Alkenes: structure and electrophilic additionEdexcel A-Level Chemistry: Revision notes

Alkenes are hydrocarbons containing a C=C double bond, with general formula CₙH₂ₙ (e.g. C₂H₄ ethene, C₃H₆ propene). Cycloalkenes such as cyclohexene also contain C=C. They are unsaturated, because they can add atoms across the double bond.

A C=C double bond is made of two different bonds:

• a sigma (σ) bond: head-on overlap of orbitals along the line between the nuclei
• a pi (π) bond: sideways overlap of p orbitals above and below the plane of the bond, giving a region of high electron density

The π bond is weaker and more exposed than the σ bond, so it is the site of reaction.

An electrophile is an electron pair acceptor. It is attracted to regions of high electron density such as the C=C bond. Examples are H in H–Br (δ+) and Br in Br₂ once a dipole is induced.

Heterolytic fission is the breaking of a covalent bond so that both electrons go to one atom, forming a positive and a negative ion: H–Br → H⁺ + Br⁻. Compare homolytic fission, which forms radicals.

In addition, the π bond breaks and two new σ bonds form. Examples with ethene or propene:

• H₂, nickel catalyst → alkane: C₂H₄ + H₂ → C₂H₆. This catalytic hydrogenation hardens unsaturated vegetable oils to make margarine.
• Br₂ or Cl₂ → dihalogenoalkane: C₂H₄ + Br₂ → CH₂BrCH₂Br
• HBr or HCl → halogenoalkane: C₂H₄ + HBr → CH₃CH₂Br
• steam with an acid catalyst (phosphoric acid) → alcohol: C₂H₄ + H₂O → CH₃CH₂OH
• acidified potassium manganate(VII) → diol: C₂H₄ → CH₂(OH)CH₂OH; the purple solution turns colourless.

Hydrogen bromide with ethene. (1) The π electrons attack the δ+ hydrogen of H–Br. A curly arrow goes from the C=C bond to the H, and another from the H–Br bond to the Br. (2) A carbocation CH₃CH₂⁺ and Br⁻ form. (3) A lone pair on Br⁻ attacks the C⁺ (curly arrow from the lone pair to the carbon), forming CH₃CH₂Br.

Bromine with ethene. The electron density of the C=C induces a dipole in Br–Br, making the nearer Br δ+. The π electrons attack it, the Br–Br bond breaks heterolytically giving Br⁻ and a carbocation, and Br⁻ then attacks to give 1,2-dibromoethane.

Adding HBr to an unsymmetrical alkene such as propene can give two carbocations. Carbocation stability is tertiary > secondary > primary, because alkyl groups release electron density towards the positive carbon (positive inductive effect) and so stabilise it.

Propene: CH₃CH⁺CH₃ (secondary) is more stable than CH₃CH₂CH₂⁺ (primary). The major product is therefore 2-bromopropane, with 1-bromopropane as the minor product.

2-methylpropene gives a tertiary carbocation, (CH₃)₃C⁺, so the major product is 2-bromo-2-methylpropane.

Shake the liquid with bromine water (or bromine). An alkene decolourises it, orange to colourless, by addition. An alkane leaves the mixture orange, because it is saturated and does not undergo addition.

Do not say 'clear': say 'colourless'.