Alkene structure isomerism and reactions

Key Definition Alkenes are unsaturated hydrocarbons with the general formula CₙH₂ₙ, characterised by a C=C double bond made of one σ bond and one π bond.

The π bond, formed by the sideways overlap of p-orbitals, is electron-rich and exposed, which makes alkenes far more reactive than alkanes.

Restricted rotation around the C=C double bond produces geometric isomers when each double-bond carbon carries two different groups. The cis–trans system fails when the substituents are too varied to define a clear main chain, and the E–Z system replaces it by assigning priority to the groups on each carbon according to atomic number (the Cahn–Ingold–Prelog rules).

The key reactions of alkenes are addition reactions:

• Hydrogen (H₂) with a nickel (Ni) catalyst gives alkanes (hydrogenation).
• Halogens (e.g. Br₂ or Cl₂) give dihalogenoalkanes.
• Hydrogen halides (e.g. HBr) give monohalogenoalkanes.
• Steam (H₂O) with an H₃PO₄ catalyst gives alcohols (hydration).
• Cold, dilute acidified KMnO₄ gives diols.

The decolourisation of bromine water — a colour change from orange to colourless — is the diagnostic test for a C=C double bond.

The mechanism of these additions is electrophilic addition, which proceeds in the following steps:

1. The π electrons attack a δ⁺ atom — either a permanent dipole, as in HBr, or a dipole induced in a non-polar molecule such as Br₂.
2. The π bond breaks and a positively charged carbocation intermediate is formed.
3. The carbocation is then attacked by the nucleophilic counterion (e.g. Br⁻) to give the product.

With unsymmetrical alkenes such as propene, the major product forms via the more stable carbocation. Carbocation stability follows the order tertiary > secondary > primary, because of the electron-donating positive inductive (+I) effect of alkyl groups (Markovnikov's rule).

Alkenes also polymerise by addition: the C=C double bonds break and the monomers link into long-chain polymers with no other product formed. Drawing repeat units, and identifying monomers from repeat units, is a routine exam skill.

The major exam connections cluster around three themes:

1. Every reaction in this topic is driven by the same idea — the π bond as the source of electrons.
2. Mechanism marks always demand correct curly arrow origins (from bonds or lone pairs) and proper handling of dipoles.
3. Environmental questions on polymer disposal must reference both the development of biodegradable polymers and the removal of toxic incineration gases (e.g. HCl) by alkaline scrubbing.