Ethers are prepared by the dehydration of alcohols in presence of protic acids at 413 K. Symmetrical and unsymmetrical ethers can also be prepared by Williamson synthesis. This reaction involves \( S_N2 \) attack of an alkoxide ion on a primary alkyl halide. If tertiary alkyl halide is used, elimination reaction occurs and alkene is formed instead of ether.
C–O bond in ethers are cleaved under drastic conditions with excess of HI. When unsymmetrical ethers react with HI, the alkyl halide is formed from the smaller alkyl group. If one of the alkyl groups is tertiary, the alkyl halide is formed from the tertiary alkyl group because tertiary carbocation is more stable than primary carbocation. Cleavage of alkyl aryl ethers takes place at the alkyl–oxygen bond due to more stable alkyl–oxygen bond.
The order of reactivity of hydrogen halides is:
\[
HI>HBr>HCl
\]
Aromatic ethers undergo electrophilic substitution reactions. The alkoxy group attached to the aromatic ring activates the ring and directs the incoming group to ortho and para positions.
Question: 1
Complete the following equation:
\[
\text{Anisole} + CH_3Cl \xrightarrow{\text{Anhyd. } AlCl_3} \; ?
\]
Show Hint
–OCH\(_3\) group is ortho–para directing and strongly activating.
Concept:
This is a Friedel–Crafts alkylation reaction. The –OCH\(_3\) group activates the benzene ring and directs substitution to ortho and para positions.
Products:
\[
\text{o-Methylanisole and p-Methylanisole (para major)}
\]
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Question: 2
Complete the following reaction:
\[
\text{Anisole} \xrightarrow{\text{Conc. } HNO_3 + H_2SO_4} \; ?
\]
Show Hint
Electrophilic substitution in anisole occurs mainly at ortho and para positions.
Concept:
Nitration is an electrophilic substitution reaction. The methoxy group directs nitration to ortho and para positions.
Products:
\[
\text{o-Nitroanisole and p-Nitroanisole (para major)}
\]
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Question: 3
Write the names of alkyl halide and sodium alkoxide used to prepare tert-butyl ethyl ether by Williamson synthesis.
Show Hint
In Williamson synthesis use primary alkyl halide to avoid elimination.
Concept:
In Williamson synthesis, primary alkyl halide should be used to avoid elimination.
To prepare tert-butyl ethyl ether:
\[
(C_2H_5)Cl + (CH_3)_3CONa
\]
Answer:
Alkyl halide: Ethyl chloride
Sodium alkoxide: Sodium tert-butoxide
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Question: 4
Anisole on reaction with HI gives phenol and CH\(_3\)I and not methanol and iodobenzene. Justify the statement.
Show Hint
Alkyl aryl ether cleavage occurs at alkyl–O bond, not aryl–O bond.
Concept:
The tetrahedral bond angle is 109.5\(^\circ\). In ethers:
Two bulky alkyl groups attached to oxygen repel each other.
This increases bond angle slightly.
Therefore:
\[
\text{C–O–C bond angle} \approx 111^\circ
\]
Conclusion:
Due to repulsion between bulky alkyl groups, bond angle is slightly greater than 109.5\(^\circ\).
