Chapter 6
Alkyl Halides,Nucleophilic
Substitution and Elimination
Organic Chemistry,5th Edition
L,G,Wade,Jr.
Jo Blackburn
Richland College,Dallas,TX
Dallas County Community College District
2003,Prentice Hall
Chapter 6 2
Classes of Halides
Alkyl,Halogen,X,is directly bonded to sp3
carbon.
Vinyl,X is bonded to sp2 carbon of alkene.
Aryl,X is bonded to sp2 carbon on benzene
ring,Examples:
C
H
H
H
C
H
H
B r
a l k y l h a l i d e
C C
H
H
H
C l
v i n y l h a l i d e
I
a r y l h a l i d e
=>
Chapter 6 3
Polarity and Reactivity
Halogens are more electronegative than C.
Carbon-halogen bond is polar,so carbon has
partial positive charge.
Carbon can be attacked by a nucleophile.
Halogen can leave with the electron pair,
=>
C
H
H
H
B r
+? -
Chapter 6 4
Classes of Alkyl Halides
Methyl halides,only one C,CH3X
Primary,C to which X is bonded has
only one C-C bond.
Secondary,C to which X is bonded has
two C-C bonds.
Tertiary,C to which X is bonded has
three C-C bonds.
=>
Chapter 6 5
Classify These:
C H 3 C H C H 3
C l
C H 3 C H 2 F
( C H 3 ) 3 C B r CH 3I
=>
Chapter 6 6
Dihalides
Geminal dihalide,two halogen atoms
are bonded to the same carbon
Vicinal dihalide,two halogen atoms are
bonded to adjacent carbons.
C
H
H
H
C
H
B r
B r
g e m i n a l d i h a l i d e
C
H
H
B r
C
H
H
B r
v i c i n a l d i h a l i d e
=>
Chapter 6 7
IUPAC Nomenclature
Name as haloalkane,
Choose the longest carbon chain,even if the
halogen is not bonded to any of those C’s.
Use lowest possible numbers for position.
C H 3 C H C H 2 C H 3
C l C H
3 ( C H 2 ) 2 C H ( C H 2 ) 2 C H 3
C H 2 C H 2 B r
2 -c h l o r o b u ta n e 4 -( 2 - b r om oe t h y l ) h e p ta n e=>
Chapter 6 8
Systematic Common
Names
Name as alkyl halide.
Useful only for small alkyl groups.
Name these:
C H 3 C H C H 2 C H 3
C l
( C H 3 ) 3 C B r
C H 3 C H
C H 3
C H 2 F
=>
Chapter 6 9
“Trivial” Names
CH2X2 called methylene halide.
CHX3 is a haloform.
CX4 is carbon tetrahalide.
Examples,
CH2Cl2 is methylene chloride
CHCl3 is chloroform
CCl4 is carbon tetrachloride,
=>
Chapter 6 10
Uses of Alkyl Halides
Solvents - degreasers and dry cleaning fluid
Reagents for synthesis of other compounds
Anesthetic,Halothane is CF3CHClBr
CHCl3 used originally (toxic and carcinogenic)
Freons,chlorofluorocarbons or CFC’s
Freon 12,CF2Cl2,now replaced with Freon 22,
CF2CHCl,not as harmful to ozone layer.
Pesticides - DDT banned in U.S,
=>
Chapter 6 11
Dipole Moments
m = 4.8 x? x d,where? is the charge
(proportional to DEN) and d is the distance
(bond length) in Angstroms.
Electronegativities,F > Cl > Br > I
Bond lengths,C-F < C-Cl < C-Br < C-I
Bond dipoles,C-Cl > C-F > C-Br > C-I
1.56 D 1.51 D 1.48 D 1.29 D
Molecular dipoles depend on shape,too!
=>
Chapter 6 12
Boiling Points
Greater intermolecular forces,higher b.p.
dipole-dipole attractions not significantly different
for different halides
London forces greater for larger atoms
Greater mass,higher b.p.
Spherical shape decreases b.p.
(CH3)3CBr CH3(CH2)3Br
73?C 102?C =>
Chapter 6 13
Densities
Alkyl fluorides and chlorides less dense
than water.
Alkyl dichlorides,bromides,and iodides
more dense than water.
=>
Chapter 6 14
Preparation of RX
Free radical halogenation (Chapter 4)
produces mixtures,not good lab synthesis
unless,all H’s are equivalent,or
halogenation is highly selective.
Free radical allylic halogenation
produces alkyl halide with double bond on
the neighboring carbon,
=>
Chapter 6 15
Halogenation of Alkanes
All H’s equivalent,Restrict amount of
halogen to prevent di- or trihalide formation
Highly selective,bromination of 3? C =>
+ H B r
H
B r
h?B r
2+
H
H
9 0 %
+ H B rC H 3 C
C H 3
C H 3
B r
h?
B r 2+C H 3 C
C H 3
C H 3
H
Chapter 6 16
Allylic Halogenation
Allylic radical is resonance stabilized.
Bromination occurs with good yield at the
allylic position (sp3 C next to C=C).
Avoid a large excess of Br2 by using
N-bromosuccinimide (NBS) to generate
Br2 as product HBr is formed.
N
O
O
B r + H B r N
O
O
H + B r2=>
Chapter 6 17
Reaction Mechanism
Free radical chain reaction
initiation,propagation,termination.
H H
B r
H
+ H B r
B r
B r
H B r
+ B r
=>
2 B rB r 2 h?
Chapter 6 18
Substitution Reactions
The halogen atom on the alkyl halide is
replaced with another group.
Since the halogen is more electronegative
than carbon,the C-X bond breaks
heterolytically and X- leaves.
The group replacing X- is a nucleophile,=>
C C
H X
+ N u c,- C C
H N u c
+ X,-
Chapter 6 19
Elimination Reactions
The alkyl halide loses halogen as a halide
ion,and also loses H+ on the adjacent
carbon to a base.
A pi bond is formed,Product is alkene.
Also called dehydrohalogenation (-HX),
=>
C C
H X
+ B,- + X,- + H B C C
Chapter 6 20
SN2 Mechanism
Bimolecular nucleophilic substitution.
Concerted reaction,new bond forming
and old bond breaking at same time.
Rate is first order in each reactant.
Walden inversion,
=>
C
H
B r
H
H
H O CH O B r
H
HH
CH O
H
H
H
+ B r -
Chapter 6 21
SN2 Energy Diagram
One-step reaction.
Transition state is highest in energy,=>
Chapter 6 22
Uses for SN2 Reactions
Synthesis of other classes of compounds.
Halogen exchange reaction.
Nu cl eo p h il e P r o d u ct Clas s o f P r o d u ct
R- X + I
-
R- I akyl h ali de
R- X + OH
-
R- OH al coh o l
R- X +
-
OR'
R- OR' ether
R- X +
-
SH
R- S H th i o l
R- X +
-
S R'
R- S R' th i o et he r
R- X + NH 3?
R- NH 3
+
X
-
am i ne sa l t
R- X + N 3
-
R- N 3 azi de
R- X +
-
C? C- R'
R- C? C- R' al kyne
R- X +
-
C? N
R- C? N ni t r i l e
R- X + R- COO
-
R- COO- R' est er
=>
Chapter 6 23
SN2,Nucleophilic Strength
Stronger nucleophiles react faster.
Strong bases are strong nucleophiles,but
not all strong nucleophiles are basic.
=>
Chapter 6 24
Trends in Nuc,Strength
Of a conjugate acid-base pair,the base is
stronger,OH- > H2O,NH2- > NH3
Decreases left to right on Periodic Table,
More electronegative atoms less likely to
form new bond,OH- > F-,NH3 > H2O
Increases down Periodic Table,as size
and polarizability increase,I- > Br- > Cl-
=>
Chapter 6 25
Polarizability Effect
=>
Chapter 6 26
Bulky Nucleophiles
Sterically hindered for attack on carbon,
so weaker nucleophiles.
C H 3 C H 2 O
e t h o x i de ( u n h i n d e r e d )
w e a k e r b a s e,bu t s t r o n g e r n u c l e o ph i l e
C
C H 3
H 3 C
C H 3
O
t - b u t o x i d e ( h i n de r e d)
s t r o n g e r b a s e,bu t w e a k e r n u c l e o p h i l e
=>
Chapter 6 27
Solvent Effects (1)
Polar protic solvents (O-H or N-H) reduce
the strength of the nucleophile,
Hydrogen bonds must be broken before
nucleophile can attack the carbon.
=>
Chapter 6 28
Solvent Effects (2)
Polar aprotic solvents (no O-H or N-H) do not
form hydrogen bonds with nucleophile
Examples,
C H 3 C N
a c e t o n i t r i l e C
O
H 3 C C H 3
a c e t o n e
=>
d i m e t h y l f o r m a m i d e
( D M F )
C
H
O
N
C H 3
C H 3
Chapter 6 29
Crown Ethers
Solvate the cation,
so nucleophilic
strength of the anion
increases.
Fluoride becomes a
good nucleophile.
O
O
O
O
O
O
K +
1 8 - c r o w n - 6
C H 2 C l
K F,( 1 8 - c r o w n - 6 )
CH 3 CN
C H 2 F
=>
Chapter 6 30
SN2,Reactivity of
Substrate
Carbon must be partially positive.
Must have a good leaving group
Carbon must not be sterically hindered.
=>
Chapter 6 31
Leaving Group Ability
Electron-withdrawing
Stable once it has left (not a strong base)
Polarizable to stabilize the transition state.
=>
Chapter 6 32
Structure of Substrate
Relative rates for SN2,
CH3X > 1° > 2° >>
3°
Tertiary halides do not react via the
SN2 mechanism,due to steric
hindrance,=>
Chapter 6 33
Stereochemistry of SN2
Walden inversion
=>
Chapter 6 34
SN1 Reaction
Unimolecular nucleophilic substitution.
Two step reaction with carbocation
intermediate.
Rate is first order in the alkyl halide,
zero order in the nucleophile.
Racemization occurs.
=>
Chapter 6 35
SN1 Mechanism (1)
Formation of carbocation (slow)
( C H 3 ) 3 C B r ( C H 3 ) 3 C + + B r -
=>
Chapter 6 36
SN1 Mechanism (2)
Nucleophilic attack
( C H 3 ) 3 C + + H O H ( C H
3 ) 3 C O H
H
( C H 3 ) 3 C O H
H
H O H+ ( C H 3 ) 3 C O H + H 3 O +
=>
Loss of H+ (if needed)
Chapter 6 37
SN1 Energy Diagram
Forming the
carbocation is
endothermic
Carbocation
intermediate is in
an energy well.
=>
Chapter 6 38
Rates of SN1 Reactions
3° > 2° > 1° >> CH3X
Order follows stability of carbocations (opposite to
SN2)
More stable ion requires less energy to form
Better leaving group,faster reaction (like SN2)
Polar protic solvent best,It solvates ions
strongly with hydrogen bonding,
=>
Chapter 6 39
Stereochemistry of SN1
Racemization,
inversion and retention
=>
Chapter 6 40
Rearrangements
Carbocations can rearrange to form a
more stable carbocation.
Hydride shift,H- on adjacent carbon
bonds with C+.
Methyl shift,CH3- moves from adjacent
carbon if no H’s are available,
=>
Chapter 6 41
Hydride Shift
C H 3 C
B r
H
C
H
C H 3
C H 3 C H 3 C
H
C
H
C H 3
C H 3
C H 3 C
H
C
H
C H 3
C H 3 C H 3 C
H
C
C H 3
C H 3
H
C H 3 C
H
C
C H 3
C H 3
H
N u c
C H 3 C
H
C
C H 3
C H 3
H N u c
=>
Chapter 6 42
Methyl Shift
C H 3 C
B r
H
C
C H 3
C H 3
C H 3 C H 3 C
H
C
C H 3
C H 3
C H 3
C H 3 C
H
C
C H 3
C H 3
C H 3 C H 3 C
H
C
C H 3
C H 3
C H 3
C H 3 C
H
C
C H 3
C H 3
C H 3
N u c
C H 3 C
H
C
C H 3
C H 3
C H 3 N u c
=>
Chapter 6 43
SN2 or SN1?
Primary or methyl
Strong nucleophile
Polar aprotic solvent
Rate = k[halide][Nuc]
Inversion at chiral
carbon
No rearrangements
Tertiary
Weak nucleophile (may
also be solvent)
Polar protic solvent,silver
salts
Rate = k[halide]
Racemization of optically
active compound
Rearranged products
=>
Chapter 6 44
E1 Reaction
Unimolecular elimination
Two groups lost (usually X- and H+)
Nucleophile acts as base
Also have SN1 products (mixture)
=>
Chapter 6 45
E1 Mechanism
Halide ion leaves,forming carbocation.
Base removes H+ from adjacent carbon.
Pi bond forms,=>
H C
H
H
C
C H 3
C H 3
B r
C
H
H
H
C C H 3
C H 3
O
H
H
C
H
H
H
C C H 3
C H 3
C C
H
C H 3
C H 3
H
+ H 3 O
+
Chapter 6 46
A Closer Look
O
H
H
C
H
H
H
C C H 3
C H 3
C C
H
C H 3
C H 3
H
+ H 3 O
+
=>
Chapter 6 47
E1 Energy Diagram
Note,first step is same as SN1
=>
Chapter 6 48
E2 Reaction
Bimolecular elimination
Requires a strong base
Halide leaving and proton abstraction
happens simultaneously - no
intermediate,
=>
Chapter 6 49
E2 Mechanism
H C
H
H
C
C H 3
C H 3
B r
C C
H
C H 3
C H 3
H
O
H
+ H 2 O B r
-
+
=>
Chapter 6 50
Saytzeff’s Rule
If more than one elimination product is possible,
the most-substituted alkene is the major product
(most stable).
R2C=CR2>R2C=CHR>RHC=CHR>H2C=CHR
tetra > tri > di > mono
C C
B r
H
C
H
C H 3
H
H
H
C H 3
O H
-
C C
H
H
C
H H
C H 3
C H 3
C
H
H
H
C
H
C
C H 3
C H 3
+
=>
minor major
Chapter 6 51
E2 Stereochemistry
=>
Chapter 6 52
E1 or E2?
Tertiary > Secondary
Weak base
Good ionizing solvent
Rate = k[halide]
Saytzeff product
No required geometry
Rearranged products
Tertiary > Secondary
Strong base required
Solvent polarity not
important
Rate = k[halide][base]
Saytzeff product
Coplanar leaving
groups (usually anti)
No rearrangements
=>
Chapter 6 53
Substitution or
Elimination?
Strength of the nucleophile determines
order,Strong nuc,will go SN2 or E2.
Primary halide usually SN2.
Tertiary halide mixture of SN1,E1 or E2
High temperature favors elimination.
Bulky bases favor elimination.
Good nucleophiles,but weak bases,
favor substitution,=>
Chapter 6 54
Secondary Halides?
Mixtures of products are common.
=>
Chapter 6 55
End of Chapter 6
Alkyl Halides,Nucleophilic
Substitution and Elimination
Organic Chemistry,5th Edition
L,G,Wade,Jr.
Jo Blackburn
Richland College,Dallas,TX
Dallas County Community College District
2003,Prentice Hall
Chapter 6 2
Classes of Halides
Alkyl,Halogen,X,is directly bonded to sp3
carbon.
Vinyl,X is bonded to sp2 carbon of alkene.
Aryl,X is bonded to sp2 carbon on benzene
ring,Examples:
C
H
H
H
C
H
H
B r
a l k y l h a l i d e
C C
H
H
H
C l
v i n y l h a l i d e
I
a r y l h a l i d e
=>
Chapter 6 3
Polarity and Reactivity
Halogens are more electronegative than C.
Carbon-halogen bond is polar,so carbon has
partial positive charge.
Carbon can be attacked by a nucleophile.
Halogen can leave with the electron pair,
=>
C
H
H
H
B r
+? -
Chapter 6 4
Classes of Alkyl Halides
Methyl halides,only one C,CH3X
Primary,C to which X is bonded has
only one C-C bond.
Secondary,C to which X is bonded has
two C-C bonds.
Tertiary,C to which X is bonded has
three C-C bonds.
=>
Chapter 6 5
Classify These:
C H 3 C H C H 3
C l
C H 3 C H 2 F
( C H 3 ) 3 C B r CH 3I
=>
Chapter 6 6
Dihalides
Geminal dihalide,two halogen atoms
are bonded to the same carbon
Vicinal dihalide,two halogen atoms are
bonded to adjacent carbons.
C
H
H
H
C
H
B r
B r
g e m i n a l d i h a l i d e
C
H
H
B r
C
H
H
B r
v i c i n a l d i h a l i d e
=>
Chapter 6 7
IUPAC Nomenclature
Name as haloalkane,
Choose the longest carbon chain,even if the
halogen is not bonded to any of those C’s.
Use lowest possible numbers for position.
C H 3 C H C H 2 C H 3
C l C H
3 ( C H 2 ) 2 C H ( C H 2 ) 2 C H 3
C H 2 C H 2 B r
2 -c h l o r o b u ta n e 4 -( 2 - b r om oe t h y l ) h e p ta n e=>
Chapter 6 8
Systematic Common
Names
Name as alkyl halide.
Useful only for small alkyl groups.
Name these:
C H 3 C H C H 2 C H 3
C l
( C H 3 ) 3 C B r
C H 3 C H
C H 3
C H 2 F
=>
Chapter 6 9
“Trivial” Names
CH2X2 called methylene halide.
CHX3 is a haloform.
CX4 is carbon tetrahalide.
Examples,
CH2Cl2 is methylene chloride
CHCl3 is chloroform
CCl4 is carbon tetrachloride,
=>
Chapter 6 10
Uses of Alkyl Halides
Solvents - degreasers and dry cleaning fluid
Reagents for synthesis of other compounds
Anesthetic,Halothane is CF3CHClBr
CHCl3 used originally (toxic and carcinogenic)
Freons,chlorofluorocarbons or CFC’s
Freon 12,CF2Cl2,now replaced with Freon 22,
CF2CHCl,not as harmful to ozone layer.
Pesticides - DDT banned in U.S,
=>
Chapter 6 11
Dipole Moments
m = 4.8 x? x d,where? is the charge
(proportional to DEN) and d is the distance
(bond length) in Angstroms.
Electronegativities,F > Cl > Br > I
Bond lengths,C-F < C-Cl < C-Br < C-I
Bond dipoles,C-Cl > C-F > C-Br > C-I
1.56 D 1.51 D 1.48 D 1.29 D
Molecular dipoles depend on shape,too!
=>
Chapter 6 12
Boiling Points
Greater intermolecular forces,higher b.p.
dipole-dipole attractions not significantly different
for different halides
London forces greater for larger atoms
Greater mass,higher b.p.
Spherical shape decreases b.p.
(CH3)3CBr CH3(CH2)3Br
73?C 102?C =>
Chapter 6 13
Densities
Alkyl fluorides and chlorides less dense
than water.
Alkyl dichlorides,bromides,and iodides
more dense than water.
=>
Chapter 6 14
Preparation of RX
Free radical halogenation (Chapter 4)
produces mixtures,not good lab synthesis
unless,all H’s are equivalent,or
halogenation is highly selective.
Free radical allylic halogenation
produces alkyl halide with double bond on
the neighboring carbon,
=>
Chapter 6 15
Halogenation of Alkanes
All H’s equivalent,Restrict amount of
halogen to prevent di- or trihalide formation
Highly selective,bromination of 3? C =>
+ H B r
H
B r
h?B r
2+
H
H
9 0 %
+ H B rC H 3 C
C H 3
C H 3
B r
h?
B r 2+C H 3 C
C H 3
C H 3
H
Chapter 6 16
Allylic Halogenation
Allylic radical is resonance stabilized.
Bromination occurs with good yield at the
allylic position (sp3 C next to C=C).
Avoid a large excess of Br2 by using
N-bromosuccinimide (NBS) to generate
Br2 as product HBr is formed.
N
O
O
B r + H B r N
O
O
H + B r2=>
Chapter 6 17
Reaction Mechanism
Free radical chain reaction
initiation,propagation,termination.
H H
B r
H
+ H B r
B r
B r
H B r
+ B r
=>
2 B rB r 2 h?
Chapter 6 18
Substitution Reactions
The halogen atom on the alkyl halide is
replaced with another group.
Since the halogen is more electronegative
than carbon,the C-X bond breaks
heterolytically and X- leaves.
The group replacing X- is a nucleophile,=>
C C
H X
+ N u c,- C C
H N u c
+ X,-
Chapter 6 19
Elimination Reactions
The alkyl halide loses halogen as a halide
ion,and also loses H+ on the adjacent
carbon to a base.
A pi bond is formed,Product is alkene.
Also called dehydrohalogenation (-HX),
=>
C C
H X
+ B,- + X,- + H B C C
Chapter 6 20
SN2 Mechanism
Bimolecular nucleophilic substitution.
Concerted reaction,new bond forming
and old bond breaking at same time.
Rate is first order in each reactant.
Walden inversion,
=>
C
H
B r
H
H
H O CH O B r
H
HH
CH O
H
H
H
+ B r -
Chapter 6 21
SN2 Energy Diagram
One-step reaction.
Transition state is highest in energy,=>
Chapter 6 22
Uses for SN2 Reactions
Synthesis of other classes of compounds.
Halogen exchange reaction.
Nu cl eo p h il e P r o d u ct Clas s o f P r o d u ct
R- X + I
-
R- I akyl h ali de
R- X + OH
-
R- OH al coh o l
R- X +
-
OR'
R- OR' ether
R- X +
-
SH
R- S H th i o l
R- X +
-
S R'
R- S R' th i o et he r
R- X + NH 3?
R- NH 3
+
X
-
am i ne sa l t
R- X + N 3
-
R- N 3 azi de
R- X +
-
C? C- R'
R- C? C- R' al kyne
R- X +
-
C? N
R- C? N ni t r i l e
R- X + R- COO
-
R- COO- R' est er
=>
Chapter 6 23
SN2,Nucleophilic Strength
Stronger nucleophiles react faster.
Strong bases are strong nucleophiles,but
not all strong nucleophiles are basic.
=>
Chapter 6 24
Trends in Nuc,Strength
Of a conjugate acid-base pair,the base is
stronger,OH- > H2O,NH2- > NH3
Decreases left to right on Periodic Table,
More electronegative atoms less likely to
form new bond,OH- > F-,NH3 > H2O
Increases down Periodic Table,as size
and polarizability increase,I- > Br- > Cl-
=>
Chapter 6 25
Polarizability Effect
=>
Chapter 6 26
Bulky Nucleophiles
Sterically hindered for attack on carbon,
so weaker nucleophiles.
C H 3 C H 2 O
e t h o x i de ( u n h i n d e r e d )
w e a k e r b a s e,bu t s t r o n g e r n u c l e o ph i l e
C
C H 3
H 3 C
C H 3
O
t - b u t o x i d e ( h i n de r e d)
s t r o n g e r b a s e,bu t w e a k e r n u c l e o p h i l e
=>
Chapter 6 27
Solvent Effects (1)
Polar protic solvents (O-H or N-H) reduce
the strength of the nucleophile,
Hydrogen bonds must be broken before
nucleophile can attack the carbon.
=>
Chapter 6 28
Solvent Effects (2)
Polar aprotic solvents (no O-H or N-H) do not
form hydrogen bonds with nucleophile
Examples,
C H 3 C N
a c e t o n i t r i l e C
O
H 3 C C H 3
a c e t o n e
=>
d i m e t h y l f o r m a m i d e
( D M F )
C
H
O
N
C H 3
C H 3
Chapter 6 29
Crown Ethers
Solvate the cation,
so nucleophilic
strength of the anion
increases.
Fluoride becomes a
good nucleophile.
O
O
O
O
O
O
K +
1 8 - c r o w n - 6
C H 2 C l
K F,( 1 8 - c r o w n - 6 )
CH 3 CN
C H 2 F
=>
Chapter 6 30
SN2,Reactivity of
Substrate
Carbon must be partially positive.
Must have a good leaving group
Carbon must not be sterically hindered.
=>
Chapter 6 31
Leaving Group Ability
Electron-withdrawing
Stable once it has left (not a strong base)
Polarizable to stabilize the transition state.
=>
Chapter 6 32
Structure of Substrate
Relative rates for SN2,
CH3X > 1° > 2° >>
3°
Tertiary halides do not react via the
SN2 mechanism,due to steric
hindrance,=>
Chapter 6 33
Stereochemistry of SN2
Walden inversion
=>
Chapter 6 34
SN1 Reaction
Unimolecular nucleophilic substitution.
Two step reaction with carbocation
intermediate.
Rate is first order in the alkyl halide,
zero order in the nucleophile.
Racemization occurs.
=>
Chapter 6 35
SN1 Mechanism (1)
Formation of carbocation (slow)
( C H 3 ) 3 C B r ( C H 3 ) 3 C + + B r -
=>
Chapter 6 36
SN1 Mechanism (2)
Nucleophilic attack
( C H 3 ) 3 C + + H O H ( C H
3 ) 3 C O H
H
( C H 3 ) 3 C O H
H
H O H+ ( C H 3 ) 3 C O H + H 3 O +
=>
Loss of H+ (if needed)
Chapter 6 37
SN1 Energy Diagram
Forming the
carbocation is
endothermic
Carbocation
intermediate is in
an energy well.
=>
Chapter 6 38
Rates of SN1 Reactions
3° > 2° > 1° >> CH3X
Order follows stability of carbocations (opposite to
SN2)
More stable ion requires less energy to form
Better leaving group,faster reaction (like SN2)
Polar protic solvent best,It solvates ions
strongly with hydrogen bonding,
=>
Chapter 6 39
Stereochemistry of SN1
Racemization,
inversion and retention
=>
Chapter 6 40
Rearrangements
Carbocations can rearrange to form a
more stable carbocation.
Hydride shift,H- on adjacent carbon
bonds with C+.
Methyl shift,CH3- moves from adjacent
carbon if no H’s are available,
=>
Chapter 6 41
Hydride Shift
C H 3 C
B r
H
C
H
C H 3
C H 3 C H 3 C
H
C
H
C H 3
C H 3
C H 3 C
H
C
H
C H 3
C H 3 C H 3 C
H
C
C H 3
C H 3
H
C H 3 C
H
C
C H 3
C H 3
H
N u c
C H 3 C
H
C
C H 3
C H 3
H N u c
=>
Chapter 6 42
Methyl Shift
C H 3 C
B r
H
C
C H 3
C H 3
C H 3 C H 3 C
H
C
C H 3
C H 3
C H 3
C H 3 C
H
C
C H 3
C H 3
C H 3 C H 3 C
H
C
C H 3
C H 3
C H 3
C H 3 C
H
C
C H 3
C H 3
C H 3
N u c
C H 3 C
H
C
C H 3
C H 3
C H 3 N u c
=>
Chapter 6 43
SN2 or SN1?
Primary or methyl
Strong nucleophile
Polar aprotic solvent
Rate = k[halide][Nuc]
Inversion at chiral
carbon
No rearrangements
Tertiary
Weak nucleophile (may
also be solvent)
Polar protic solvent,silver
salts
Rate = k[halide]
Racemization of optically
active compound
Rearranged products
=>
Chapter 6 44
E1 Reaction
Unimolecular elimination
Two groups lost (usually X- and H+)
Nucleophile acts as base
Also have SN1 products (mixture)
=>
Chapter 6 45
E1 Mechanism
Halide ion leaves,forming carbocation.
Base removes H+ from adjacent carbon.
Pi bond forms,=>
H C
H
H
C
C H 3
C H 3
B r
C
H
H
H
C C H 3
C H 3
O
H
H
C
H
H
H
C C H 3
C H 3
C C
H
C H 3
C H 3
H
+ H 3 O
+
Chapter 6 46
A Closer Look
O
H
H
C
H
H
H
C C H 3
C H 3
C C
H
C H 3
C H 3
H
+ H 3 O
+
=>
Chapter 6 47
E1 Energy Diagram
Note,first step is same as SN1
=>
Chapter 6 48
E2 Reaction
Bimolecular elimination
Requires a strong base
Halide leaving and proton abstraction
happens simultaneously - no
intermediate,
=>
Chapter 6 49
E2 Mechanism
H C
H
H
C
C H 3
C H 3
B r
C C
H
C H 3
C H 3
H
O
H
+ H 2 O B r
-
+
=>
Chapter 6 50
Saytzeff’s Rule
If more than one elimination product is possible,
the most-substituted alkene is the major product
(most stable).
R2C=CR2>R2C=CHR>RHC=CHR>H2C=CHR
tetra > tri > di > mono
C C
B r
H
C
H
C H 3
H
H
H
C H 3
O H
-
C C
H
H
C
H H
C H 3
C H 3
C
H
H
H
C
H
C
C H 3
C H 3
+
=>
minor major
Chapter 6 51
E2 Stereochemistry
=>
Chapter 6 52
E1 or E2?
Tertiary > Secondary
Weak base
Good ionizing solvent
Rate = k[halide]
Saytzeff product
No required geometry
Rearranged products
Tertiary > Secondary
Strong base required
Solvent polarity not
important
Rate = k[halide][base]
Saytzeff product
Coplanar leaving
groups (usually anti)
No rearrangements
=>
Chapter 6 53
Substitution or
Elimination?
Strength of the nucleophile determines
order,Strong nuc,will go SN2 or E2.
Primary halide usually SN2.
Tertiary halide mixture of SN1,E1 or E2
High temperature favors elimination.
Bulky bases favor elimination.
Good nucleophiles,but weak bases,
favor substitution,=>
Chapter 6 54
Secondary Halides?
Mixtures of products are common.
=>
Chapter 6 55
End of Chapter 6
