14.2.2—Reactions of alkenes

The electron-rich C=C attacks an electrophile, then a nucleophile bonds to the carbocation-like intermediate. Hydrogenation adds H₂ with Pt/Ni, hydration adds steam with phosphoric acid, and HX adds a hydrogen halide at room temperature.

The π bond is consumed and each alkene carbon gains a new atom or group. Write the reagent, catalyst and product rather than treating every addition as hydrogenation.

CH₂=CH₂ + H₂ → CH₃CH₃; CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH with H₃PO₄. Both use the same alkene but different reagents and products.

Electrophilic addition is not substitution, and steam hydration is reversible whereas catalytic hydrogenation is not described by the same equilibrium.