Tertiary alcohols prefer the SN1 reaction because the SN1 mechanism involves the formation of a carbocation intermediate. In tertiary alcohols, the carbon attached to the hydroxyl group (OH) is bonded to three other carbon atoms, making it highly substituted. This high substitution stabilizes the carbocation intermediate through hyperconjugation and inductive effects, making the formation of the carbocation more favorable.
On the other hand, primary alcohols prefer the SN2 reaction. The SN2 mechanism is a one-step process where the nucleophile attacks the carbon center at the same time as the leaving group departs. In primary alcohols, the carbon attached to the hydroxyl group is bonded to only one other carbon atom, making it less sterically hindered. This lack of steric hindrance allows the nucleophile to approach the carbon center more easily, facilitating the SN2 reaction.
In summary, the preference for SN1 or SN2 reactions in alcohols depends on the stability of the carbocation intermediate and the steric hindrance around the carbon center. Tertiary alcohols favor the SN1 reaction due to the stability of the carbocation, while primary alcohols favor the SN2 reaction due to minimal steric hindrance.