Alkenes

Alkenes ($C_nH_{2n}$) are unsaturated hydrocarbons with a reactive C=C double bond (a σ and a π bond). The π bond is a region of high electron density, so alkenes undergo electrophilic addition.

Electrophilic addition

The π electrons attack an electrophile (electron-poor species). General mechanism (e.g. HBr + ethene):

1. The C=C π electrons attack the δ+ end of HBr, forming a carbocation and a bromide ion.
2. The bromide ion attacks the carbocation, forming the product.

Use curly arrows (movement of an electron pair) in the mechanism.