elementary steps

proton transfer

a proton is transferred from an acid to a base

biomolecular nucelophilic subsitution

Sn2

substrate undergoes subsitution

coordination

bond formation

heterolysis 

bond is broken and the two electrons end up on one of the atoms initially involved in the bond 

nucleophilic addition

nucleophile is added using breaking of pi bond

nucleophile elimination

nucleophile leaving creates pi bond

biomolecular elimination

E2

strong base attacks a substrate in which a leaving group and a hydrogen atom are on adjacent carbons 

electrophilic addition

nonpolar pi bond donates electrons to strongly electron deficient species (E+) forming a bond

electrophile elimination

electrophile eliminated from carbocation, generating a stable, uncharged, organic species

carbocation rearrangement

1,2 hydride shift

1,2 alkyl shift

Migration of an atom or group to an adjacent atom