Dehydration

Dehydration is the removal of a water molecule from an alcohol to form an alkene. It is an elimination reaction: the -OH group and an H atom on the neighbouring carbon are removed, and a C=C double bond forms between the two carbons.CH₃CH₂OH → CH₂=CH₂ + H₂OEthanol forms ethene.Conditions: heat the alcohol with a concentrated phosphoric acid catalyst (concentrated sulfuric acid can also be used, but it causes more side reactions, such as charring). Alternatively, the alcohol vapour can be passed over hot aluminium oxide.Practical example: cyclohexanol, heated with concentrated phosphoric acid, forms cyclohexene. Cyclohexene has a lower boiling point than cyclohexanol, so it distils off as it forms and is collected. It is then purified: washed, dried with an anhydrous salt and redistilled.Testing the product: the alkene decolourises bromine water (orange to colourless), showing that a C=C bond has formed.More than one product: if the -OH carbon has two different neighbouring carbons carrying H atoms, water can be removed in more than one way. Butan-2-ol, CH₃CH(OH)CH₂CH₃, gives but-1-ene and but-2-ene, and but-2-ene forms as both E and Z isomers.Why it matters: ethanol can be made by fermenting sugars from plants, so dehydrating it provides ethene from a renewable source instead of crude oil. This ethene can be used to make polymers such as poly(ethene).