dehydration of alcohols

An alcohol is a carbon chain wearing an -OH group. Heat it with a strong acid and it can be persuaded to throw off that oxygen along with a neighbouring hydrogen, leaving behind a molecule of water and a brand-new double bond. This loss of H and OH (together, the elements of H2O) from an alcohol to make an alkene is called dehydration — literally the removal of water.

There is a catch: hydroxide (OH-) is a terrible leaving group, so the alcohol cannot simply expel it. The acid solves this. First the oxygen is protonated to make an oxonium ion (R-OH2+), which can now leave as neutral water — a fine leaving group. For secondary and tertiary alcohols this departure gives a carbocation (an E1 path), which then loses a beta proton to a weak base in solution, forming the alkene. Primary alcohols, which would need an unstable primary carbocation, tend instead toward a concerted E2-like loss of water. Common acids are sulfuric or phosphoric acid with heat.

Dehydration is the standard way to convert an abundant, cheap alcohol into a more reactive alkene for further chemistry, and it is the reverse of acid-catalyzed hydration (adding water across a double bond). Because the carbocation intermediate can rearrange and because it gives the more stable alkene, dehydration usually follows Zaitsev's rule, favouring the more substituted product — and tertiary alcohols dehydrate far more easily than primary ones.