Alkene Reactions.
1. Electrophilic Addition. An important feature of alkene
reactivity is an ability to react with a variety of electrophilic reagents,
those reagents attracted to the source of extra electron density. In an
alkene, the 2pz-2pz  bond (a relatively weak covalent bond due to the
poor side-to-side overlap of the p orbitals), serves as such a source. It
is concentrated above and below the molecular plane and is therefore
accessible for the reactions. Formation of variety of acyclic and
cyclic intermediates is possible.
2.
Examples of cyclic intermediates. (Note that the following
alkene is assumed to have reacted :
H
R
'R
H
a. Epoxides. This intermediate is formed in a reaction of an
alkene and a peroxyacid, RCO3H. A protonated epoxide is an
intermediate in formation of diols.
H
O
O+
H
R
H
R'
H
R
H
R'
b. Mercurinium ion. This ion is formed in the reaction of an
alkene and mercuric acetate.
Symmetrical mercurinium ion
As ymmetrical mercurinium ion
-
+
(CH 3CO2 )
 + Hg
(CH3 CO2 )
Hg
H
R
H
R'
H
R
H
 +
R'
If one of the alkene carbons is best able to sustain the positive
charge, the intermediate will be asymmetrical, with only a partial
bond between that carbon and mercury ion. The carbon best able to
sustain the positive charge will bear partial positive charge.
In the nucleophilic opening of the mercurinium ion, this carbon
will be the preferred site of nucleophile attack. Thus, the opening
of the ion is regioselective.
c. Halonium ions. These are formed when an alkene displaces a
halide anion from a halogen molecule. The bromonium ion is
shown. Just like mercurinium ions (see 2b above), asymmetrical
halonium ions exist.
Symmetrical bromonium ion
As ymmetrical bromonium ion
 +
+
Br
Br
H
R
H
H
R
H
R'
 +
R'
d. Oxomanganese anion. This intermediate is formed in
reaction of an alkene and a permanganate anion, MnO4-. Just like an
epoxide, this intermediate is hydrolyzed to a diol.
O
O
Mn
O
H
-
O
H
R
R'
e. Oxoosmium complex. This intermediate is formed in reaction
of an alkene and a osmium tetroxide, OsO4. Just like an epoxides or
oxomanganese anions, this intermediate is hydrolyzed to a diol.
O
O
Os
O
H
O
H
R
R'
3.
Examples of acyclic intermediates.
a. Carbocations. These are formed in reaction of an alkene and
an acidic electrophilic reagent. May be present in electrophilic
additions of other reagents when a particularly stable carbocation is
formed.
H
R
+
H
H
R'
b. Alkyl hydrogen sulfates. These are easily hydrolyzed in
neutral H2O to yield alcohols. They are esters, products of a reaction
of alkenes and concentrated sulfuric acid, when the amount of water
available for hydrolysis of the ester is low, and are the result of
trapping of the intermediate carbocation with the hydrosulfate, a
conjugate base of the sulfuric acid. The ease of hydrolysis of
hydrogen sulfate is entirely due to the fact that the hydrosulfate is a
weak base and hence a good leaving group.
H
R
H
H
R'
O
O
S
H
O
O
c. Organomercuric salts. These are formed in a reaction of an
intermediate mercurinium ion with an H2O nucleophile. They are,
after a loss of a proton, reduced by sodium borohydride, NaBH4, to
yield alcohols.
Hg(OAc)
R
H
H
R'
OH2+
d. Alkylboranes. Formed in a reaction of an alkene with
diborane, (BH3)2. Subsequently oxidized by H2O2/HO- to yield
alcohols.
BH2
H
H
R
H
R'
4. General mechanism of electrophilic addition of acidic
reagents. a. Scheme:
1. Slow, usually reversible protonation of the alkene. The
reverse step, elimination, is effective when the conjugate base of the
acid is strong.
H
R
H
R
H
'R
H
B
H
H
+
+
B-
R'
2. Fast trapping of the carbocation intermediate with a base.
H
H
R
BH
+
H
R'
R
H
H
R'
B
b. Defining characteristics:
i. Acidic reagent must be used. Alkenes are weak bases and
require strong acids for the protonation to be effective.
ii. Rearrangements are common. Just like in substitution and
elimination reactions, the rearrangements reflect the presence of an
carbocation intermediate.
iii. Orientation . Markovnikov addition, with the proton found
on the least substituted carbon, is observed. Again, the most stable
carbocation is produced in Step 1, leading to the positive charge being
developed on the most substituted carbon.
5. Other synthetic reactions involving the electrophilic addition
to a double bond.
a. Acid-catalyzed hydration. Alkenes are easily hydrated by
water in the presence of a strong mineral acid, usually sulfuric, to give
alcohols. Since the formation of the alcohol is the result of trapping
of the carbocation intermediate with the water nucleophile, the
reaction is reversible and rearrangements are possible. Thus, this
reaction is not stereoselective or specific. But, it is regioselective,
since the Markovnikov product is formed, with incoming H+ placed on
the least-substituted carbon giving a most stable carbocation
intermediate.
b. Oxymercuration-demercuration. Alkenes are capable of
adding mercuric acetate yielding cyclic mercurinium ion. If the
reaction is carried out in the presence of moisture, water will serve as
an added nucleophile and open the mercurinium ion to give a
protonated hydroxymercuric salt which is, after a loss of proton,
reduced with NaBH4 to give an alcohol.
+
Hg(OAc)
Hg (OAc)
H
R
Hg(OAc) 2
R
- OAc-
H
R
R
H
R
OH 2+
H
R
+
R
H2O
R
OH 2+
Hg(OAc)
Hg(OAc)
H
H
H
H
-H+
H
R
H
R
OH
H
NaBH 4
H
R
H
R
OH
The opening of the mercurinium ion proceeds in the
Markovnikov fashion, with the incoming H (this time as a hydride )
being placed on the least substituted carbon. The reaction is
regioselective since the water nucleophile will attack the carbon best
able to sustain the positive charge (see 2b above). It is also
diastereoselective and stereospecific since it represents an overall
anti-addition to the double bond.
c. Hydroboration-oxidation. Alkenes are capable of adding
diborane to yield alkylboranes. Diborane is the dimer of a
hypothetical borane, BH3, a structure featuring an unusual 2 electron,
3 center bond , where hydrogen exists as a hydride to satisfy the octet
requirement of boron. Intermediate alkyl borane is not isolated but is
oxidized in situ with alkaline H2O2 to yield an alcohol. As in 5b, this
corresponds to formal hydration of the alkene. Mechanism :
†
H
R
-
H
B
H
H
B2 H6
H
H
THF
R
H
 H
B
+
H
R +
H
R
R
H
H
R
H
B
H
H
OH
H 2O2
H
H
OH
R
R
H
H
H
-
R
R
Note that the TS features a carbon with partial carbocation
character. Thus, BH2 fragment will be directed onto the least
substituted carbon and the hydride onto the most substituted carbon
(in order to stabilize the incipient carbocation). In the final product
(alcohol) the hydroxy group will replace the BH2 group giving rise to
an overall anti-Markovnikov addition of H2O to an alkene. Thus, the
reaction is regioselective. It is also diastereoselective and
stereospecific since it represents an overall syn-addition to the double
bond.
d. Addition of halogens. This reaction is used to prepare vicinal
dihalides.
Br
+
Br
R
H
Br 2
'R
H
R
H
R
H
H
Br
R'
H
R'
-
Br
Only Br2 and Cl2 are reactive towards alkenes by this
mechanism. The evidence for the formation of the halonium ions
include:
i. Stereochemistry. If the acyclic carbocation
intermediate is formed, the reaction is expected to be nonstereoselective or specific, contrary to the observed results.
ii. Formation of mixed addition products. When
competing nucleophiles (in the form of nucleophilic solvents or added
salts) are present during halogen addition, mixed addition products are
obtained. This is known as the effect of added nucleophiles. See 5e
below.
The reaction is regioselective since the incoming nucleophile
will attack the carbon best able to sustain the positive charge (see 2b
and 2c above). It is also diastereoselective and stereospecific since it
represents an overall anti-addition to the double bond.
trans-1,2-dibromo-1-methylcyclopentane
+
Br
Br
CH3
CH3
Br 2
H
+
H
CH3
H
Br Br
e. Formation of mixed addition products.
+
H
H
Cl
Cl
Cl2
H 2O
H
H
H
H
H
H
H
H 2O
H
-H +
H
H
H
H
Cl
H
H
OH 2+
OH
Chlorohydrin
In this case the addition mode is also Markovnikov and it is
diastereoselective and stereospecific, with the halogen of the
halonium ion moved to the least substituted carbon. A variety of
nucleophiles such as water, alcohols, halide or cyanide anions may be
used.
6. Oxidation.
a. Epoxidation-hydroxylation. Alkenes react with peroxyacids
to give another cyclic intermediates - epoxides. These are hydrolyzed
under acidic conditions via protonated epoxide to give diols.
Step 1. Formation of the epoxide; mCPBA = meta-chloroperoxybenzoic acid
O
mCPBA
O
H
C
O
Cl
H
C
O
H
Cl
O
O
O+
H
R
H
R'
H
R'
H
O
H
R
H
R'
OH
OH
+
H
R
H
H+
H
R
R'
R
H2O
H
H
R'
-H+
R
H
OH2+
Step 2. Acid-catalyzed hydration and ring-opening of the epoxide.
H
R'
OH
Note that the reaction represents a anti-addition pathway and is
a diastereoselective and stereospecific route to diols. It is also
regioselective since the incoming water nucleophile will attack the
side of the epoxide best able to sustain positive charge.
b. KMnO4 hydroxylation. Alkenes react with cold alkaline
KMnO4 to form dihydroxy compounds - diols. The reaction involves
formation of the cyclic oxomanganese intermediate that is
subsequently hydrolyzed in alkaline medium to give the desired
product. Note that the reaction represents a syn-addition pathway
and is a diastereoselective and stereospecific route to diols.
O
OH
O
R
Mn
H
O
OH
MnO 4 -
R
OH O
H
-
R
H
R'
H
H
H
R'
R'
c. OsO4 hydroxylation. Alkenes will also react with OsO4 to
form diols. The reaction involves formation of the cyclic osmate ester
intermediate subsequently hydrolyzed in alkaline medium to give the
desired product. Note that the reaction represents a syn-addition
pathway and is a diastereoselective and stereospecific route to diols.
O
OH
O
R
Os
H
OH
O
OsO4
O
H
R
OH H
R
H
R'
H
H
R'
R'
d. Oxidation with hot KMnO4 . This reagent breaks the double
bond, and the reaction products depend on the structure of the alkene.
H2 CH3 C
H
MnO4 -
O
H
+
H2 CH3 C
HOT
H2 O +
CO2
OH
H
O
MnO4
HOT
CH(CH 3 )2
-
HO
CH(CH 3 )2
O
Note that the purpose of the reagent is to replace the double bond
with two oxygen atoms. Therefore, one may expect to see two types
of products: aldehydes and ketones. Hot KMnO4 is a harsh oxidizer.
As the result, the oxidation is carried out further with aldehydes being
oxidized to carboxylic acids and formaldehyde (CH2O) to CO2 and
water. Ketones are resistant to oxidation and are therefore found as
products under these conditions. Both acyclic and cyclic alkenes are
subject to degradative ozonolysis.
e. Degradative ozonolysis. Ozone, O3, adds to double bonds
forming a number of intermediates the most important of which is
ozonide. This species is reduced in situ by Zn in the presence of
water to give aldehydes and/or ketones depending on the degree of
substitution of the double bond. Each double bond yields two
fragments if it is substituted asymmetrically. Both acyclic and cyclic
alkenes are subject to degradative ozonolysis.
O
H2 CH3 C
H
H
H2 CH3 C
O3
H
CH3
CH3 O
H
O
O
Zn
H2O
+
H2 CH3 C
H3 C
H
H
O
O
O3
CH(CH 3 )2
O
O
O
CH(CH 3 )2
Zn
H2O
H
CH(CH 3 )2
O