Illustrate with examples the limitations of Williamson's synthesis for the preparation of certain types of ethers.

Explanation

Williamson's synthesis is a versatile methose for the synthesis of both symmetrical and unsymmetrical ethers. However, for the synthesis of unsymmetrical ethers, a proper choice of reactants is necessary. Since Williamson's synthesis occurs by `SN_(2)` mechanism and primary alkyl halides are most reacive i n`Sn_(2)` reaction, therefore, best yield of unsymmetrical ethers are obtained when the alkyl halides are primary and the alkoxide may be primary, secondary or tertiary. For example, tert-butylethyl ether is prepared by trating ethyl bromine with sodium tert-butylethyl ether is prepared by treating ethyl bromide with sodium tert-butoxide.
`underset((3^(@)"Alkoxide"))(CH_(3)-overset(CH_(3))overset("| ")underset(CH_(3))underset("| ")("C ")-O^(-))Na^(+)+CH_(3)overset(delta+)(C )H_(2)-overset(delta-)(Br)overset(Delta)rarrCH_(3)-overset(CH_(3))overset("| ")underset(CH_(3))underset("| ")("C ")-OCH_(2)CH_(3)+Na^(+)Br^(-)`
The above ether cannot be prepared by treating sodium ethoxide with tert-butyl chloride or bromide since under these condition an alkene, i.e., isobutylene is the main product.
`(CH_(3))_(3)-C-Br+C_(2)H_(5)O^(-)Na^(+)rarr underset("2-Methylpropene")(CH_(3)-overset(CH_(3))overset("| ")("C ")=CH_(2))+NaBr+C_(2)H_(5)OH`
Aryl and vinyl halides cannot be used as substrates because they are less reactive in nucleophilic substitution.