Chapter Seven Part TWO
Haloalkanes
Key words
 SN1
 SN2
 E1
 E2
 Racemic mixture and racemization
β-Elimination
•
-Elimination: Removal of atoms or groups of atoms from adjacent
carbons to form a carbon-carbon double bond.
• We study a type of β-elimination called dehydrohalogenation (the elimination of
HX).
β-Elimination
β-Elimination Energy Profiles
β-Elimination
• Zaitsev rule: The major product of a β-elimination is the more
stable (the more highly substituted) alkene.
-
Br
CH3 CH2 O Na
2-Bromo-2methylbutan e
+
+
CH3 CH2 OH
Br
-
2-Meth yl-2-bu tene
(major p roduct)
+
CH3 O Na
+
CH3 OH
1-Bromo-1-methylcyclopentane
2-Meth yl-1b utene
1-Methylcyclopen tene
(major product)
Methylenecyclop entane
β-Elimination Product Distribution
H 2O
+
E1
Major
(Zaitsev product)
E2
minor
KOtBu
Hoffman product (anti-Zaitsev)
Only
I
E2
+
NaOMe
Major
(Zaitsev product)
minor
β-Elimination
• There are two limiting mechanisms for β-elimination reactions.
• E1 mechanism: at one extreme, breaking of the C-X bond is
complete before reaction with base breaks the C-H bond.
• Only R-X is involved in the rate-determining step.
• E2 mechanism: at the other extreme, breaking of the C-X and
C-H bonds is concerted.
• Both R-X and base are involved in the rate-determining step.
E1 Mechanism
• Step 1: Rate-determining ionization of C-X gives a carbocation
intermediate.
CH3
CH2 -C-CH3
s low , rate
determin ing
Br
CH3
CH3 -C-CH3 +
+
(A carb ocation
in termediate)
Br
–
• Step 2: Proton transfer from the carbocation intermediate to a
base (in this case, the solvent) gives the alkene.
H
O
H3 C
CH3
+ H-CH2 -C-CH3
+
fas t
CH3
H +
O H + CH2 =C-CH3
H3 C
E2 Mechanism
• A one-step mechanism; all bond-breaking and bond-forming
steps are concerted.
E2 Mechanism Stereochemistry Demands
Predict the E2 major organic product of the following reactions
Br
NaOMe
MeOH
Br
NaOMe
MeOH
NaOH
MeOH
Br
Predict the E2 major organic product of the following reactions
B r
N a O H
M e O H
B r
K O tB u
H O tB u
B r
K O tB u
H O tB u
Elimination Reactions
• Table 7.6 Summary of E1 versus E2 Reactions for haloalkanes
Substitution versus Elimination
• Because many nucleophiles are also strong bases (OH- and RO-),
SN and E reactions often compete.
• The ratio of SN/E products depends on the relative rates of the
two reactions.
SN1 versus E1
• Reactions of 2° and 3° haloalkanes in polar protic solvents
give mixtures of substitution and elimination products. Product
ratios are difficult to predict.
H 2O
E1
Major
(Zaitsev product)
SN1
H 2O
I
OH
tert butyl iodide
SN 1
CH3CH2OH
OCH2CH3
SN2 versus E2
• It is considerably easier to predict the ratio of SN2 to E2 products.
Attack of base on a -hydrogen by
E2 is only sligh tly affected by
bran chin g at the -carbon; alken e
formation is accelerated
R R
H
C
C leavin g grou p
SN 2 attack of a nu cleop hile is R
R
imped ed by branching at the
- and -carbons
Summary of S versus E for Haloalkanes
For methyl and 1° haloalkanes
Methyl
CH3 X
SN2
The only sub stitution reactions observed
SN1
SN 1 reactions of methyl halid es are n ever observed.
The meth yl cation is so uns table that it is never
formed in solution .
Primary
RCH2 X
SN2
The main reaction w ith strong bases such as OH - an d
EtO -. Als o, th e main reaction w ith good
nu cleophiles/w eak bas es, su ch as I- and CH 3 COO -.
E2
The main reaction with stron g, bu lky bases , such as
potas sium t ert-butoxide.
SN 1 / E1
Primary cation s are n ever formed in s olution; th erefore,
S N 1 and E1 reaction s of p rimary h alid es are never obs erved.
Summary of S versus E for Haloalkanes
For 2°and 3°haloalkanes
Secondary SN 2
R2 CHX
Tertiary
R3 CX
The main reaction w ith w eak b ases/good nu cleophiles,
such as I- and CH 3COO -.
E2
Th e main reaction w ith s trong bases /good nucleoph iles
su ch as OH - an d CH 3CH 2O -.
SN1 / E1
Common in reactions w ith w eak nucleop hiles in p olar
protic s olvents, s uch as w ater, methan ol, an d ethanol.
SN 2
SN 2 reactions of tertiary halides are n ever observed
because of th e extreme crow ding aroun d the 3° carbon.
Main reaction w ith strong bas es, su ch as HO - an d RO -.
E2
SN1 / E1
Main reactions w ith poor nu cleophiles/w eak b ases.
• Examples: Predict the major product and the mechanism for
each reaction.
SHMe
Br
DMSO
Br
KOtBu
HOtBu
NaOH
MeOH
Br
• Examples: Predict the major product and the mechanism for
each reaction.
B r
N a O E t
E tO H , 8 0 o C
B r
B r
N a O H
H
2
O
End of Chapter Seven
Haloalkanes
Key words
 SN1
 SN2
 E1
 E2
 Racemic mixture and racemization