Reactions of Alkenes (AQA A-Level Chemistry): Revision Notes

3.4.3 Reactions of Alkenes

Alkenes, with their carbon-carbon double bonds ($C=C$), are highly reactive and undergo a range of additional reactions. These reactions involve the breaking of the $π$-bond in the double bond, allowing new atoms to bond to the carbon atoms.

Formation of Major and Minor Products in Unsymmetrical Alkenes

When unsymmetrical alkenes (alkenes where the carbon atoms in the double bond are attached to different groups) react with electrophiles like $HBr$, the products can include major and minor products. This is due to the formation of different carbocation intermediates.

Mechanism of Electrophilic Addition with $HBr$

1. Polarisation: The π-electrons in the double bond polarise the $H-Br$ bond, making $H$ slightly positive ($δ⁺$).
2. Step 1: The π-electrons attack the $δ⁺$ hydrogen atom, breaking the $H-Br$ bond and creating a carbocation intermediate on one of the carbons.
3. Step 2: The $Br⁻$ ion (from $HBr$) attacks the carbocation, resulting in the formation of a haloalkane.

Major vs Minor Product

Major Product: Formed from the more stable carbocation.

• The stability of a carbocation increases as more alkyl groups are attached to the positively charged carbon atom (due to the inductive effect).
• Tertiary carbocations (3 alkyl groups) > Secondary carbocations (2 alkyl groups) > Primary carbocations (1 alkyl group). Minor Product: Formed from the less stable carbocation.

Stability of Carbocation Intermediates

The formation of major and minor products depends on the stability of the carbocation formed during the reaction:

• Tertiary carbocation: Most stable, as it is stabilised by three electron-donating alkyl groups.
• Secondary carbocation: Moderately stable, stabilised by two alkyl groups.
• Primary carbocation: Least stable, with only one alkyl group providing stability. This explains why major products form through more stable carbocation intermediates, while minor products form through less stable intermediates.