The correct order for the rate of $\alpha ,\beta$-dehydrohalogenation of the following compounds is: (i) $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{CH}{}_2\mathrm{CH}{}_2\mathrm{I}$ (1-iodobutane, 1${}^{\circ }$) (ii) $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{CHI}-\mathrm{CH}{}_3$ (2-iodobutane, 2${}^{\circ }$) (iii) $(\mathrm{CH}{}_3){}_3\mathrm{C}-\mathrm{I}$ (2-iodo-2-methylpropane, 3${}^{\circ }$)

Solution & Explanation

Dehydrohalogenation rate increases with the degree of substitution of the carbon bearing the halogen because more $\beta$-hydrogens are available and the resulting alkene is more stable: $3{}^{\circ }>2{}^{\circ }>1{}^{\circ }$. So the increasing order is (i) 1${}^{\circ }$ < (ii) 2${}^{\circ }$ < (iii) 3${}^{\circ }$.