Electrophilic addition

Addition of hydrogen across the C=C double bond in the presence of heat and a nickel or platinum catalyst to form an alkane.

For example, the reaction of ethene with hydrogen produces ethane:

C₂H₄ + H₂ ➔ C₂H₆

Addition of halogens such as chlorine or bromine across the C=C double bond at room temperature to form haloalkanes.

For example, the reaction of ethene with bromine produces 1,2-dibromoethane:

C₂H₄ + Br₂ ➔ C₂H₄Br₂

Addition of steam across the C=C double bond in the presence of a phosphoric acid catalyst to form an alcohol.

For example, the reaction of ethene with steam produces ethanol:

C₂H₄ + H₂O ➔ C₂H₅OH

Bromine water turns from orange to colourless in the presence of an alkene. 

This colour change occurs because orange bromine is used up in the reaction, as it adds across the C=C bond of the alkene, and a colourless bromoalkane is produced.

Heterolytic fission

The high electron density of the π-electrons which polarises the Br-Br bond causing heterolytic fission.

The H-Br bond is polar, so the partially positive hydrogen atom acts as an electrophile because it is attracted to the electron rich double bond.

The hydrogen of the hydrogen halide adds to the carbon atom of the unsymmetrical alkene with the greater number of hydrogen atoms already attached.

Markownikoff's rule determines the major product in the reaction between a hydrogen halide and an unsymmetrical alkene.

The major product is one which is formed via the most stable carbocation intermediate. 

The more substituted a carbocation is, the greater its stability.

An electrophile is an electron-pair acceptor.

An electrophile is an electron-deficient species that accepts a pair of electrons to form a new covalent bond. Examples of electrophiles include AlCl₃, Br⁺ and H₃O⁺.

A carbocation is an ion that contains a positively charged carbon atom.

For example, the structure of the C(CH₃)H₂⁺ carbocation is:

The stability of carbocations increases in the order: primary < secondary < tertiary.