Allyl bromide $\mathrm{CH}{}_2=\mathrm{CH}-\mathrm{CH}{}_2-\mathrm{Br}$ reacts with iodide in acetone considerably faster than 1-bromopropane although both are primary. The best explanation is that in the SN2 transition state

1. Athe C=C bond forces the reaction to follow an SN1 pathway through a vinyl cation
2. Bthe p orbital on the reacting carbon overlaps with the $\pi$ system of the C=C bond, which stabilises the transition state
3. Cthe double bond donates electrons into the C-Br sigma bond and makes bromide a poorer leaving group
4. Dthe double bond makes the carbon bearing bromine more hindered to the nucleophile