Some alcohols undergo rearrangement or other unwanted side reactions when they dehydrate in acid. Alcohols may be dehydrated under mildly basic conditions using phosphorus oxy-chloride (POCl₃) in pyridine. The alcohol reacts with phosphorus oxychlorides much like it reacts with tosyl chloride (Section 11-5), displacing a chloride ion from phosphorus to give an alkyl dichlorophosphate ester. The dichlorophosphate group is an outstanding leaving group. Pyridine reacts as a base with the dichlorophosphate ester to give an E2 elimination. Propose a mechanism for the dehydration of cyclohexanol by POCl₃ in pyridine.

Rearrangement reactions constitute an important class of organic reactions.A few examples of rearrangement reactions are Beckmann rearrangement and Hofmann rearrangement.

In the dehydration of alcohols, protonation transforms the hydroxyl group into a good leaving group. This forms a carbocation, and the leaving of water molecules happens. In the next step, the loss of proton creates an alkene.

Alcohols get dehydrated under mild basic conditions by POCl₃ in pyridine. The mechanism of the reaction of alcohols with POCl₃ in pyridine is similar to that of tosyl chloride. The alcohol displaces a chloride ion from phosphorus to generate an alkyl dichlorophosphate ester.

The dichlorophosphate is an excellent leaving group. Pyridine serves as a base with dichlorophosphate ester to generate an E2 elimination product. The mechanism can be given as:

The mechanism for the dehydration of cyclohexanol by in pyridine