Alkenes: Addition Reactions

Alkenes contain a carbon–carbon double bond (C=C). They are called unsaturated because more atoms can add on. In an addition reaction, atoms add across the double bond. The C=C opens and becomes a single bond, making one larger product.

Why addition happens

The C=C bond is more reactive than a single bond. When new bonds form, energy is released, so most addition reactions are exothermic. Bond making releases energy; this helps the reaction happen.

Key addition reactions of alkenes

• Hydrogenation: alkene + hydrogen → alkane (nickel catalyst, about 150 °C). Used to harden vegetable oils into margarine. [ $\mathrm{C_2H_4 + H_2 \rightarrow C_2H_6}$ ]
• Halogenation: alkene + bromine/chlorine → dihalogenoalkane. Bromine water turns from orange to colourless with alkenes (test for C=C). [ $\mathrm{C_2H_4 + Br_2 \rightarrow C_2H_4Br_2}$ ]
• Hydrohalogenation: alkene + hydrogen halide (HCl/HBr) → halogenoalkane. One H and one halogen add across C=C. Example: ethene + HCl → chloroethane. [ $\mathrm{C_2H_4 + HCl \rightarrow C_2H_5Cl}$ ]
• Hydration: alkene + steam → alcohol (phosphoric acid catalyst on silica, high temperature and pressure). Example: ethene → ethanol. [ $\mathrm{C_2H_4 + H_2O \rightarrow C_2H_5OH}$ ]
• Addition polymerisation: many alkene molecules (monomers) join to make a long-chain polymer. Example: ethene → poly(ethene). [ $$\mathrm{n\,CH_2{=}CH_2 \rightarrow (-CH_2-CH_2-)_n}$$ ]

A helpful picture in words

Imagine the C=C like a springy double handshake. It can “open” so two new hands (atoms) can hold on, turning the double handshake into two single ones.

Common misconceptions

• “The double bond disappears.” It changes into a single bond; it does not vanish.
• “Alkanes also decolourise bromine water.” Alkanes do not, unless UV light is used (that is substitution, not addition).
• “No conditions are needed.” Many additions (like hydrogenation, hydration) need catalysts and heat.