Ethers are prepared by the dehydration of alcohols in presence of protic acids at $413\text{ K}$. Symmetrical and unsymmetrical ethers can also be prepared by Williamson synthesis. This reaction involves $\text{S}_\text{N}2$ 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. $\text{C} - \text{O}$ bond in ethers are cleaved under drastic conditions with excess of $\text{HI}$. When unsymmetrical ethers react with $\text{HI}$, 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 $\text{HI} > \text{HBr} > \text{HCl}$. 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.