Topic 9: Substitution and Elimination

Sn2 reaction occurs with

strong nucleophile

Sn1 reaction occurs with

weak nucelophile

E2 reaction occurs with

strong base

E1 reaction occurs with

weak base

NaH

strong base, poor Nu- (E2)

KOtBu

strong base, poor Nu- (E2)

Cl-

weak base, good Nu- (Sn2)

Br-

weak base, good Nu- (Sn2)

I-

weak base, good Nu- (Sn2)

HS-

weak base, good Nu- (Sn2)

(R)S-

weak base, good Nu- (Sn2)

N3-

weak base, good Nu- (Sn2)

CH3CO2-

weak base, good Nu- (Sn2)

acetylide

weak base, good Nu- (Sn2)

HO-

strong base, good Nu- (Sn2 + E2)

MeO-

strong base, good Nu- (Sn2 + E2)

EtO-

strong base, good Nu- (Sn2 + E2)

H2O

weak base, weak Nu- (Sn1 and E1)

MeOH

weak base, weak Nu- (Sn1 and E1)

EtOH

weak base, weak Nu- (Sn1 and E1)

SH2

weak base, weak Nu- (Sn1 only)

HS(R)

weak base, weak Nu- (Sn1 only)

strong base, weak Nu- AND 1 degree

E2

strong base, weak Nu- AND 2 degree

E2

strong base, weak Nu- AND 3 degree

E2

strong base, strong Nu- AND 1 degree

E2 or Sn2

strong base, strong Nu- AND 2 degree

E2 or Sn2

strong base, strong Nu- AND 3 degree

E2

weak base, strong Nu- AND 1 degree

Sn2

weak base, strong Nu- AND 2 degree

Sn2

weak base, strong Nu- AND 3 degree

Sn1

weak base, weak Nu- AND 1 degree

none

weak base, weak Nu- AND 2 degree

none

weak base, weak Nu- AND 3 degree

Sn1 or E1

what reaction does heat favor?

elimination

Sn2 vs E2: bulky base/Nu-

E2

Sn2 vs E2: small base/Nu-

Sn2

Sn2 vs E2: 1 degree haloalkane

Sn2

Sn2 vs E2: 3 degree haloalkane

E2 ONLY

Sn2 vs E2: branching near leaving group

E2

Sn2 vs E2: 2 degree haloalkane/weak base

slows down Sn2, but cannot prevent it completely

do polar aprotic solvents make strong base/nucleophile more or less nucleophilic

more

steps in Sn1

2

steps in Sn2

1

Sn1 is ___ order

first

Sn2 is ___ order

second

Sn2 can only occur with what halides

methyl and primary are favored, sometimes can occur in secondary

Sn1 can only occur with what halides

can occur with secondary, prefers tertiary

protic

has ability to give up a proton

aprotic

no H+

protic solvents can

hydrogen bond, solvate carbocation intermediate (good for Sn1), solvate anion nucleophiles (bad for Sn2)

polar aprotic solvents can

not hydrogen bond, solvate carbocation intermediate, not solvate anion nucleophiles (good for Sn2)

in ___ solvents, stronger bases are stronger Nu-

aprotic

in ___ solvents, weaker bases are stronger Nu-

protic

stable leaving groups for Sn1 or Sn2

H2O, Cl-, Br-, TsO-

The stereochemistry at a chiral electrophilic carbon is ___in a SN2 reaction.

inverted (this is called the walden inversion)

The stereochemistry at a chiral electrophilic carbon is ___in a SN1 reaction.

scrambled

can carbocation intermediates rearrange in Sn1

yes

can carbocation intermediates rearrange in Sn2

no because there is no intermediate

steps in E1

2

steps in E2

1

E1 is ___ order

first

E2 is ___ order

second

E1 is favored by ____ solvents

protic

E2 is favored by solvents that

increase base strength

Both E1 and E2 favor ____ substituted halides

3 degree

Zaitsev's product is ___

more substituated

How to make Hoffman product?

use a bulky base through E2 reaction

E1 reaction product is always ___

Zaitsev, most stable, trans

for E2 reaction to occur, the leaving group and the hydrogen atom must be

anti-perplanar to each other (sometimes the cis product can be formed)