Alkenes: preparation and reactions

Preparing alkenes:

• Partial hydrogenation of alkynes: with Lindlar's catalyst (palladium on charcoal, partly poisoned), the product is the cis alkene; with sodium in liquid ammonia, the trans alkene.
• Dehydrohalogenation: an alkyl halide is heated with alcoholic KOH, which removes HX from neighbouring carbons (β-elimination). The rate is I > Br > Cl for the halogen and 3° > 2° > 1° for the alkyl group.
• Dehalogenation of vicinal dihalides (two halogens on neighbouring carbons) with zinc.
• Acidic dehydration of alcohols: heating with concentrated sulphuric acid removes water.

The π electrons attract electron-seeking reagents, so alkenes undergo electrophilic addition: H₂ (with Ni), halogens, hydrogen halides (HX), cold concentrated H₂SO₄, and water (with acid). Adding bromine: the reddish-orange colour of bromine disappears as it adds across the double bond, a simple test for unsaturation.

When an unsymmetrical reagent such as HBr adds to an unsymmetrical alkene, which way round does it go? Markovnikov's rule: the negative part of the reagent goes to the carbon of the double bond that has fewer hydrogen atoms. The reason: the reaction goes through the more stable carbocation. So propene + HBr → 2-bromopropane is the main product.

The peroxide effect (Kharasch effect): in the presence of a peroxide, HBr adds the other way round (anti-Markovnikov), because the mechanism changes to a free radical chain. Propene + HBr with peroxide → 1-bromopropane. This happens with HBr only.

Oxidation: cold, dilute potassium permanganate (Baeyer's reagent) adds two –OH groups, giving a glycol, and its purple colour disappears, another test for unsaturation. Hot, acidic KMnO₄ breaks the C=C completely. Ozonolysis (ozone followed by zinc and water) also splits the C=C, into two C=O compounds; by identifying them, we can tell where the double bond was.

Alkenes can join into long chains: polymerisation. Ethene gives polythene; propene gives polypropene.