An optically active alkyl bromide $\mathrm{C}{}_5\mathrm{H}{}_{11}\mathrm{Br}$ is heated with alcoholic KOH (E2, no rearrangement) and gives exactly three alkenes, when E and Z isomers are counted separately. The bromide is

2-Bromopentane is chiral and gives pent-1-ene, (E)-pent-2-ene and (Z)-pent-2-ene, three alkenes. 1-Bromo-2-methylbutane is chiral but gives only 2-methylbut-1-ene. 2-Bromo-3-methylbutane is chiral but gives 3-methylbut-1-ene and 2-methylbut-2-ene, two alkenes. 3-Bromopentane is achiral.