Ch 11 Substitution Reactions

Chemistry233
Chapter11Handout–SubstitutionReactions
SN2 Reaction - Substitution Nucleophilic Bimolecular
+
H 3C X
H 3C Nu
Nu
Only a single step in the reaction
This single step must then = slow step
+
X
Rate = k H 3C X
Nu -
Reaction Coordinate:
TS
G‡
H
H
SM
E rel
partial bond
broken
partial bond
formed
Br
+
HO
HO
Br
HO
HH
H
+ Br
H
HH
TS
(Transition State)
SM
H
P
P
Rxn
G‡ = energy required to get to the transition state
SN1 Reaction - Substitution Nucleophilic Unimolecular
X
+
Nu
Nu
+
X
"slow step"
More than one step involved in the reaction with discrete intermediate
Carbocation formation is the rate determining step
Only a single reactant (the alkyl) halide is involved in the rate determining step = unimolecular
Rate = k
Br
Changing the concentration of R-X changes the reaction rate
Chaing the concentration of any other reactants will not affect the rate
X
H 2O
+ Br
TS-1
SM
TS-2
Int 1
TS-1
G‡1
G ‡2
Int 1
H
O
H
Int 2
H 2O
TS-3
OH +
H 3O
P
TS-2
G ‡3
E rel
G‡1 > G‡2 and G‡3
TS-3
Int 2
Requires the most energy therefore = slow step of the reaction
SM
P
Rxn
AlkylHalideReactivity
X
X
H 3C X
methyl
primary
secondary
allylic
SN 2 Only
Br
I
S
HS
RS
H 2S
O
HO
RO
C
N C
R C
H 2O
RSH
ROH GoodLeavingGroups
R Cl
R OTs
(we will talk about this one later on) R OH2
CommonSolvents
PolarProtic
H
H
O
H 3C
H
MeOH
PolarAprotic
O
H N H
H
O
F
Cl
O
SN1 Only
Weak
X
R Br
tertiary
benzilic
SN1 or SN 2
CommonNucleophiles
Strong
R I
X
X
X
H
dimethylsulfoxide
(DMSO)
H
EtOH
O
O
S
OH
AcOH
hexane
CCl4
carbontetrachloride O
H 3C
UsedinSN1Reactions
N
N,N-dimethylformamide
(DMF)
O
acetone
Non-Polar
N
acetonitrile
tetrahydrofuran
(THF)
UsedinSN2Reactions
DifferentiatingBetweenSN1andSN2
Factor
FavoringSN2
Substrate
Methylorprimary
Nucleophile
Strongnucleophile
LeavingGroup
Goodleavinggroup
Solvent
PolarAprotic
NotUsedinSubstitution
Reactions
FavoringSN1
Tertiary
WeakNucleophile
Verygoodleavinggroup
PolarProtic