Read the passage carefully and answer the questions.

Replacement of a hydrogen atom in a hydrocarbon by an alkoxy or carboxyl group yields a class of compounds known as ethers. Ethers are classified as symmetrical or unsymmetrical on the basis of groups attached to the oxygen atoms. Diethyl ether, a symmetrical ether, has been widely used as an inhalation anesthetic. Ethers can be prepared by acid catalyzed intermolecular dehydration of alcohols and Williamson's synthesis. Acid catalyzed dehydration of alcohols is not generally preferred as it gives a mixture of elimination and substitution products. In Williamson's synthesis, an alkyl halide is allowed to react with sodium alkoxide. Ethers containing substituted Alkyl groups may also be prepared by this method. The C-O bond in ether is weakly polar and is cleaved under drastic conditions with excess of hydrogen halides. In electrophilic substitution, the alkoxy group deactivates the aromatic ring and directs the incoming group to ortho and para positions.

In Williamson synthesis, the alkoxide ion attacks the alkyl halide via which pathway?

$S_N2$

The correct answer is Option (1) → $S_N2$

Williamson ether synthesis involves reaction:

$RO^-+R'X→ROR'+X^-$

Alkoxide ion ($RO^-$) is a strong nucleophile, and the reaction proceeds through bimolecular nucleophilic substitution ($SN^2$).

Why $SN^2$ mechanism

• Strong nucleophile (alkoxide)
• One-step backside attack
• Rate depends on both alkoxide and alkyl halide
• Inversion of configuration occurs
• Favored with primary alkyl halides

Option-wise Detailed Explanation

1. $SN^2$

Williamson ether synthesis proceeds through a single-step bimolecular nucleophilic substitution. The alkoxide ion is a strong nucleophile and attacks the carbon bearing the halogen from the backside, displacing the halide ion. This gives inversion of configuration and works best with primary alkyl halides.

2. $SN^1$

$SN^1$ requires formation of a carbocation intermediate, which is not favored here. Alkoxide ions are strong bases and strong nucleophiles; under $SN^1$-favorable conditions they instead cause elimination rather than substitution. Therefore $SN^1$ is not the mechanism for Williamson synthesis.

3. Depends on nature of alkoxide ion

The mechanism does not change with different alkoxides because all alkoxides are strong nucleophiles. Regardless of whether it is methoxide, ethoxide, or tert-butoxide, substitution (when it occurs) still proceeds via $SN^2$. The nature of alkoxide mainly affects steric hindrance and elimination tendency, not the pathway type.

4. Depends on the nature of alkyl halide

The success of the reaction depends on the alkyl halide (primary is best, tertiary gives elimination), but the substitution pathway itself remains $SN^2$. If the alkyl halide is tertiary, $SN^2$ cannot occur due to steric hindrance, and elimination dominates. Thus this option is misleading regarding the mechanism.