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Synthesis of Alkenes by Dehydrohalogenationof Alkyl Halides
When an alkyl halide is treated with a strong base such as
sodium or potassium hydroxide (NaOH, KOH).
Sodium methoxide (NaOCH3), sodium ethoxide (NaOC2H5)
or potassium tertiary butoxide, (KOC(CH3)3) an
elimination reaction takes place and one or more alkenes is
formed.
C C
X
H
CCstrong base
-HX
X = Br, Cl, I
An Elimination Reaction
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The E2 Mechanism of Dehydrohalogenation
C C
X
H
CC X
B
strong base ( B ) BH+ +
?The mechanism is concerted i.e. bond making
and bond breaking occur simultaneously.
?Rate of Reaction = k[base][alkyl halide]
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This type of elimination is referred to as a 1,2- elimination or
as a ? -elimination. The hydrogen that is lost is attached to thecarbon that is in the ? -position with respect to the halogen
atom.
C C
Br
H
?
?
12
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Stereochemistry of the E2 Mechanism
For the E2 mechanism to take place, the alkyl halide mustadopt a conformation in which the halogen atom and the ? -
hydrogen are anti to each other.
H
X
H
X
This is called the anti-periplanar or anti-coplanarconformation
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Note!
Dehydrohalogenation reactions can also take place under
conditions that favour an E1 mechanism. However, the
synthesis of alkenes is better achieved by way of an E2
mechanism.
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Dehydrobromination of 2-Bromopentane
CH3 CH2 CH CH CH2
H
Br
HH3C CH2 CH2 CH
CH2
CH3 CH2CH CH3CH
2-bromopentane +
1-pentene
2-pentene (disubstituted alkene)
B
(monosubstituted alkene)
Zaitsevs rule would dictate that the more highly
substituted 2-pentene be the preferred product and thisis so when bases such as sodium hydroxide, potassium
hydroxide, sodium methoxide and sodium ethoxide are
used.
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The Hofmann Product
When potassium tertiary butoxide which is a much bulkier
molecule than sodium or potassium hydroxide; sodium
methoxide and sodium ethoxide, is used, the less substituted
alkene, 1-pentene is the one that predominates and is referredto as the Hofmann product.
C
CH3
CH3
CH3
OK
potassium tertiary butoxide
C O
CHH
H
C
H
HH
CHH
H
K
potassium tertiary butoxide
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Hofmann Product contd
Bulky bases abstract the least hindered H+
Least substituted alkene is major product.
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Reactions of Alkenes
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Reactivity of C=C
Electrons in pi bond are loosely held.
Electrophiles are attracted to the pi electrons.
Carbocation intermediate forms (typically).
Nucleophile adds to the carbocation.
Net result is addition to the double bond.
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Addition Reactions
The typical mode of reaction of alkenes is that of
addition. In addition reactions, two molecules combine to
give a single molecule of product.
Notice the bonding changes that occur.
C C + A B C CA B
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Addition Reactions Contd
The reaction is energetically favourable. The amount of
energy released by the formation of two sigma bonds is in excess of
that needed to break one sigma bond and one pi bond. As a result,
addition reactions of alkenes are usually exothermic (heat is given
off).
C C
C CA B
B
Reaction Coordinate
G
G negative (exothermic reaction)
G H T S
Energy
A,
Energy Profile of Addition Reactions
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What is the Mechanism of AdditionReactions of Alkenes?
Remember that the term Reaction Mechanism refers to
the sequence and timing of the bond breaking and bond
making processes that bring about the conversion of
starting materials into products.
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The Reaction Mechanism tells us theorder in which the following bond
changes occur:
C C + A B C CA B
Bond Breaking
One pibond and one sigmabond are broken.
Bond Making
Two new sigmabonds are formed
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The Mechanism of AdditionReactions of Alkenes
Some addition reactions of alkenes are stepwise. They occurby way of a series of steps and involve the formation ofintermediate species.
Some addition reactions are concerted. The conversion ofstarting material to product takes place in one step . Bondsare broken and formed simultaneously with no intermediate
species being formed.
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Electrophilic Addition
An alkene is a nucleophile by virtue of itspi bond. The
two lobes of the pi bonding orbital above and below the
molecular framework contain a pair of fairly loosely held
electrons which can be donated to an electrophile. We say
that the electrons are loosely held because the pi bond isweaker than the sigma.
CC lobes of pibonding orbitalabove and below the molecularframework
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Electrophilic Addition Contd
The addition is called Electrophilic Addition when the
molecule, E Nu that adds to the alkene is highly
polarized such that an electrophile E is provided.
C C E Nu C CNu E?? ??
+
An electrophile is an electron deficient species that will accept an electron pair into an empty
orbital to form a bond.
The nucleophile is the electron rich species that will donate a pair of electrons to the
electrophile
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REMINDER!Representing Reaction Mechanisms
on PaperUse of Curly Arrows
A curly arrow represents the movement of a pair of
electrons. The arrow originates at the source of the
electrons and terminates at the place where the electronsend up.
source ofelectrons
destination ofelectrons
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A B A B
CC
A
CC
A
CC
A B
CC
A
B
+
+
Electrons usually end up either on an atom itself or
in the gap between two atoms where a new bond is
being formed.
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Step 1: Pi electrons attack the electrophile.
The Mechanism of Electrophilic
Addition is a Stepwise Mechanism.
Step 2: Nucleophile attacks the carbocation.
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Typical Highly Polarized Molecules thatadd to Alkenes
H X
H OSO3H
H
H
OH
????
?? ??
Hydrogen halides, (X= Br, Cl, I)
Sulfuric acid (H2SO4)
??
Hydronium ion
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C C
H Br
C C
H
Br
C C
H
Br
C C
H
Br
+Fast
Slow
Mechanism of Addition of HBr toAlkenes
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Electrophilic Addition Reactions
When a symmetrical alkene undergoes electrophilic
addition of an unsymmetrical molecule, HY, only one
product is possible regardless of the direction in which
HY adds.
C C
H
H H
H
C C
H
HH
Y
H
H
C C
H
HH
Y
H
H
HY
2 1 2 1 2 1
ethene, symmetrical alkene
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Regioselectivity in Electrophilic AdditionReactions Contd
If the alkene is unsymmetrical, for example propene, the
addition of HY can in theory lead to two different products
I and II which are structural or constitutional isomers
C C
H
H H
CH3
C C
CH3
HH
Y
H
H
C C
CH3
HH
Y
H
H
HY+
I II
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The Two Possible Carbocations
C C
H
H H
CH3
C C
CH3
HH
Y
H
H
C C
CH3
HH
Y
H
H
Y H
HY
+
I II
+
C C
CH3
HH
H
H
IIY
Compound
carbocation B
C C
CH3
HH
H
H
IY
Compound
carbocation A
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Markovnikovs Rule
(The Modern Statement)
When an unsymmetrical reagent adds to a double bond by
way of an ionic mechanism, the positive portion of the
reagent (the electrophile) attaches itself to a carbon atom of
the double bond so that the more stable of the two possible
carbocations is formed as the intermediate.
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Markovnikovs Rule Obeyed
C C
CH3
HH
H
H
C C
H
H H
CH3
C C
CH3
HH
H
HY
ICompound
HY
A
secondary carbocationformed preferentiallyfrom more stablecarbocation
C C
CH3
HH
H
H
IICompound
HY
B
primary carbocation
Not formed or formedin a minor amount
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The MarkovnikovProduct is formed byway of the energeticallymore favorable route.
Markovnikov
product
AntiMarkovnikov
product
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Markovnikovs Rule(The Original Statement)
In the addition of HX to an alkene, the hydrogen atom adds
to the carbon atom of the double bond that already has the
greater number of hydrogens.
C C
H
H H
CH3
C C
CH3
HH
X
H
H
C C
CH3
X
HH
H
H
HX+
I II
When a reaction such as the one above has the potential to
produce two or more constitutional isomers and yet only one is
formed or a mixture in which one predominates, the reaction is
said to be regioselective.
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Electrophilic Addition With Rearrangement
CH3 CH
CH3
CH2
CH CH3 CH
CH3
CH3
CH
CH3 C
CH3
CH3
CH2
Cl
Cl
HCl
2-chloro-3-methylbutane
2-chloro-2-methylbutane
3-methyl-1-butene
+
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The Expected Outcome
CH3 C
CH3
H
C C
H
H
HH
CH3 C
CH3
H
C C
H
H
HH
Cl
CH3 C
CH3
H
C C H
HH
H Cl
Cl
2-chloro-3-methylbutaneCarbocation formed in keepingwith Markovnikov's Rule
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Rearrangement
CH3 C
CH3
H
C C
H
H
HH
CH3 C
CH3
C C
H
H
H
H
H
1,2-hydride shift
Cl
CH3 C
CH3
C C
H
H
H
H
HCl
2 31 4
2-chloro-2-methylbutane
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Addition of HBr to alkenes
C C
H
H H
CH3
HBrC C
CH3
HH
Br
H
H
Markovnikov Product
II
C C
H
H H
CH3
HBr
Anti-Markovnikov Product
(under certaincircumstances)
C C
CH3
HH
Br
H
H
Sometimes the expected Markovnikov addition was observed,
yet at other times the regioselectivity was just the opposite.
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Anti-Markovnikov addition of HBr toalkenes
II
C C
H
H H
CH3
HBr
Anti-Markovnikov Product
(under certain
circumstances)
C C
CH3
HH
Br
H
H
It was discovered that the anti-Markovnikovaddition occurred when there happened to beorganic peroxides present in the reaction
mixture.
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Anti-Markovnikov Addition of HBr toAlkenes contd.
Organic peroxides have the following structures:
O O RR
O O HR
alkyl peroxide
alkyl hydroperoxide
II
C C
H
H H
CH3
HBr
Anti-Markovnikov Product
C C
CH3
HH
Br
H
HROOR
orROOH
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Anti-Markovnikov Addition of HBr toAlkenes contd.
In the presence of organic peroxides, addition of HBr
proceeds by way of a Free Radical Mechanism.
STEP 1 (Initiation)
The weak oxygenoxygen bond in the organic peroxideundergoes homolysis to produce alkoxy radicals.
O OR R +R O O Rheat
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STEP 2 (Propogation)
An alkoxy radical abstracts a hydrogen atom
from H-Br thus producing a bromine radical.
Anti-Markovnikov Addition of HBr toAlkenes contd.
+R O H Br R O H + Br
Electrophile
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Anti-Markovnikov Addition of HBr toAlkenes contd.
STEP 3 (Propogation)
C C
H
H
H3C
H
Br
C C
Br
H
HCH3
H
2 alkyl radical
The bromine radical then adds to the double bond of thealkene in such a way as to give the more stable secondaryalkyl radical.
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Anti-Markovnikov Addition of HBr toAlkenes contd.
In the product forming fourth step, the secondary radicalabstracts a hydrogen atom from HBr to give an alkylbromide.
STEP 4 (Propogation)
C C
H
H
Br
CH3
H
H Br
C C
H
H
Br
CH3
H
H
+ Br
Electrophile
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Anti-Markovnikov ??
Tertiary radical is more stable, so that intermediate forms
faster.
CH3 C
CH3
CH CH3 Br+
CH3 C
CH3
CH CH3
Br
CH3 C
CH3
CH CH3
Br
X