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Reactions of Alkenes

The pi bond is weaker than the sigma bond so is broken more readily. The C-C bond has an enthalpy of 347kJmol-1 whilst the C=C bond has an enthalpy of 612 kJmol-1Alkenes undergo addition reactions. These include the addition of hydrogen, hydrogen halides, halogens and steam. Hydrogen is mixed with an alkene and passed over a hot nickel catalyst at 423K to form an alkane. Hydrogen is added across the double bond. This is known as hydrogenation.Halogenation occurs at room temperature. Alkenes react with halogens or hydrogen halides to form halogenoalkanes. Alkenes react with steam to form alcohols. The mechanism of the reactions is electrophilic addition.

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Specification: OCR Chemistry A (H432), A-level, 4.1.3 Alkenes. Topic: Alkenes.

Key terms in this lesson

addition
A reaction in which a pi bond is broken and atoms or groups are added across it to form a saturated product, with no atoms lost.
hydrogenation
The addition of hydrogen across a carbon-carbon double bond, using a nickel catalyst, to form a saturated compound.
electrophilic addition
A reaction mechanism in which an electrophile is attracted to and attacks an electron-rich double bond, and a nucleophile then bonds to the resulting carbocation.
halogenation
A reaction in which a halogen atom is substituted for a hydroxyl group or a hydrogen atom in a molecule.

More in Alkenes

All 11 lessons in Alkenes · All OCR A-level Organic Chemistry topics