4. Predict the reactivity of alkyl halides (primary, secondary and tertiary) in each substitution reaction below. List them in the order of increasing reactivity for Sn1 and Sn2

Answer 1

For reaction speed, Primary>Secondary>Tertiary for SN2 reactions. This is due to the fact that SN2 reaction speed depends on sterics (crowding), and less sterics are better for SN2 since there is a backside attack. Therefore, Primary would be fastest for SN2 and Tertiary slowest (Tertiary would not occur at all in reality due to steric hindrance being too high).

Reaction speed for SN1 is exactly opposite SN2: SN1 speed is Tertiary>Secondary>Primary. This is due to the fact that SN1 forms a carbocation, and it prefers to form the most stable (most stubstituted/stabilized) carbocation.