Explain the following with an example.
$(i)$ Kolbe's reaction.
$(ii)$ Reimer-Tiemann reaction.
$(iii)$ Williamson ether synthesis.
$(iv)$ Unsymmetrical ether.

(N/A) $(i)$ Kolbe's reaction: When phenol is treated with sodium hydroxide,sodium phenoxide is produced. This sodium phenoxide,when treated with carbon dioxide followed by acidification,undergoes electrophilic substitution to give ortho-hydroxybenzoic acid (salicylic acid) as the main product.
$(ii)$ Reimer-Tiemann reaction: When phenol is treated with chloroform $(CHCl_{3})$ in the presence of aqueous sodium hydroxide,a $-CHO$ group is introduced at the ortho position of the benzene ring to form salicylaldehyde.
$(iii)$ Williamson ether synthesis: This is a laboratory method to prepare symmetrical and unsymmetrical ethers by reacting an alkyl halide with a sodium alkoxide. It involves an $S_{N}2$ attack of the alkoxide ion on the alkyl halide. For example: $CH_{3}CH_{2}ONa + CH_{3}I \rightarrow CH_{3}CH_{2}OCH_{3} + NaI$.
$(iv)$ Unsymmetrical ether: An ether where the two alkyl or aryl groups attached to the oxygen atom are different. For example: ethyl methyl ether $(CH_{3}OCH_{2}CH_{3})$.