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.
Q) In Williamson synthesis, the alkoxide ion attacks the alkyl halide via which pathway?
Correct Answer :
SN²
Solution :
The correct option is SN2.
Step-by-step Explanation:
Williamson's synthesis is an organic reaction used for the preparation of symmetrical and unsymmetrical ethers. The reaction involves a nucleophilic substitution reaction where an alkoxide ion acts as a strong nucleophile and attacks an alkyl halide.
This reaction proceeds via a biomolecular nucleophilic substitution pathway, commonly known as the SN2 mechanism. In this pathway:
1. The alkoxide ion () performs a backside attack on the sp3-hybridized carbon atom bonded to the leaving group (halide ion) in the alkyl halide molecule.
2. The bond-forming (between the carbon and the alkoxide oxygen) and the bond-breaking (between the carbon and the halogen) steps occur simultaneously in a single transition state.
3. Because the mechanism is concerted and happens in a single step with a backside attack, it is highly sensitive to steric hindrance. Therefore, this SN2 pathway is most efficient when the alkyl halide is primary (1°).
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