Ab initio MO studies are carried out on the identity gas phase nucleophilic substitution reactions of CH2-CHCH(2)X by X(-) with X = H, F, and Cl using the 6-31++G** basis sets including electron correlation at the MP2 level. With the weakest nucleofuge, X = H, the reaction proceeds by a stepwise S(N)2’ mechanism in which the breakdown of a stable intermediate is rate-limiting, whereas with the strongest nucleofuge, X = Cl, the concerted S(N)2 reaction is the most favored. For X = F all three pathways, i.e., anti-S(N)2’, syn-S(N)2’, and S(N)2, are competitive, with a little preference for the anti-S(N)2’ path. In all cases, the syn-S(N)2’ reaction is found to be the least favored. The possibility of 1,3-sigmatropic rearrangement within the allylic system taking place in parallel with nucleophilic displacement can be precluded.