I've been looking into activation of alcohols for nucleophilic substitution, and I've come across a series of reactions that differentiate between primary, secondary and tertiary alcohols. I've seen it written in multiple places that reagents like thionyl chloride and phosphorus trihalides aren't used for tertiary alcohols, but that strong acid is used instead. I was wondering why this was the case, because surely an activated alcohol has a good enough leaving group to progress through an SN1 mechanism.
The only thing I can think of is that the alcohol is too sterically hindered to effectively react with the activating agents. But I have seen a reaction scheme that substitutes a tosyl group from a tertiary alcohol via SN1, which confused me even more.
Any help is greatly appreciated.
Edit: I forgot to mention that I've also seen specified that the Mitsunobu reaction works only on primary and secondary alcohols, and I really hope all of these are linked in some way because that would make this a lot easier.