Arrange the following alkyl halides in decreasing order of the rate of  -elimination reaction with alcoholic KOH

i) ${\left({\mathrm{CH}}_3\right)}_2\mathrm{CH}-{\mathrm{CH}}_2\mathrm{Br}$

 ii) ${\mathrm{CH}}_3-{\mathrm{CH}}_2-\mathrm{Br}$

 iii) ${\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-\mathrm{Br}$

Stability of alkene after β-elimination directly depends on the number of β-substituents.

Since number of β- methyl substituent in (i) is 2, (iii) is 1 and (ii) is 0, thus the correct order of stability is: (i)>(iii)>(ii).