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 oxychloride 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.
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