction

Choose the correct answer from the options given below:

(B) and (D) only

The correct answer is Option (1) → (B) and (D) only.

Let us explore each of the reactions in detail to understand how they work and whether they involve the formation of new carbon-carbon (C-C) bonds.

(A) Cannizzaro Reaction

The Cannizzaro reaction is a redox reaction that occurs with non-enolizable aldehydes when treated with a strong base, such as sodium hydroxide (NaOH). It leads to the disproportionation of the aldehyde.

Let us look at the reaction between formaldehyde and benzaldehyde:

Mechanism:

The reaction involves a nucleophilic acyl substitution on an aldehyde, with the leaving group concurrently attacking another aldehyde in the second step. First, hydroxide attacks a carbonyl. The resulting tetrahedral intermediate then collapses, re-forming the carbonyl and transferring hydride to attack another carbonyl. In the final step of the reaction, the acid and alkoxide ions formed exchange a proton. In the presence of a very high concentration of base, the aldehyde first forms a doubly charged anion from which a hydride ion is transferred to the second molecule of aldehyde to form carboxylate and alkoxide ions. Subsequently, the alkoxide ion acquires a proton from the solvent.

Conclusion:

C-C Bond Formation: No. The reaction does not create any new C-C bonds.

(B) Friedel-Crafts Alkylation

The Friedel-Crafts alkylation involves the addition of an alkyl group to an aromatic ring using an alkyl halide and a Lewis acid catalyst, such as aluminum chloride (AlCl₃). Let us take the example of Friedel-Crafts alkylation of benzene:

Mechanism

Conclusion:

C-C Bond Formation: Yes. A new C-C bond is formed between the aromatic ring and the alkyl group.

(C) Clemmensen Reduction

The Clemmensen reduction is a method for reducing carbonyl compounds (ketones and aldehydes) to alkanes using zinc amalgam (Zn(Hg)) and hydrochloric acid (HCl).

Let us consider an example with the reactant 3-oxobutanoic acid:

Mechanism:

Conclusion:

C-C Bond Formation: No. The reaction reduces the carbonyl compound to an alkane without forming new C-C bonds; it only reduces existing functional groups.

(D) Riemer-Tiemann Reaction

The Riemer-Tiemann reaction is a method for the ortho-formylation of phenols using chloroform (CHCl₃) and a strong base like sodium hydroxide (NaOH).

Conclusion:

C-C Bond Formation: Yes. A new C-C bond is formed when the ortho-hydroxybenzaldehyde product is generated from the phenol.

Thus, the reactions that result in the formation of new C-C bonds are:

Friedel-Crafts Alkylation (B)

Riemer-Tiemann Reaction (D)

So the correct answer is: (B) and (D) only.