ChemistryMediumClass 12

Williamson Synthesis — Preparation of ethers

Alcohols, Phenols & Ethers

JEE Qs

Hard

min

For Williamson synthesis, always prioritize a primary alkyl halide and a strong alkoxide/phenoxide to achieve optimal ether yields and avoid elimination side products.

🧮 Key Formulas

R-X + R'-O⁻Na⁺ → R-O-R' + NaX (where R is primary alkyl group, R' can be any alkyl or aryl group)

R-OH + Na → R-O⁻Na⁺ + 1/2 H₂

Ar-OH + NaOH → Ar-O⁻Na⁺ + H₂O

1° R-X + Nu⁻ → R-Nu (SN2 mechanism)

✅ Key Points for JEE

1Williamson synthesis is an SN2 reaction; therefore, the alkyl halide (R-X) *must* be primary (or methyl) for good yields of ether. Secondary and tertiary alkyl halides preferentially undergo E2 elimination.
2The alkoxide/phenoxide (R'-O⁻Na⁺) can be primary, secondary, tertiary, or aryl. The steric bulk on the alkoxide/phenoxide does not hinder the SN2 attack.
3Aryl halides (e.g., chlorobenzene) cannot be used as the alkyl halide component due to the partial double bond character of the C-X bond and the instability of aryl carbocations, making them unreactive towards SN2 (or SN1) mechanisms.
4When preparing unsymmetrical ethers (R-O-R'), always choose the less hindered alkyl group as the alkyl halide and the more hindered group (if any) as the alkoxide to maximize ether formation and minimize elimination.
5The reaction proceeds best with strong nucleophiles (alkoxides/phenoxides) and polar aprotic solvents (like DMF, DMSO, acetone) to favor SN2 over E2.

⚠️ Common Mistakes

✕Attempting to synthesize ethers using secondary or tertiary alkyl halides as the electrophile, leading to predominantly alkene (elimination) products instead of ethers.
✕Using aryl halides (e.g., bromobenzene) as the electrophile (R-X component) in the reaction.
✕Incorrectly identifying which component should be the alkyl halide and which should be the alkoxide when designing the synthesis of a specific ether.

NCERT Chapters

Class 12 Chemistry Ch 11: Alcohols, Phenols and Ethers
Class 12 Chemistry Ch 10: Haloalkanes and Haloarenes