Topic 20.3: Elimination reactions

20.3.1 Describe, using equations, the elimination of HBr from bromoalkanes

Bromoalkane give alkene by dehydrohalogenation using heat and concentrated sulphuric acid

20.3.2 Describe and explain the mechanism for the elimination of HBr from bromoalkanes

The mechanism first involves removal of a photon (abstraction) by the base. This happens to the hydrogen atom which is on the carbon adjacent to the carbon holding the bromine atom. This hydrogen atom is said to be an 'alpha-hydrogen'

This produces a negatively charged species (a carbocation), which then rearranges by moving the negatively charge lone pair into a pi orbital between the two carbon atoms with the loss of the bromine atom as bromide ion