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Also known as: Bimolecular Nucleophilic Substitution, Walden Inversion
A one-step concerted mechanism where the nucleophile attacks the electrophilic carbon from the back side (180°) while the leaving group departs simultaneously. The reaction proceeds with inversion of configuration (Walden inversion) at the stereocenter.
Nucleophile approaches the electrophilic carbon from the back side (180° to the leaving group). The C-Nu bond forms while the C-LG bond breaks simultaneously in a single transition state.
Concerted — one step, no intermediate
Product forms with complete inversion of stereochemistry at the carbon center (Walden inversion). If starting material is R, product is S, and vice versa.
Backside attack causes umbrella-like inversion
CH₃Br
NaI in acetone (Finkelstein)
CH₃I
(R)-2-Bromobutane
NaCN in DMSO
(S)-2-Cyanobutane
CH₃CH₂Cl
NaOH in H₂O/DMSO
CH₃CH₂OH
Fundamental method for forming C-C (with CN⁻), C-O, C-N, and C-S bonds. Essential for functional group interconversion.