CCB1013 Organic ChemistryBy Dr. Oh Pei Ching
1
Chapter 8:
Alkenes and Alkynes II –
Addition Reactions
2
Alkenes and Alkynes II
CCB1013 Organic Chemistry
� By the end of this lecture, you should be able to:
• Use curved arrow notation in addition mechanism
• Understand additions to alkenes and alkynes
(hydrogen halides, sulfuric acid, water and halogens)
• Identify proton donors and acceptors in proton
transfer steps
• Understand stereochemistry, regiochemistry, and
conditions for use of a variety of reagents that add to
alkenes and alkynes
3
Learning Outcomes
CCB1013 Organic Chemistry
� Example:
4
Introduction: Addition Reactions to Alkenes
CCB1013 Organic Chemistry
Electrophilic Nucleophilic
5
Introduction: Addition Reactions to Alkenes
CCB1013 Organic Chemistry
� How to Understand Additions to Alkenes
� An addition reaction results in the conversion of one π
bond and one σ bond into two σ bonds. Generally,
addition reactions are exothermic.
� How to Understand Additions to Alkenes
� Since π bonds are formed from the overlapping of p
orbitals, π electron clouds are above and below the
plane of the double bond. The electrons are loosely held
and are a source of electron density (nucleophilic).
6
Introduction: Addition Reactions to Alkenes
CCB1013 Organic Chemistry
C C π electron
clouds
7
Introduction: Addition Reactions to Alkenes
CCB1013 Organic Chemistry
� Electrophilic Addition
� C=C and C≡C π bonds are particularly susceptible to
electrophilic reagents (electrophiles).
� Common electrophiles: H+, X+ (X = Cl, Br, I), Hg2+, etc.
� In addition reactions, the alkene changes from a nucleophile
in the first step to an electrophile in the second.
Recall:
Electrophiles can accept e‒
Nucleophile can donate e‒
8
Electrophilic Addition of Hydrogen Halides to
Alkenes: Mechanism and Markovnikov’s Rule
CCB1013 Organic Chemistry
� Hydrogen halides add to the double bond of alkenes:
� Markovnikov’s Rule:
� For symmetrical substrates, no problem for regiochemistry.
C C
H
H
H
HE Nu
E
CC H
H
H
H
E
C CH
H
H
H
E
C CH
H
H
H
Nu E
CC H
H
H
H
Nu
Nu
same
as
same
as
9
Electrophilic Addition of Hydrogen Halides to
Alkenes: Mechanism and Markovnikov’s Rule
CCB1013 Organic Chemistry
� Markovnikov’s Rule:
� But for unsymmetrical substrates, two regioisomers are
possible.
E NuC C
H
H3C
H
H E
CC H
H
CH3
H
E
C CCH3
H
H
H
E
C CCH3
H
H
H
Nu E
CC H
H
CH3
H
Nu
Nu Nu
or
different
from
10
Electrophilic Addition of Hydrogen Halides to
Alkenes: Mechanism and Markovnikov’s Rule
CCB1013 Organic Chemistry
� Markovnikov’s Rule:
� Addition of HX to an alkene proceeds so that the hydrogen
atom adds to the carbon that already has the most hydrogen
atoms.
� Example:
� Addition of HBr to propene occurs to give 2-bromopropane as
the major product.
11
Electrophilic Addition of Hydrogen Halides to
Alkenes: Mechanism and Markovnikov’s Rule
CCB1013 Organic Chemistry
� Mechanism for addition of hydrogen halide to alkene
Rate-
determining
step
12
Electrophilic Addition of Hydrogen Halides to
Alkenes: Mechanism and Markovnikov’s Rule
CCB1013 Organic Chemistry
� Free-energy diagram for addition of hydrogen halide to alkene
� The reaction has a highly endergonic first step (rate
determining) and a highly exergonic second step.
13
Electrophilic Addition of Hydrogen Halides to
Alkenes: Mechanism and Markovnikov’s Rule
CCB1013 Organic Chemistry
� Theoretical Explanation of Markovnikov’s Rule:
� The product with the more stable carbocation intermediate
predominates (because it is formed faster).
� The most stable carbocation is formed fastest because it has a
lower ∆G‡.
� The transition state for the rate determining step (first step) resembles a
carbocation and is stabilized by factors which stabilize carbocations.
14
Electrophilic Addition of Hydrogen Halides to
Alkenes: Mechanism and Markovnikov’s Rule
CCB1013 Organic Chemistry
� Theoretical Explanation of Markovnikov’s Rule:
� The product with the more stable carbocation intermediate
predominates (because it is formed faster).
� The most stable carbocation is formed fastest because it has a
lower ∆G‡.
� The transition state for the rate determining step (first step) resembles a
carbocation and is stabilized by factors which stabilize carbocations.
∆G‡(2o) < ∆G‡(1o)
→ 2o carbocation forms faster
15
Electrophilic Addition of Hydrogen Halides to
Alkenes: Mechanism and Markovnikov’s Rule
CCB1013 Organic Chemistry
� Mechanism for addition of HBr to 2-methylpropene
This
reaction
takes place
This
reaction
does not
occur to any
appreciable
extent
16
Electrophilic Addition of Hydrogen Halides to
Alkenes: Mechanism and Markovnikov’s Rule
CCB1013 Organic Chemistry
� Modern Statement of Markovnikov’s Rule:
� In the ionic addition of an unsymmetrical reagent to a double
bond, the positive portion of the adding reagent attaches itself
to a carbon atom of the double bond so as to yield the more
stable carbocation as an intermediate.
� Regioselective Reactions:
� A reaction that can potentially yield two or more
constitutional isomers actually produces only one or a
predominance of one isomer.
� Markovnikov additions of hydrogen halides to alkenes are
regioselective.
17
Electrophilic Addition of Hydrogen Halides to
Alkenes: Mechanism and Markovnikov’s Rule
CCB1013 Organic Chemistry
� Exception to Markovnikov’s Rule:
� Anti-Markovnikov addition occurs when alkenes are treated
with HBr in the presence of peroxides (ROOR).
� Addition occurs via a radical mechanism.
� This anti-Markovnikov addition does not take place with HI,
HCl, and HF, even when peroxides are present.
Anti-
Markovnikov
Product
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Stereochemistry of the Ionic Addition to an Alkene
CCB1013 Organic Chemistry
� Addition of HX to 1-hexene yields a chiral molecule.
� A racemic mixture is produced because the intermediate
carbocation is achiral.
C C
H
H
Bu
HH X
C CH2 HH
Bu
X
X
achiral
trigonal planar
carbocation
C CH3H
X
Bu
(S)-2-Halohexane(50%)
attack from top
rac
em
ate
C CH3
H
X
Bu
(R)-2-Halohexane(50%)
(R)-2-Halohexane
(50%)
1-Hexene donates a
pair of electrons to
the proton of HX to
form an achiral
carbocation.attack from bottom
19
Addition of Sulfuric Acid to Alkenes
CCB1013 Organic Chemistry
� Addition of cold concentrated sulfuric acid to alkenes leads to
alkyl hydrogen sulfates which are soluble in the acid.
� The addition follows Markovnikov’s rule.
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Addition of Sulfuric Acid to Alkenes
CCB1013 Organic Chemistry
� Alcohols from Alkyl Hydrogen Sulfates:
� Alkyl hydrogen sulfates can be hydrolyzed by heating with
water.
� The net result is Markovnikov addition of water to an alkene.
21
Addition of Water to Alkenes:
Acid-Catalyzed Hydration
CCB1013 Organic Chemistry
� The reaction of alkenes with dilute aqueous acid (e.g. dilute
H2SO2 and H3PO4) leads to Markovnikov addition of water.
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Addition of Water to Alkenes:
Acid-Catalyzed Hydration
CCB1013 Organic Chemistry
� Mechanism:
Rate-
determining
step
23
Addition of Water to Alkenes:
Acid-Catalyzed Hydration
CCB1013 Organic Chemistry
� The hydration of alkenes and the dehydration of alcohols are
simply reverse reactions of one other.
H The reaction is governed by the position of all the equilibria.
H Hydration is favored by addition of a small amount of acid and a large amount
of water.
H Dehydration is favored by concentrated acid with very little water present
(removal of water produced also helps favor dehydration).
� Rearrangements:
� One complication associated with alkene hydration is the
occurrence of rearrangements, as carbocation formed initially
may rearrange to a more stable one.
How to avoid
rearrangement?
Oxymercuration
-demercuration
24
Alcohols from Alkenes through Oxymercuration-
Demercuration: Markovnikov Addition
CCB1013 Organic Chemistry
� Oxymercuration-demercuration procedure gives high yields of
alcohols and avoids rearrangements.
Alkene mercuric acetate (hydroxyalkyl)mercury
Water and mercuric acetate add to the double bond
sodium alcohol
borohydride
Sodium borohydride reduces the acetoxymercury group and replaces it with hydrogen
OAc
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Alcohols from Alkenes through Oxymercuration-
Demercuration: Markovnikov Addition
CCB1013 Organic Chemistry
� Oxymercuration–demercuration is also highly regioselective and
follows Markovnikov’s rule.
� Specific Examples:
No
rearrange-
ment
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Alcohols from Alkenes through Oxymercuration-
Demercuration: Markovnikov Addition
CCB1013 Organic Chemistry
� Mechanism of Oxymercuration
� The mechanism involves formation of a bridged mercurinium ion.
27
Alcohols from Alkenes through Oxymercuration-
Demercuration: Markovnikov Addition
CCB1013 Organic Chemistry
� Mechanism of Oxymercuration
� The mechanism involves formation of a bridged mercurinium ion.
28
Alcohols from Alkenes through Hydroboration-
Oxidation: Anti-Markovnikov Syn Addition
CCB1013 Organic Chemistry
� Hydroboration-Oxidation:
� The reaction leads to syn and anti-Markovnikov addition of water to
alkenes.
� Hydroboration: Synthesis of Alkylboranes
� The elements of hydrogen and boron are added across the double bond.
� In practice, a borane complex with the solvent tetrahydrofuran (THF) is often used.
29
Alcohols from Alkenes through Hydroboration-
Oxidation: Anti-Markovnikov Syn Addition
CCB1013 Organic Chemistry
� Hydroboration: Mechanism
� When terminal alkene (e.g. propene) is treated with solution
containing BH3:THF, boron hydride adds successively to three
molecules of alkene.
� Boron becomes attached to the least substituted carbon of the
double bond.
� The bulky boron group can approach the least sterically hindered carbon more
easily.
� This orientation also allows a �+ charge in the transition state to reside at the most
substituted carbon.
� This orientation leads to anti-Markovnikov product.
30
Alcohols from Alkenes through Hydroboration-
Oxidation: Anti-Markovnikov Syn Addition
CCB1013 Organic Chemistry
� Hydroboration: Mechanism
� Boron becomes attached to the least substituted carbon of the double
bond.
� The boron and hydride add with syn stereochemistry.
Boron attachment
31
Alcohols from Alkenes through Hydroboration-
Oxidation: Anti-Markovnikov Syn Addition
CCB1013 Organic Chemistry
� Oxidation and Hydrolysis of Alkylboranes
� Oxidation and hydrolysis to the alcohol takes place with retention of
stereochemistry at the carbon bonded to boron.
32
Alcohols from Alkenes through Hydroboration-
Oxidation: Anti-Markovnikov Syn Addition
CCB1013 Organic Chemistry
� Oxidation of trialkylboranes: Mechanism
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Alcohols from Alkenes through Hydroboration-
Oxidation: Anti-Markovnikov Syn Addition
CCB1013 Organic Chemistry
� The hydroboration-oxidation of 1-methylcyclopentene gives the anti-
Markovnikov product with syn addition of the elements of water.
� Summary of Alkene Hydration Methods:
� Acid-catalyzed hydrolysis: Markovnikov addition
� Oxymercuration-demercuration: Markovnikov addition
� Hydroboration-oxidation: anti-Markovnikov and syn addition
34
Addition of Bromine and Chlorine to Alkenes
CCB1013 Organic Chemistry
� Addition produces vicinal dihalides.
� The addition of halogens is an anti addition to the double bond.
35
Addition of Bromine and Chlorine to Alkenes
CCB1013 Organic Chemistry
� Bromine serve as a test for the presence of carbon-carbon multiple
bonds.
� If bromine is added to an alkene (or alkyne), the red color of the bromine
reagent disappears when an alkene (or alkyne) is present in excess.
� Alkanes do not react appreciably with bromine or chlorine at room
temperature in the dark.
36
Addition of Bromine and Chlorine to Alkenes
CCB1013 Organic Chemistry
� Mechanism:
� A bromonium ion intermediate results instead of the carbocation seen in
other addition reactions.
37
Addition of Bromine and Chlorine to Alkenes
CCB1013 Organic Chemistry
� Stereochemistry:
� The net result is anti addition because of SN2 attack on the bromonium
ion intermediate.
� When cyclopentene reacts the product is a racemic mixture of trans-1,2-
dibromocyclopentane enantiomers.
38
Addition of Bromine and Chlorine to Alkenes
CCB1013 Organic Chemistry
� Stereospecific Reactions:
� A reaction is stereospecific if a particular stereoisomeric form of the
starting material reacts in such a way that it gives a specific
stereoisomeric form of the product.
� Example: cis- and trans-2-butene give stereoisomeric products when
halogenated.
� Halogenation of double bonds is stereospecific.
Reaction 1
Reaction 2
39
Addition of Bromine and Chlorine to Alkenes
CCB1013 Organic Chemistry
� Halohydrin Formation:
� If halogenation is carried out in aqueous solvent (not nucleophilic
solvent such as CCl4), the water molecule can act as a nucleophile to open
the halonium ion.
� The product is a halohydrin.
40
Addition of Bromine and Chlorine to Alkenes
CCB1013 Organic Chemistry
� Halohydrin Formation Mechanism:
41
Addition of Bromine and Chlorine to Alkenes
CCB1013 Organic Chemistry
� Halohydrin Formation:
� In unsymmetrical alkenes, the bromonium ion will have some of its δ +
charge density on the most substituted of the two carbons.
� The most substituted carbon can best accommodate δ+ charge.
� The water nucleophile will tend to react at the carbon with the most δ+
charge.
42
Addition of Bromine and Chlorine to Alkynes
CCB1013 Organic Chemistry
� Addition of halogen to alkynes can occur once or twice depending on
how many equivalents of the halogen are added.
� Addition of one equivalent usually proceeds to give the trans dihalide
through anti addition.
43
Addition of Hydrogen Halides to Alkynes
CCB1013 Organic Chemistry
� Addition of hydrogen halides occurs once or twice depending on how
many molar equivalent of hydrogen halide are added.
� Both additions are Markovnikov and give gem-halides.
44
Addition of Hydrogen Halides to Alkynes
CCB1013 Organic Chemistry
� HBr can be generated by reaction of acetyl bromide and alumina.
� Anti-Markovnikov addition of HBr occurs in the presence of peroxide.