TOPIC 3: ALKENES: REACTIVITY AND SYNTHESIS
The chemistry of alkenes is dominated by addition reactions,
many of which occur through carbocation intermediates. The
information covered in the “Reaction Supplement” is vital to helping
you understand, not simply just memorize, the reactions of alkenes.
H
H
H
OH
First we will cover the general
C
C
C
C
mechanism of an electrophilic
alcohol
alkane
addition reaction and then look
at the regiospecificity of the
reaction before we examine
X
OH
C
HO
C
C
halohydrin
X
X
C
C
1,2-diol
C
specific alkene addition reactions.
OH
C
alkene
C
C
carbonyl
compound
1,2-dihalide
H
X
C
C
C
C
halide
O
C
cyclopropane
I. ELECTROPHILIC ADDITION REACTION
* In this section, we will examine reactions in which an alkene
reacts with an electrophile to form a new compound. One of
the key factors we will consider is that of regioselectivity, which
will help us to determine on which alkene carbon the addition
reaction occurs.
 NUCLEOPHILE:
- A “nucleus lover”
- Species that donates an electron pair to an electrophile
- Nucleophiles are also Lewis bases
- Alkenes (the π bond) behave as nucleophiles
Remember that the nucleus has a positive charge, so nucleophiles
have negative electron density and are in search of a positive
charge source
 ELECTROPHILE:
- An “electron lover”
- Species that accepts an electron pair from a nucleophile
- Electrophiles are also Lewis acids
Electrophiles lack electron density and are in search of a negative
electron source
 ELECTROPHILIC ADDITION REACTION:
- The addition of an electrophile to an alkene to yield a
saturated product
- 2 e─ from the π bond form a new σ bond
- A carbocation intermediate is formed
Reaction Mechanism:
H
Br
Br
H3C
H3C
C
H
H
C
H
The electrophile HBr is
attacked by the nucleophile
alkene. A new C – H σ bond
& carbocation intermediate
form.
H3C
C
H
C
H3C
H
The carbocation is now
attacked by the Br─
nucleophile. A C – Br bond
is formed
H
Br
H3C
H3C
C
H
C
H
The neutral addition product
II. ORIENTATION OF ELECTROPHILIC ADDITION:
MARKOVNIKOV’S RULE
* In this section, we will examine how unsymetrically
substituted alkenes give rise to a single addition product even
though two possible products can form. We say that such
reactions are regiospecific when only one of the two possible
orientations occur.
 REGIOSPECIFIC:
- Describes a reaction that occurs when an unsymmetrical
reactant yields a specific product (addition at a specific
“region”) rather than a mixture of products
Cl
H3C
C
H3C
A
CH2 +
B
HCl
H3C
C
A
H3C
H
H3C
C
B
Cl
H
H
H
H3C
C
C
A
B
H
H
2-chloro-2-methylpropane
1-chloro-2-methylpropane
(only product)
(NOT formed)
 MARKOVNIKOV’S RULE:
- In the addition of HY to an alkene, the H attaches to the
carbon w/ fewer alkyl substituents and the Y attaches to the
carbon w/ more alkyl substituents
- REASON: In the 1st step of the addition, the C – H bond
forms and a carbocation results. If the H adds to the less
substituted carbon, the more stable carbocation intermediate
will form.
Cl
H
Cl
H3C
C
B
H3C
C
H3C
A
CH2 +
HCl
H3C
C
A
H3C
H
-
C
A
H
H3C
H
C
B
H
H
3o carbocation;
2-chloro-2-methylpropane
more stable
(only product)
B
H
H3C
C
A
H3C
Cl
H
Cl
C
B
H
H
-
H3C
H3C
C
C
A
B
H
H
1o carbocation;
1-chloro-2-methylpropane
less stable
(NOT formed)
III. FORMATION OF HALIDES: ADDITION OF HX TO ALKENES
* When HX (X = halogen) is added to an alkene, a halocarbon
(halide) forms. This reaction occurs through a carbocation
intermediate and follows Markovnikov’s Rule.
IV. FORMATION OF 1,2-DIHALIDES: ADDITION OF X2 TO ALKENES
* When X2 (X = halogen) is added to an alkene, a dihalide
forms. This reaction occurs through a carbocation
intermediate. Markovnikov’s Rule does not apply here because
addition of X occurs at both alkene carbon atoms. Bromine
and chlorine both add readily to alkenes. Fluorine is too
reactive to control in most laboratory situations and iodine
does not react with most alkenes.
H
Cl
Cl
C
C
H
H
H
1,2-dichloroethane
Approximately 6 million tons per year of
1,2-dichloroethane are synthesized by
addition of Cl2 to ethylene. The product
is used in the manufacture of polyvinyl
chloride (PVC).
V. FORMATION OF HALOHYDRINS
* When alkenes add HO – Cl or HO – Br under suitable
contions, a 1,2-halo alcohol or halohydrin forms. Formation of
the halohydrin does not occur by direct addition of HOBr or
HOCl, but rather by the addition of Cl2 or Br2 in the presence
of water. Because the halogen adds first, the
-OH addition site follows Markovnikov’s Rule.
 HALOHYDRIN:
- a “halo alcohol” : - OH and –X groups on adjacent carbons
Overview of Reaction:
HO
Cl2
H2O
Cl
addition of -OH occurs at
the 2o carbon of the alkene
Reaction Mechanism:
The reaction occurs via addition of X (from X2), followed by
addition of H2O, and finally deprotination of water to yield – OH.
*See board for mechanism.
VI. FORMATION OF ALCOHOLS:
ADDITION OF WATER TO ALKENES
* Water adds to alkenes to yield alcohols in a process known as
hydration. Depending on the reaction conditions used to form
the alcohol, either the Markovnikov or anti-Markovnikov
product can be selected for.
HO
H2O
H+
OH
1. BH3
2. H2O2, OH-
Markovnikov
Product
anti- Markovnikov
Product
 HYDRATION REACTION:
- a reaction in which water is added
A. Acid- Catalyzed Alcohol Formation: Markovnikov Product
- Reaction of an alkene w/ H2O in the presence of acid to form
alcohol
- Yields Markonikov product
- Occurs via carbocation intermediate (electrophilic addition
reaction mechansim)
* See board for mechanism
B. Hydroboration Alcohol Formation: anti - Markovnikov Product
- Reaction of an alkene w/ BH3 (borane) in the presence of
H2O2/ OH─ to form alcohol
- Yields anti - Markonikov product
- Addition of BH3 occurs in a single step via a 4-membered
ring where B forms bond to less substituted carbon
- BH3 is then substituted by OH ─ to form the alcohol
* See board for mechanism
VII. FORMATION OF DIOLS: HYDROXYLATION
* Hydroxylation of of an alkene occurs when an –OH group
adds to each of the alkene carbons. The product is called a
diol (short for “di – alcohol”), which is also known as a glycol,
a term seen quite often in reference to boimolecules. The
reaction does not occur via the traditional electrophilic
addition mechanism because no carbocation is formed.
C
C
1. OsO4
2. NaHSO3
an alkene
HO
C
a diol
OH
C
 HYDROXYLATION:
- a reaction in which – OH is added to both alkene carbons
 DIOL:
- a dialcohol (2 – OH groups)
Alkene hydroxylation does not involve a carbocation intermediate
but rather is thought to occur through an intermediate cyclic osmate
formed by the addition of OsO4 to the alkene. The cyclic osmate is
then cleaved (broken) in a seperate step using aqueous sodium
bisulfate, NaHSO3.
H3C
CH3
CH3
alkene
O
1. OsO4
OH
2. NaHSO3
Os
O
H3C
O
O
H3C
cyclic osmate
intermediate
OH
H3C
diol
VIII. FORMATION OF CARBONYLS: ALKENE CLEAVAGE
* In the alkene addition reactions we have seen thus far, the
carbon-carbon double bond has been converted to a single
bond but the carbon skeleton has remained intact. There are
however powerful oxidizing agents that will cleave or break C
= C bonds and produce two fragments.
O3
C
1
C
2
Zn, H3O+
+
C
1
O
O
C
2
While we won’t go into much detail about the specific mechanism of
this reaction, it is important to note that it does not occur via a
carbocation intermediate. Rather, it forms a cyclic intermediate
similar to that seen for hydroxylation. This reaction is of special
interest because it is an example of an organic oxidation/ reduction
reaction. Addition of O3 (ozone) forms oxidation product called an
ozonide. Oxidation products show an increase in bonds to oxygen.
The ozonide, which is explosive and therefore never isolated, is then
reduced (reduction in the number of bonds to oxygen) to form the
carbonyl compounds.
O
O3
C
C
O
C
oxidation
Zn, H3O+
C
O
step
An ozonide
reduction
step
+
C
O
O
C