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This document provides detailed notes on alkenes, covering their key properties, characteristic reactions, and associated reaction mechanisms. These notes are designed for Cambridge A-Level Chemistry (9701) students.
Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond ($C=C$). The presence of this double bond makes alkenes more reactive than alkanes.
Alkenes are named using the IUPAC system. The parent chain is identified, and the position of the double bond is indicated by a number prefix.
Example: $CH_2=CH_2$ is named ethene, and $CH_3-CH=CH_2$ is named propene.
Alkenes generally have lower boiling points and melting points compared to alkanes with similar molecular weights. This is due to the weaker intermolecular forces (London dispersion forces) present in alkenes.
Boiling points are lower than alkanes due to less polar C=C bond and weaker London dispersion forces.
Alkenes are non-polar and therefore soluble in non-polar solvents but insoluble in water.
Generally less dense than alkanes.
The carbon-carbon double bond in alkenes is the site of most chemical reactivity. Alkenes undergo addition reactions readily.
Addition reactions involve the addition of atoms or groups of atoms across the double bond, converting an alkene into a saturated hydrocarbon.
Addition of hydrogen ($H_2$) across the double bond in the presence of a metal catalyst (e.g., Pt, Pd, Ni) to form an alkane.
Reaction Mechanism: The alkene adsorbs onto the metal surface, followed by the adsorption and dissociation of hydrogen. The hydrogen atoms then add across the double bond.
Addition of a halogen (e.g., Cl2, Br2) across the double bond. This is a very fast reaction.
Reaction Mechanism: The halogen molecule is polarized by the π electrons of the alkene, forming a cyclic halonium ion intermediate. This intermediate is then attacked by a halide ion.
Addition of a hydrogen halide (e.g., HCl, HBr) across the double bond. Follows Markovnikov's rule.
Markovnikov's Rule: In the addition of HX to an alkene, the hydrogen atom attaches to the carbon with more hydrogen atoms, and the halide attaches to the carbon with fewer hydrogen atoms.
Reaction Mechanism: Protonation of the alkene to form a carbocation intermediate, followed by nucleophilic attack by the halide ion.
Addition of water across the double bond in the presence of an acid catalyst (e.g., H2SO4). Follows Markovnikov's rule.
Reaction Mechanism: Protonation of the alkene to form a carbocation intermediate, followed by nucleophilic attack by water.
Alkenes are readily oxidized.
Reaction with a peroxyacid (e.g., m-CPBA) to form an epoxide (oxirane). The oxygen atom forms a three-membered ring with one of the carbon atoms of the double bond.
Reaction Mechanism: A concerted mechanism involving a 'butterfly' transition state.
Reaction with potassium permanganate ($KMnO_4$) or potassium dichromate ($K_2Cr_2O_7$) to form diols (vicinal diols) or dicarboxylic acids (if the alkene is terminal).
Reaction Mechanism: The alkene is oxidized to a diol, which is further oxidized to a diol or dicarboxylic acid.
Alkenes can undergo polymerization to form polymers. This process involves the joining of many alkene monomers to form a long chain.
The most common type of polymerization, where monomers add to each other without the loss of any atoms.
Example: Polymerization of ethene to form polyethylene.
Reaction Mechanism: Initiation, propagation, and termination steps involving free radicals.
Monomers join together with the loss of a small molecule (e.g., water).
Example: Polymerization of alkenes with functional groups (e.g., carboxylic acids).
Alkenes are important organic compounds due to their reactivity. Their characteristic addition reactions, particularly those involving the double bond, are fundamental to organic chemistry. Understanding Markovnikov's rule and the mechanisms of these reactions is crucial for A-Level Chemistry.