<Dt> X ⋅ + R − H ⟶ X − H + R ⋅ (\ displaystyle (\ ce (X . + R-H -> X-H + R .))) </Dt> <Dt> R ⋅ + X 2 ⟶ R − X + X ⋅ (\ displaystyle (\ ce (R. + X2 -> R-X + X .))) </Dt> <Dd> Reactions during the chain reaction of radical substitution </Dd> <P> The addition and its counterpart, the elimination, are reactions which change the number of substitutents on the carbon atom, and form or cleave multiple bonds . Double and triple bonds can be produced by eliminating a suitable leaving group . Similar to the nucleophilic substitution, there are several possible reaction mechanisms which are named after the respective reaction order . In the E1 mechanism, the leaving group is ejected first, forming a carbocation . The next step, formation of the double bond, takes place with elimination of a proton (deprotonation). The leaving order is reversed in the E1cb mechanism, that is the proton is split off first . This mechanism requires participation of a base . Because of the similar conditions, both reactions in the E1 or E1cb elimination always compete with the S 1 substitution . </P>

Chemical reactions involve what part of the atom