CHE 240
Unit IV
Stereochemistry, Substitution
and Elimination Reactions
CHAPTER SIX
Terrence P. Sherlock
Burlington County College
2004
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.
=>
H + H C Br
H
Chapter 6
2
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
3
Classify These:
CH3
CH CH3
CH3CH2F
Cl
CH3I
(CH3)3CBr
Chapter 6
=>
4
Dihalides
• Geminal dihalide: two halogen atoms
are bonded to the same carbon
• Vicinal dihalide: two halogen atoms are
bonded to adjacent carbons.
H
H
H
C
C
H H
H C C Br
Br
H Br
geminal dihalide
Br H
vicinal dihalide
Chapter 6
=>
5
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.
CH3
CH CH2CH3
Cl
2-chlorobutane
CH2CH2Br
CH3(CH2)2CH(CH2)2CH3
4-(2-bromoethyl)heptane
=>
Chapter 6
6
Systematic Common
Names
• Name as alkyl halide.
• Useful only for small alkyl groups.
• Name these:
(CH3)3CBr
CH3
CH CH2CH3
Cl
CH3
CH3
Chapter 6
CH CH2F
=>
7
“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
8
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
9
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
10
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
=>
11
Densities
• Alkyl fluorides and chlorides less dense
than water.
• Alkyl dichlorides, bromides, and iodides
more dense than water.
=>
Chapter 6
12
Halogenation of Alkanes
• All H’s equivalent. Restrict amount of
halogen to prevent di- or trihalide formation
H
H
H
+ Br2
h
Br
+
HBr
• Highly selective: bromination of 3 C =>
CH3
CH3
C H
+ Br2
h
CH3
CH3
CH3
C Br +
HBr
CH3
Chapter 6
90%
13
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.
O
N Br
O
O
N H
+ HBr
Chapter 6
O
+
Br2
=>
14
Reaction Mechanism
Free radical chain reaction
initiation, propagation, termination.
h
Br2
2 Br
H
H
H
Br
+
HBr
Br
Br
H
=>
Br
+
Chapter 6
Br
15
Substitution Reactions
C C
+
Nuc:
-
C C
H X
+
X:
-
H Nuc
• 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. =>
Chapter 6
16
Elimination Reactions
C C
+
-
B:
C C
+
X:
-
+ HB
H X
• 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).
=>
Chapter 6
17
SN2 Mechanism
H
H
H O
H
C Br
H
HO C Br
H H
H
HO C
H
-
+
Br
H
• Bimolecular nucleophilic substitution.
• Concerted reaction: new bond forming
and old bond breaking at same time.
• Rate is first order in each reactant.
• Walden inversion.
=>
Chapter 6
18
SN2 Energy Diagram
• One-step reaction.
• Transition state is highest in energy. =>
Chapter 6
19
Uses for SN2 Reactions
• Synthesis of other classes of compounds.
• Halogen exchange reaction.
Nucleophile

Product
R-I
Class of Product
akyl halide
-

R-OH
alcohol
R-X + OR'
-

R-OR'
ether
-

R-SH
thiol
-

R-SR'
thioether

amine salt

R-NH3+X
R- N3
R-X + CC-R'
-

R-CC-R'
alkyne
-

R-CN
R-COO-R'
nitrile
R-X + I
-
R-X + OH
R-X + SH
R-X + SR'
R-X + NH3
-
R-X + N3
R-X + CN
R-X + R-COO
-

-
Chapter 6
azide
=>
ester
20
SN2: Nucleophilic Strength
• Stronger nucleophiles react faster.
• Strong bases are strong nucleophiles, but
not all strong nucleophiles are basic.
=>
Chapter 6
21
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
22
Polarizability Effect
=>
Chapter 6
23
Bulky Nucleophiles
Sterically hindered for attack on carbon,
so weaker nucleophiles.
CH3 CH2 O
ethoxide (unhindered)
weaker base, but stronger nucleophile
CH3
H3C
C
O
CH3
=>
t-butoxide (hindered)
stronger base, but
weaker nucleophile
Chapter 6
24
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
25
Solvent Effects (2)
• Polar aprotic solvents (no O-H or N-H) do not
form hydrogen bonds with nucleophile
• Examples:
O
CH3 C N
acetonitrile
H
C
N
CH3
CH3
dimethylformamide
(DMF)
Chapter 6
O
C
H3C
CH3
acetone
=>
26
Crown Ethers
• Solvate the cation,
so nucleophilic
strength of the anion
increases.
• Fluoride becomes a
good nucleophile.
O
O
O
K+
O
O
O
18-crown-6
CH2Cl
CH2F
KF, (18-crown-6)
CH3CN
Chapter 6
=>
27
SN2: Reactivity of
Substrate
• Carbon must be partially positive.
• Must have a good leaving group
• Carbon must not be sterically hindered.
=>
Chapter 6
28
Leaving Group Ability
• Electron-withdrawing
• Stable once it has left (not a strong base)
• Polarizable to stabilize the transition state.
=>
Chapter 6
29
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
30
Stereochemistry of SN2
Walden inversion
=>
Chapter 6
31
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
32
SN1 Mechanism (1)
Formation of carbocation (slow)
+
(CH3)3C
(CH3)3C Br
-
+ Br
=>
Chapter 6
33
SN1 Mechanism (2)
• Nucleophilic attack
+
(CH3)3C
+ H O H
(CH3)3C O H
H
• Loss of H+ (if needed)
(CH3)3C O H
(CH3)3C O H + H O H
+
+ H3O
=>
H
Chapter 6
34
SN1 Energy Diagram
• Forming the
carbocation is
endothermic
• Carbocation
intermediate is in
an energy well.
=>
Chapter 6
35
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
36
Stereochemistry of SN1
Racemization:
inversion and retention
=>
Chapter 6
37
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
38
Hydride Shift
H
Br H
CH3
CH3
C C CH3
C C CH3
H CH3
H CH3
H
H
CH3
CH3
C C CH3
C C CH3
H CH3
H CH3
H
CH3
C C CH3
H Nuc
Nuc
H CH3
CH3
C C CH3
=>
H CH3
Chapter 6
39
Methyl Shift
CH3
Br CH3
CH3
CH3
C C CH3
H CH3
H CH3
CH3
CH3
CH3
C C CH3
CH3
C C CH3
C C CH3
H CH3
H CH3
CH3
CH3
C C CH3
Nuc
H CH3
CH3
CH3 Nuc
C C CH3
=>
H CH3
Chapter 6
40
SN2
or
SN1?
• Primary or methyl
• Strong nucleophile
• Tertiary
• Weak nucleophile (may
also be solvent)
• Polar aprotic solvent
• Polar protic solvent, silver
salts
• Rate = k[halide][Nuc]
• Inversion at chiral
carbon
• Rate = k[halide]
• Racemization of optically
active compound
• No rearrangements
• Rearranged products
=>
Chapter 6
41
E1 Reaction
•
•
•
•
Unimolecular elimination
Two groups lost (usually X- and H+)
Nucleophile acts as base
Also have SN1 products (mixture)
=>
Chapter 6
42
E1 Mechanism
H
H Br
H C C CH3
H C C CH3
H CH3
H CH3
H
H O
H
CH3
H
+
C C
H C C CH3
H
H CH3
+
H3O
CH3
• Halide ion leaves, forming carbocation.
• Base removes H+ from adjacent carbon.
• Pi bond forms.
=>
Chapter 6
43
A Closer Look
H
H O
H
CH3
H
+
C C
H C C CH3
H
H CH3
Chapter 6
+
H3O
CH3
44
=>
E1 Energy Diagram
• Note: first step is same as SN1
Chapter 6
=>
45
E2 Reaction
• Bimolecular elimination
• Requires a strong base
• Halide leaving and proton abstraction
happens simultaneously - no
intermediate.
=>
Chapter 6
46
E2 Mechanism
H Br
CH3
H
C C
H C C CH3
H
O
H
H CH3
-
+ H2O + Br
CH3
=>
Chapter 6
47
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
H Br CH3
H C C C CH3
H H H
-
OH
CH3
H
H
CH3
C C C CH3 + H C C C
H
CH3
H H
H H
minor
Chapter 6
major
48
=>
E2 Stereochemistry
=>
Chapter 6
49
E1 or
• Tertiary > Secondary
• Weak base
• Good ionizing solvent
• Rate = k[halide]
• Saytzeff product
• No required geometry
• Rearranged products
E2?
• Tertiary > Secondary
• Strong base required
• Solvent polarity not
important
• Rate = k[halide][base]
• Saytzeff product
• Coplanar leaving
groups (usually anti)
• No rearrangements
=>
Chapter 6
50
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
51
Secondary Halides?
Mixtures of products are common.
=>
Chapter 6
52
Chapter 6
53
POWER POINT IMAGES FROM
“ORGANIC CHEMISTRY, 5TH EDITION”
L.G. WADE
ALL MATERIALS USED WITH PERMISSION OF AUTHOR
PRESENTATION ADAPTED FOR BURLINGTON COUNTY COLLEGE
ORGANIC CHEMISTRY COURSE
BY:
ANNALICIA POEHLER STEFANIE LAYMAN
CALY MARTIN
Chapter 6
54