Alkenes revision notes

Alkenes: Key Concepts

Definition

Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond (C=C). This double bond is responsible for the unique chemical properties of alkenes compared to alkanes, which are saturated.

Bonding in Alkenes

Alkenes feature both sigma (σ) and pi (π) bonds. The σ bond is formed by the head-on overlap of orbitals, while the π bond results from the side-on overlap of p orbitals. This arrangement gives alkenes their reactivity, particularly in electrophilic addition reactions.

Electrophilic Attack

The C=C bond is electron-rich, making it susceptible to attack by electrophiles, which are species that seek electrons. This reaction is fundamental in organic synthesis and can lead to various products depending on the electrophile used.

Testing for Unsaturation

Bromine water can be used to test for unsaturation in alkenes. When bromine water is added to an alkene, the solution changes from brown to colorless, indicating the presence of a double bond.

Electrophilic Addition Reactions

One common reaction is the addition of hydrogen bromide (HBr) to alkenes. The reaction proceeds through the formation of a carbocation intermediate, where the stability of the carbocation influences the product formed. More stable carbocations (tertiary > secondary > primary) will form preferentially.

Major Products from Unsymmetrical Alkenes

When adding HBr to unsymmetrical alkenes, the Markovnikov rule applies: the hydrogen atom from HBr will attach to the carbon with the greater number of hydrogen atoms, leading to the formation of the more stable carbocation.

Hydration of Alkenes

Alkenes can be converted to alcohols through hydration, typically using water and an acid catalyst. This reaction also follows the Markovnikov rule.

Polymerization

Alkenes can undergo addition polymerization, where the double bonds open up to form long chains. The repeat unit of the polymer can be derived from the alkene monomer. It is important to distinguish between addition polymers, formed from alkenes, and condensation polymers, which involve the elimination of small molecules during polymer formation.