Common elementary Steps

Association of a nucleophile(An) non bonding pair (n) —→ empty atomic orbital (a)

Nucleophillic addition to pi bond (Adn) non bonding pair (n) —→ pi antibond (pi*)

Bimolecular nucleophillic substitution (SN2) or proton transfer(pt) Non bonding pair(n) —→ sigma antibond (sigma*)

Association of an electrophile (Ae) pi bond(pi) —→ empty atomic orbital(a)

Pi bond(pi) —→ pi antibond(pi*)

Beta elimination (Eb) Pi bond (pi) —→ sigma antibond(sigma*)

Sigma bond(sigma) —→ empty atomic orbital (a)

Dissociation of nucelofuge(Dn)/electrofuge(De) or 1,2 rearrangement(1,2R), Sigma bond(sigma) —→ pi antibond(pi*)

Bio molecular Elimination(E2) sigma bond (sigma)—→sigma antibond(sigma*)