alcohol dehydration

Dehydration is the reaction that turns an alcohol into an alkene by stripping out the elements of water — one -OH and one neighbouring hydrogen leave together, and a new carbon-carbon double bond closes the gap. It is the reverse of acid-catalysed hydration: where hydration adds water across a double bond, dehydration takes water back out.

Most alcohol dehydrations run with a strong acid catalyst (concentrated H2SO4 or H3PO4) and heat. The acid first protonates the -OH, converting it from a hopeless leaving group into water, which can then depart. For a secondary or tertiary alcohol this gives a carbocation, and a base then plucks a proton from a neighbouring (beta) carbon to form the double bond — an E1 mechanism. The product follows Zaitsev's rule, favouring the more substituted, more stable alkene. Reactivity tracks carbocation stability: tertiary alcohols dehydrate easily, secondary need more forcing conditions, and primary alcohols are sluggish (going by a different, concerted E2-like route to avoid an unstable primary cation).

Dehydration is a standard way to install a double bond, but it has two honest caveats. First, because it runs through a carbocation, skeletal rearrangements (hydride or alkyl shifts) can scramble the product into something unexpected. Second, when more than one alkene can form, you usually get a mixture, just enriched in the Zaitsev product. For cleaner, rearrangement-free results, chemists often dehydrate indirectly — convert the -OH to a tosylate or halide and run an E2 with a base instead.