Final answer:
The choice between SN1 and SN2 mechanisms in the reaction of alcohol with PCl5 depends on the substrate and the solvent. Primary or methyl substrates are more likely to undergo SN2 reactions, while tertiary substrates are more likely to undergo SN1 reactions. The solvent also plays a role, with polar aprotic solvents favoring SN2 reactions and polar protic solvents favoring SN1 reactions.
Step-by-step explanation:
In the case of alcohol+PCl5, the reaction can proceed via both SN1 and SN2 mechanisms. In an SN1 reaction, the inversion of configuration does not occur, while in an SN2 reaction, the inversion of configuration does occur. The choice between SN1 and SN2 depends on the specific conditions of the reaction, including the substrate and the solvent.
For primary or methyl substrates, the reaction is more likely to proceed via the SN2 mechanism. This is because primary and methyl substrates favor backside attack by the nucleophile, leading to inversion of configuration. On the other hand, tertiary substrates are more likely to undergo an SN1 reaction due to steric hindrance, where the nucleophile attacks a carbocation intermediate from either side, leading to a racemic mixture of products.
The solvent also plays a role in determining the mechanism. SN2 reactions tend to occur in polar aprotic solvents, which allow the nucleophile to approach the electrophilic carbon more freely. On the other hand, SN1 reactions are favored in polar protic solvents, which solvate both cations and anions and solvating ability can overcome steric hindrance in the case of tertiary substrates.