21.2 - Electrophilic Addition Reactions in Alkenes

Electrophiles are electron-pair acceptors

An electrophile is an electron-deficient species that readily accepts a pair of electrons from a nucleophile to form a covalent bond.

Types of electrophiles include:

• Cations - Positively charged ions e.g. the methyl cation, CH₃⁺.
• Neutral molecules with a partial positive charge (δ⁺) - Electron deficiency is caused by the polarisation of a bond due to an electronegative atom, e.g., boron trifluoride, BF₃.
• Compounds with carbonyl or carboxyl groups - The carbon atom of the carbonyl group is electron-deficient and susceptible to nucleophilic attack, e.g., aldehydes, ketones, and carboxylic acids.

Electrophilic addition reactions of alkenes

Alkenes are unsaturated hydrocarbons containing a carbon-carbon double bond. The high electron density of the double bond makes it susceptible to electrophilic attack, resulting in electrophilic addition reactions.

Electrophilic addition of halogens:

Halogens (X₂) add across the C=C bond to form disubstituted halogenoalkanes (C_nH_2nX₂).

For example, ethene reacts with bromine to form 1,2-dibromoethane:

Electrophilic addition of hydrogen halides:

Hydrogen halides (HX) add across the C=C bond to form monosubstituted halogenoalkanes (C_nH_2n+1X).

Symmetrical alkenes produce one product e.g., ethene reacts with hydrogen bromide to form bromoethane.

Unsymmetrical alkenes produce two products e.g., propene reacts with hydrogen bromide to form a mixture of 1-bromopropane and 2-bromopropane.

Electrophilic addition of water (hydration):

Water adds across the C=C bond in acidic conditions to form alcohols (C_nH_2n+1OH).

Symmetrical alkenes produce one product e.g., ethene reacts with water to form ethanol:

Unsymmetrical alkenes produce two products e.g., propene reacts with water to form a mixture of propan-1-ol and propan-2-ol.