Biomolecular Nucleophilic Substitution (SN2)

Electrophile

Electron deficient

Nucleophile

Electron rich

Leaving group

Group on the substrate that leaves with its bonded pair of electrons

Reaction mechanism

• Nucleophile attacks the substrate from behind

• Nucleophile attaches itself to the substrate

• Leaving group leaves with electrons 

Bulkiness of electrophile

R groups attached to the substrate cannot be bulky and difficult for the nucleophile to pass through

Nucleophile strength

• Negatively charged species

• Less electronegative atoms are less stable with a negative charge

• Larger atoms are more polarizable

• Nucleophile is as free of bulky groups as possible

• Leaving group should be electron withdrawing, stable once it leaves, and polarizable

• Polar aprotic solvent

Polar aprotic solvent

Dissolves most species in S_N2 and does not react strongly with the nucleophile; no O-H or N-H bonds