In acetone with $\mathrm{NaI}$ (SN2 conditions), which of the following comparisons of reaction rates are correct?

Increasing substitution at the reacting carbon, or at the adjacent carbon (neopentyl), strongly retards SN2, so the first two are true. Fluoride is a much poorer leaving group than chloride, so the ethyl fluoride is slower, not faster. Allyl bromide is activated by the adjacent $\pi$ system in the SN2 transition state and reacts faster than the n-propyl bromide.