Asymmetrical alkenes react with concentrated sulfuric acid, and then water, to give a major and a minor alcohol. The rule is the same as with HBr: the major product forms via the more stable carbocation (tertiary > secondary > primary).Propene. The δ+ H of H2SO4 can add to either carbon of the C=C:• H to carbon 1 gives the secondary carbocation CH3CH+CH3. HSO4– bonds to carbon 2, so the major hydrogensulfate is CH3CH(OSO2OH)CH3, with the OSO2OH group on carbon 2.
• H to carbon 2 gives the less stable primary carbocation and the minor product CH3CH2CH2OSO2OH.Warming with water replaces the OSO2OH group with OH, on the same carbon. So the major alcohol is propan-2-ol, CH3CH(OH)CH3, and the minor alcohol is propan-1-ol, CH3CH2CH2OH.Worked example 1: but-1-ene, CH2=CHCH2CH3. H adding to carbon 1 puts the positive charge on carbon 2 (secondary); H adding to carbon 2 gives a primary carbocation. The major alcohol is butan-2-ol, and the minor is butan-1-ol.Worked example 2: 2-methylpropene, (CH3)2C=CH2. H adding to the CH2 carbon gives a tertiary carbocation, (CH3)3C+, so the major alcohol is 2-methylpropan-2-ol, (CH3)3COH.Quick check: in the major product the OH is on the C=C carbon that had fewer hydrogen atoms.
• H to carbon 2 gives the less stable primary carbocation and the minor product CH3CH2CH2OSO2OH.Warming with water replaces the OSO2OH group with OH, on the same carbon. So the major alcohol is propan-2-ol, CH3CH(OH)CH3, and the minor alcohol is propan-1-ol, CH3CH2CH2OH.Worked example 1: but-1-ene, CH2=CHCH2CH3. H adding to carbon 1 puts the positive charge on carbon 2 (secondary); H adding to carbon 2 gives a primary carbocation. The major alcohol is butan-2-ol, and the minor is butan-1-ol.Worked example 2: 2-methylpropene, (CH3)2C=CH2. H adding to the CH2 carbon gives a tertiary carbocation, (CH3)3C+, so the major alcohol is 2-methylpropan-2-ol, (CH3)3COH.Quick check: in the major product the OH is on the C=C carbon that had fewer hydrogen atoms.