Dehydration is an elimination reaction in which a molecule of water is removed from an alcohol to form an alkene:CH3CH2OH → CH2=CH2 + H2O Conditions: heat with a concentrated acid catalyst, either concentrated phosphoric acid (H3PO4) or concentrated sulfuric acid (H2SO4).The -OH group and a hydrogen atom on an adjacent carbon are removed, and a C=C double bond forms between these two carbons.Why it matters: ethene made by fermentation then dehydration of ethanol can be used to make polymers such as poly(ethene) without using monomers derived from crude oil.More than one product
If the C-OH carbon has two different neighbouring carbons that carry hydrogen atoms, more than one alkene can form.Worked example: give the alkenes formed by dehydrating butan-2-ol, CH3CH(OH)CH2CH3.
Removing an H from carbon 1 gives but-1-ene, CH2=CHCH2CH3.
Removing an H from carbon 3 gives but-2-ene, CH3CH=CHCH3, which exists as E and Z stereoisomers.
So three alkenes form in total: but-1-ene, E-but-2-ene and Z-but-2-ene.Propan-2-ol, in contrast, gives only propene, because both neighbouring carbons are identical CH3 groups.A gaseous alkene such as ethene can be collected over water, while liquid alkenes are distilled off. The alkene can be confirmed with bromine water, which is decolourised.
If the C-OH carbon has two different neighbouring carbons that carry hydrogen atoms, more than one alkene can form.Worked example: give the alkenes formed by dehydrating butan-2-ol, CH3CH(OH)CH2CH3.
Removing an H from carbon 1 gives but-1-ene, CH2=CHCH2CH3.
Removing an H from carbon 3 gives but-2-ene, CH3CH=CHCH3, which exists as E and Z stereoisomers.
So three alkenes form in total: but-1-ene, E-but-2-ene and Z-but-2-ene.Propan-2-ol, in contrast, gives only propene, because both neighbouring carbons are identical CH3 groups.A gaseous alkene such as ethene can be collected over water, while liquid alkenes are distilled off. The alkene can be confirmed with bromine water, which is decolourised.