10 Halogen Derivaties

4. Distinguish between

SN1 and SN2 mechanism of substitution reaction ?

Factor SN1 SN2
Kinetics 1st order 2nd order
Molecularity Unimolecular Bimolecular
Number of steps Two steps One step
Bond making and bond breaking First the bond in the reactant breaks and then a new bond in the product is formed. Simultaneous
Transition state Two steps, two transition states One step, one transition state
Direction of attack of nucleophile Back side attack and front side attack Only back side attack
Stereochemistry Racemisation (If the substrate is optically active) Inversion of configuration (If the substrate is optically active)
Type of substrate Mainly 3° substrates Mainly 1° substrate
Polarity of solvent Polar protic solvent favorable Aprotic (non-polar) or solvent with low polarity favorable
Intermediate Intermediate involved No intermediate

10 Halogen Derivative Page 233