1-iodobutane is often omitted in SN2 reactions with NaI in acetone because iodide (I⁻) is a good nucleophile and acetone is a polar aprotic solvent, which favors the SN2 mechanism. However, 1-iodobutane is less reactive compared to other alkyl halides like 1-bromobutane or 1-chlorobutane. This is because iodine is a better leaving group due to its larger size and lower bond strength with carbon, but the steric hindrance in 1-iodobutane can slow down the reaction.
When comparing the reactivity of bromocyclopentane and bromocyclohexane in SN2 reactions, the key factor is the ring strain and steric hindrance. Bromocyclopentane is more reactive than bromocyclohexane because the five-membered ring has less steric hindrance and less ring strain, making it easier for the nucleophile to approach the carbon attached to the bromine. In contrast, bromocyclohexane has more steric hindrance due to the larger six-membered ring, which makes it harder for the nucleophile to attack the carbon-bromine bond, thus reducing its reactivity in SN2 reactions.