These occur between alkenes and electrophiles, often halogens as in halogen addition reactions. Common reactions include use of bromine water to titrate against a sample to deduce the number of double bonds present. For example, ethene + bromine → 1,2-dibromoethane:
:CH + Br → BrCHCHBr
This takes the form of 3 main steps shown below;
#Forming of a π-complex
#:The electrophilic Br-Br molecule interacts with electron-rich alkene molecule to form a π-complex 1.
#Forming of a three-membered bromonium ion
#:The alkene is working as an electron donor and bromine as an electrophile. The three-membered bromonium ion 2 consisted of two carbon atoms and a bromine atom forms with a release of Br.
#Attacking of bromide ion
#:The bromonium ion is opened by the attack of Br from the back side. This yields the vicinal dibromide with an antiperiplanar configuration. When other nucleophiles such as water or alcohol are existing, these may attack 2 to give an alcohol or an ether.
This process is called Ad2 mechanism ("addition, electrophilic, second-order"). Iodine (I), chlorine (Cl), sulfenyl ion (RS), mercury cation (Hg), and dichlorocarbene (:CCl) also react through similar pathways. The direct conversion of 1 to 3 will appear when the Br is large excess in the reaction medium. A β-bromo carbenium ion intermediate may be predominant instead of 3 if the alkene has a cation-stabilizing substituent like phenyl group. There is an example of the isolation of the bromonium ion 2.