Organic Chemistry, 6h Edition
L. G. Wade, Jr.
Chapter 6
Alkyl Halides: Nucleophilic
Substitution and Elimination
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:
H H
H C C Br
H H
alkyl halide
I
H
H
=>
C C
H
Cl
vinyl halide
Chapter 6
aryl halide
2
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.
=>
Chapter 6
3
6-2 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
4
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
=>
5
“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
6
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
7
Classify These:
CH3
CH CH3
CH3CH2F
Cl
CH3I
(CH3)3CBr
Chapter 6
=>
8
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
=>
9
6-3 Uses of Alkyl Halides
• Solvents - degreasers and dry cleaning
fluid
• Reagents for synthesis of other
compounds
• Anesthetics: 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
10
Dipole Moments
• m = 4.8 x d x d, where d 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
11
6-5 Physical Properties of RX
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
=>
12
Densities
• Alkyl fluorides and chlorides less dense
than water.
• Alkyl dichlorides, bromides, and iodides
more dense than water.
=>
Chapter 6
13
6-6 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
14
Halogenation of Alkanes
• All H’s equivalent. Restrict amount of halogen
to prevent di- or trihalide, etc formation
CH3CH3 + Cl2
h
CH3CH2Cl + CH3CHCl2 + CH3CCl3 +
ClCH2CCl3 + Cl2CHCCl3 + Cl3CCCl3
• Highly selective: bromination of 3 C
CH3
H3C
H + Br2
CH3
h
H3C
CH3
Br
CH3
Chapter 6
15
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.
=>
Chapter 6
16
Reaction Mechanism
Free radical chain reaction
 initiation, propagation, termination.
(1)
Br2
h
H
(2)
H
C
2 Br
H
.
H + Br
H
H
H
C.
H
+ HBr
H
H
(3)
C.
(4) 2R .
H
Br
Br
H
C
R R
.
Br + Br
H
Chapter 6
17
Substitution Reactions
H
R
C
H
X + Nu
R
H
C
Nu
+ X
-
H
• 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
18
Elimination Reactions
H
R
H
C
H
H
R
H
+ X
X + Base
H
-
H
• 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
19
SN2 Mechanism
• Bimolecular nucleophilic substitution.
• Concerted reaction: new bond forming
and old bond breaking at same time.
• Rate is first order in each reactant.
• Inversion of configuration.
=>
Chapter 6
20
SN2 Energy Diagram
• One-step reaction.
• Transition state is highest in energy. =>
Chapter 6
21
6-9 Generality of SN2 RXN
• Synthesis of other classes of compounds.
• Halogen exchange reaction.
=>
Chapter 6
22
6-10 Factors Affecting SN2:
Nucleophilic Strength
• Stronger nucleophiles react faster.
• Strong bases are strong nucleophiles, but
not all strong nucleophiles are basic.
=>
Chapter 6
23
Trends in
Nucleophilic 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
24
Polarizability Effect
=>
Chapter 6
25
Steric Effect
Bulky Nucleophiles
Sterically hindered for attack on carbon, so
weaker nucleophiles.
Chapter 6
26
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
27
Solvent Effects (2)
• Polar aprotic solvents (no O-H or N-H) do not form
hydrogen bonds with nucleophile
• Examples:
Chapter 6
28
Crown Ethers
• Solvate the cation, so nucleophilic strength
of the anion increases.
• Fluoride becomes a good nucleophile.
Chapter 6
29
Leaving Group Ability
• Electron-withdrawing
• Stable once it has left (not a strong base)
• Polarizable to stabilize the transition state.
Chapter 6
30
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
31
Steric Hindrance
• Nucleophile approaches from the back side.
• It must overlap the back lobe of the C-X sp3
orbital.
=>
Chapter 6
32
Stereochemistry of SN2
Walden inversion or Inversion of Configuration
=>
Chapter 6
33
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
34
SN1 Mechanism
Chapter 6
35
Chapter 6
36
SN1 Energy Diagram
• Forming the
carbocation is
endothermic
• Carbocation
intermediate is in
an energy well.
=>
Chapter 6
37
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
38
Stereochemistry of SN1
Racemization:
inversion and retention
=>
Chapter 6
39
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
40
E1 Reaction
•
•
•
•
Unimolecular elimination
Two groups lost (usually X- and H+)
Nucleophile acts as base
Also have SN1 products (mixture)
=>
Chapter 6
41
E1 Mechanism
• Halide ion leaves, forming carbocation.
• Base removes H+ from adjacent carbon.
• Pi bond forms.
=>
Chapter 6
42
A Closer Look
=>
Chapter 6
43
Example
Chapter 6
44
E1 Energy Diagram
=>
Note: first step is same as SN1
Chapter 6
45
Zaitsev’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
Chapter 6
46
Example
Chapter 6
47
E2 Reaction
• Bimolecular elimination
• Requires a strong base
• Halide leaving and proton abstraction
happens simultaneously - no
intermediate.
=>
Chapter 6
48
E2 Mechanism
• Order of reactivity: 3° > 2 ° > 1°
• Mixture may form, but Zaitsev product
predominates.
Chapter 6
=>
49
Example
Chapter 6
50
Example (2)
Chapter 6
51
E2 Stereochemistry
=>
Chapter 6
52
Chapter 6
53
E1 or
• Tertiary > Secondary
• Weak base
• Good ionizing solvent
• Rate = k[halide]
• Zaitsev product
• No required geometry
• Rearranged products
E2?
• Tertiary > Secondary
• Strong base required
• Solvent polarity not
important
• Rate = k[halide][base]
• Zaitsev product
• Coplanar leaving
groups (usually anti)
• No rearrangements
=>
Chapter 6
54
Substitution or
Elimination?
• Strength of the nucleophile determines
order: Strong nucleophile, bimolecular,
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
55
Secondary Halides?
Mixtures of products are common.
=>
Chapter 6
56
End of Chapter 6
Chapter 6
57