When mixed ethers are used, the alkyl iodide produced depends on the nature of alkyl groups. If one group is Me and the other a pri-or sec-alkyl group, then methyl iodide is produced. Here reaction occurs via $S _{ N } 2$ mechanism and because of the steric effect of the larger group, $I ^{-}$ attacks the smaller (Me) group.
$CH _{3} OC _{2} H _{5}+ HI \rightarrow CH _{3} I + C _{2} H _{5} OH$
When the substrate is a methyl $t$ -alkyl ether, the products are $t-R I$ and MeOH. Here reaction occurs by $S_{N} !$ mechanism and formation of products is controlled by the stability of carbocation. Since carbocation stability
order is $3^{\circ}>2^{\circ}>1^{\circ}> \overset{\oplus}{CH _{3}}$
$\therefore $ Alkyl halide is always derived from tert-alkyl group.
