Mechanism of Dehydration

The acid-catalysed dehydration of an alcohol is an elimination reaction. With concentrated phosphoric or sulfuric acid, the mechanism has three steps.Step 1: protonation. A lone pair of electrons on the oxygen of the -OH group forms a bond to an H⁺ ion from the acid. The alcohol becomes a protonated alcohol, with a positive charge on the oxygen.Step 2: loss of water. The C-O bond breaks heterolytically, with both electrons going to the oxygen. A water molecule leaves, and a carbocation is formed.Step 3: loss of H⁺. A C-H bond on a carbon adjacent to the positive carbon breaks. The electron pair moves to form the C=C double bond, and an H⁺ ion is released.The H⁺ used in step 1 is regenerated in step 3, so the acid is a catalyst.The curly arrows go:
from the O lone pair to the H⁺ (step 1);
from the C-O bond to the O (step 2);
from the C-H bond to the C-C bond (step 3).Worked example: for propan-2-ol, CH₃CH(OH)CH₃, the carbocation formed in step 2 is CH₃CH⁺CH₃. An H⁺ can be lost from either CH₃ group, but both give the same product, propene, CH₂=CHCH₃.When the carbocation has two different neighbouring carbons carrying hydrogen, H⁺ can be lost from either one. This is why some alcohols give a mixture of alkenes.