Sn2 Flashcards
is Sn2 stereospecific?
yes
how many steps are there?
1, concerted (same time)
what is the rxn dependent on?
structure of substrate
strength of Nu
type of solvent
basicity of L.G.
what does Sn2 stand for?
substitution, nucleophilic, bimolecular
backside or frontside attack?
backside
rate formula
rate = k[substrate][Nu] concentration based
transition state E determines rate
inversion of configuration?
yes
why not frontside?
too many e- repels
L.G. blocks from side (sterics)
Nu EN trend
across row ——–> worse Nu, slower rxn
C > N > O > F
Nu size trend
down group: better Nu, faster rxn
F < Cl < Br < I
this is opposite of the basicity trend
solvent effects on size of Nu
if small Nu, polar solvent surrounds it and makes it unable to reach E+
resonance effects on nucleophilicity
resonance makes e- less available to donate, therefore if there is resonance, worse Nu
steric hinderance
1 problem in Sn2
steric effects
Nu must be small enough to approach E+
bulky Nu are better bases, easily abstract unhindered H+
what does a leaving group do
polarizes C-X bond, making an E+
leaves with pair of e-
better L.Gs should…
be EN (to polarize bond)
be stable, less reactive conj bases (weak base)
alkyl halide L.G. trend
tosylate > I > Br > Cl > F > CN
bad L.G.s
OH- > H2N- > RO-
effect of polarization
e- are squishy, stabilizes t.s.
L.G. effect on t.s
better L.G. stabilizes t.s. by lowering Ea
substrate most important factor
steric hinderance
CH3 > 1st > 2nd > 3rd (never happens)
branching of beta carbon…
slows rxn, makes high t.s, increased Ea
why do Sn2 need polar solvents?
to dissolve Nu-salt
polar protic solvent
acidic H+
rate is lowered due to solvation
polar aprotic solvent
doesn’t solvate Nu
rate faster, nucleophilicity better
may not fully dissolve Nu-salt
