nucleophilic substitution - ch220c

nucleophilic aliphatic substituion

substitution of one group for another at a saturated, sp3 hybridized carbon atom commonly used to interconvert different functional groups

nucleophiles

bear at least one pair of non-bonding electrons and being either neutral or negatively charged

ex of typical nucleophiles

Cl-, Br-, I-, HO-, CN-, R2C-, H2O, N3-, R3N, RS-

leaving group

neutral or negatively charged; must accept the pair of bonding electrons from carbon atom as C-L bond breaks

better leaving groups

those that are conjugate bases of stronger acids

SN1

two steps; 1st - heterolytic cleavage or ionization of bond between carbon atom and leaving group (assisted by polar interactions) becoming carbocation; 2nd - combination w/ nucleophile to form substitution product

if nucleophile is the solvent, SN1 reaction known as 

solvolysis

which step of SN1 is slower why

1st step is slower bc involves breaking the C-Leaving gorup bond to form unstable carbocation (endothermic) — so rate-determining step; first-order rate constant

SN2

one step — nucleophile backside attacks the substrate causing inversion in stereochemistry; C-L bond broken concurrently with formation of C-Nuc bond; bimolecular, second-order rate constant

more alkyl groups attached to C-L

more sterically difficult for nuc to attack backside decreasing ease w/ which the SN2 process can occur

w/ increasing substitution of alkyl groups on carbon atom C-L

incipient carbocation in SN1 reaction becomes more stable, increasing its ease of formation along SN1 pathway

primary substrates undergo

SN2

tertiary substrates undergo

SN1s

secondary substrates 

react by both, but specific pathway dictated by various factors such as solvent, nature of nuc, reaction conditions 

polarizability

measure of ease w/ which the electron cloud of Lewis base is distorted by nearby center that bears partial or full positive charge