Question

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 \( \mathrm{S_N2} \) attack of an alkoxide ion on primary alkyl halide. If a tertiary alkyl halide is used, elimination reaction occurs and an alkene is formed, no ether is formed. 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 smaller alkyl group. If one of the alkyl group is tertiary, the alkyl halide is formed from the tertiary alkyl group because tertiary alkyl carbocation is more stable than the primary carbocation. Cleavage of alkyl aryl ethers takes place at the alkyl-oxygen bond due to more stable aryl-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 towards electrophilic substitution and directs the incoming group to ortho- and para-positions.

Answer the following questions :

(b) Write the names of alkyl halide and sodium alkoxide used to prepare tert-butyl ethyl ether.

Hint:

The halide must be primary for the Williamson synthesis. So the ethyl part comes from the halide and the tert-butyl part from the alkoxide.

Answer 1

Step 1: Recall the rule of Williamson synthesis:
The halide must be primary (or methyl). The alkoxide can be primary, secondary or tertiary.
The reason is that the reaction is $\mathrm{S_N2}$, which needs an open carbon for backside attack.

Step 2: Apply it to $\mathrm{(CH_3)_3C{-}O{-}C_2H_5}$:
The ether has two parts, a tert-butyl part and an ethyl part.
The ethyl part is primary, so it must come from the halide.
The tert-butyl part must come from the alkoxide.

Step 3: Write the reaction:
\[ \mathrm{(CH_3)_3CONa + CH_3CH_2Cl \longrightarrow (CH_3)_3COCH_2CH_3 + NaCl} \]
The reverse choice, tert-butyl chloride with sodium ethoxide, only gives 2-methylpropene.

Final Answer:
Use chloroethane or bromoethane (alkyl halide) and sodium tert-butoxide (sodium alkoxide).