Alkenes
What are alkenes?
Alkenes are unsaturated hydrocarbons in which at least two adjacent carbon atoms are sp2 hybridized, which causes them to form a double bond between these carbon atoms. Alkenes may be straight chains or branched chains.
Preparation of Alkenes
Dehydration of Alcohols
Dehydration is the removal of a water molecule from a compound. Alcohols on heating in the presence of a dehydrating agent such as Al2O3, P4O10, H2SO4, and H3PO4 lose a molecule of water to form an alkene.
(Note: All of the above dehydrating agents are acids. Al2O3 is a Lewis acid while P4O10, H2SO4, and H3PO4 are Bronsted acids)
The ease of dehydration of alcohols is in the following order.
Tertiary alcohols > Secondary alcohols > Primary alcohols
Dehydration with alumina (Al2O3)
When vapors of alcohol are passed over heated alumina (Al2O3), dehydration takes place with the formation of an alkene.
Dehydration with Sulphuric Acid (H2SO4)
Dehydration of primary alcohols
Dehydration of primary alcohols needs highly concentrated H2SO4 (~75%) and a temperature of around 170 oC.
Dehydration of secondary alcohols
Dehydration of secondary alcohols needs less concentrated H2SO4 (~60%) and a temperature of around 100 oC.
Dehydration of Tertiary alcohols
Dehydration of tertiary alcohols needs mild concentrated H2SO4 (~20%) and a temperature of around 85 oC.
Dehydration of 2-methyl-2-butanol forms a mixture of alkenes in accordance with Saytzev’s rule, a major (2-methyl-2-butene) and a minor (2-methyl-1-butene) product.
Orientation in Elimination and Saytzev’s Rule.
Alcohols and alkyl halides containing more than one β-carbons bearing hydrogen atoms, on dehydration and dehydrohalogenation give a mixture of alkenes in accordance with Saytzev’s rule. According to this rule, hydrogen is preferentially eliminated from the carbon atom with a fewer number of hydrogen substituents. When butanol, an alcohol with two β-carbons bearing hydrogen atoms, undergoes dehydration, it gives two products. viz. 1-butene (minor product ~ 20%) and 2-butene (major product ~ 20%).
Mechanism of dehydration of alcohols.
The dehydration of alcohols involves the loss of -OH groups from an α – carbon and the loss of -H from a β – carbon.
Step 1. Proton transfer
One of the unshared electron pairs of the oxygen atom of alcohol picks a proton from the acid to form protonated alcohol.
Step 2. Loss of water
The C-O bond breaks to form a carbonium ion, losing a water molecule.
Step 3. Formation of alkene
The carbonium ion stabilizes by transferring a β – hydrogen to a water molecule. This gives an alkene and the acid is regenerated.
