CHEM 257: reaction properties

great leaving groups

Cl-, Br-, I-, TsO-, H2O (neutral molecules are best leaving groups)

moderate leaving groups

CH3CO2- (Acetate), F- (generally won't leave)

bad leaving groups

OH-, NH2-, H3CO- (or RO-)

polar protic solvents

Acetic Acid, Water, Alcohols (EtOH, i-PrOH), NH3

polar aprotic solvents

DMF. DMSO, HMPA, CH3CN, ethyl acetate, acetone

non-polar solvents

alkanes (hexanes, etc), toluene

very good nucleophiles

I- HS-, RS

good nucleophiles

Br- HO-, RO-, CN-, N3-

fair nucleophiles

NH3, Cl-, RCO2-

weak nucleophiles

H2O, Alcohols (ROH, EtOH, etc)

very weak nucleophiles

acetic acid, carboxylic acids

very strong bases

NH2-, H-, HC≡C- (note acetylide anion only undergoes SN2 for methyl or primary substrates, otherwise, it gives E2 elimination)

strong bases

RO-, HO-

moderate bases

RNH2

weak bases

NH3, CN-, RCO2-, halogens (iodine, bromide, chloride)

SN1 mechanism

2-step reaction, forms carbocation intermediate

SN1 nucleophile strength

moderate, non-basic

SN1 leaving group

good-great

SN1 solvent

polar-protic

SN1 reactant

methyl- no reaction

primary- no reaction

secondary- non-basic nucleophile and if carbocation is resonance stabilized (benzylic/allylic)

tertiary- favored, non-basic nucleophile

SN1 stereochemistry

racemic mixture, planar carbocation intermediate

SN2 mechanism

1-step reaction, concerted (no intermediate)

SN2 nucleophile strength

strong, not too basic (preferred non-basic)

SN2 leaving group

good

SN2 solvent

polar-aprotic

SN2 reactant

methyl- highly likely (favored)

primary- highly favored

secondary- non-basic nucleophile, inversion of stereochemistry

tertiary- no reaction

SN2 stereochemistry

inversion

E1 mechanism

2-step reaction, forms carbocation intermediate

E1 nucleophile strength

basic, strong-moderate

E1 leaving group

good-great

E1 solvent

polar-protic

E1 reactant

methyl- no alkene formed

primary- no alkene formed

secondary- if carbocation intermediate is resonance stabilized, okay base

tertiary- often seen with SN1

E1 stereochemistry

Zaistev's rule (form most halyl substituted)

E2 mechanism

one-step reaction, concerted (no intermediates)

E2 nucleophile strength

basic, strong

E2 leaving group

good

E2 solvent

polar-aprotic

E2 reactant

methyl- no alkene formed

primary- favored with a strong base

secondary- favored with a strong base

tertiary- strong base needed

E2 stereochemistry

anti periplanar arrangement of H and X