Final answer:
A secondary alcohol reacts with SOCl2 or PCl5 typically via an SN2 mechanism, which is a single-step substitution where the bond formation and breaking occur simultaneously, favored due to steric hindrance and the presence of good leaving groups.
Step-by-step explanation:
When a secondary alcohol reacts with SOCl₂ or PCl₅, the reaction typically proceeds via an SN2 mechanism. In an SN2 reaction, a nucleophile attacks the electrophile (in this case, the carbon of the alcohol) from the opposite side of the leaving group, resulting in a single-step mechanism where the bond formation and bond breaking occur simultaneously.
On the other hand, SNi reactions, referring to substitution nucleophilic internal, involve the retention of configuration at the reacting center, which is less common and not typically observed with these reagents. Considering secondary alcohols, the steric hindrance is moderate, so it generally prefers SN2 over SN1 or E1 mechanisms, which would involve a carbocation intermediate that is more suitable for tertiary substrates.