A QUICK REVIEW OF

1
A QUICK REVIEW OF TWO SUBSTITUTION
REACTIONS
SN1 / SN2
2
RECALL FROM LAST LECTURE
Two similar reactions, different kinetics.
1)
First Case
55oC
primary
3
RECALL FROM LAST LECTURE
2)
Second Case
55oC
tertiary
4
QUESTION
SN1
SN2
rate k2 RBr NaOH
Is this difference significant, or is it just a
mathematical illusion of some kind?
5
THE WORK OF HUGHES AND INGOLD
6
HUGHES AND INGOLD ca. 1940
It was found experimentally that
Most primary compounds undergo SN2
reactions with second order kinetics.
Most tertiary compounds undergo SN1
reactions with first order kinetics.
Most nucleophilic substitution reactions fit
one or the other of these two patterns.
Is this significant, or is it just a
mathematical illusion of some kind?
7
2-BROMOOCTANE
A STUDY IN STEREOCHEMISTRY
Lets use an optically-active compound.
8
2-BROMOOCTANE

abbreviation

R
CH
CH3
Br
secondary
9
DIFFERENT KINETICS AT DIFFERENT BASE
CONCENTRATIONS
k1RBr
SN1
low conc OH-
lt 0.1 M
high conc OH- gt 1 M
SN2
1) H2O 2) -H
The two mechanisms compete.
k2RBrOH
10
SN1
SN2
Total Rate k1 RBr k2 RBr OH-
TOTAL
SN2
dominates at low OH-
k2 RBr OH-
dominates at high OH-
Rate
SN1
k1 RBr
OH-
RBr constant
11
STEREOCHEMICAL RESULTS

2-Bromooctane is chiral .....
What if we start with an enantiomerically
pure compound (one enantiomer only)?
Is the stereohemical result the same in each
case
SN1 and SN2 ?
12
STEREOCHEMISTRY CHANGES
R-(-)-2-bromooctane
aD -36.0o
complete inversion
SN2 conditions
high OH-
S
R
S-()-2-octanol
SN1 conditions
aD 10.3o
low OH-
R
S

complete racemization
enantiomers
aD 0o
racemic mixture
13
THE MECHANISMS
Can we explain the stereochemical results with
our proposed mechanisms for SN1 and SN2
reactions?
Lets Try .......
14
SN1
15
SN1 MECHANISM
50
sp2

-
planar carbocation
attacks top and bottom equally
(R)
50
RACEMIZATION
(S)
enantiomers
(R)
racemic mixture
16
SN2
17
SN2 MECHANISM
nucleophilic attack
(R)-config
O
R
attacks back lobe
..

C
H O
..
CH3
INVERSION
(S)-config
H
18
CONCEPTUAL ANALOGY 1
LIKE POOL OR BILLIARDS
1) two balls at rest and touching
X
2) forceful shot directly on axis
Similar in concept to an attack from the back
forcing the nucleophile to leave.
Nu
X
3) straight-on collision
Nu
X
4) momentum transfer
Nu
X
19
CONCEPTUAL ANALOGY 2
INVERSION OF AN UMBRELLA IN THE WIND
Inversion of the umbrella is similar in
concept to the inversion of an SN2 atom.
20
R
THE INVERSION PROCESS
2p
HO C B
sp2
partial bonding
Br
HO
C
activated complex is trigonal planar (sp2 )
CH3
H
configuration is inverted
sp3
sp3
Ea
(R)-configuration
(S)-configuration
21
ACTIVATED COMPLEX FOR SN2
MIDPOINT OF THE REACTION
trigonal planar (sp2)
R
breaking
forming
C
Br
HO
5 bonds to Carbon
CH3
but the bonds to Br and OH are only half-formed
and are not full bonds.
H
NOT A STABLE SPECIES