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CH221 CLASS 13
CHAPTER 7: ALKENES – REACTIONS AND SYNTHESIS
Synopsis. Class 13 considers some important alkene addition reactions, from
both the mechanistic and the synthetic viewpoint. Halogenation, halohydrin
formation and hydration reactions are all discussed here. Hydration focuses on
the oxymercuration and hydroboration /oxidation procedures. Orientation
(regioselectivity) and stereoselectivity are discussed for each addition reaction
and are closely compared for the two kinds of hydration procedures.
Introduction
Electrophilic addition reactions of alkenes are useful, making possible the
synthesis of many types of compounds, as illustrated below. Some of these
addition reactions are easily reversed and thus can be used to prepare alkenes.
The most common of these are indicated by broken back-arrows in the diagram
below and are discussed briefly on the next page.
Synthesis of Alkenes – a Preview of Elimination
Elimination is the opposite of addition and it has already been stated that alkenes
can be produced by dehydrohalogenation of alkyl halides and by dehydration of
alcohols. Although these are not necessarily the best routes to alkenes, two
examples are considered here briefly, as a contrast to addition and as a
preliminary to chapters 11 and 17 (Organic Chemistry II).
E2 elimination of HHal from alkyl halides
Br
CH3
H
KOH
+
KBr
+
H2O
CH3
C2H5OH
Note the anti (or trans) stereoselectivity of elimination: this is normal for E2 reactions
Note also the orientation (regioselectivity) of elimination, to give the more highly substituted
(more stable) alkene product
E1 elimination of H2O from tertiary alcohols
conc H2SO4,
CH3
H2O
OH
THF 50oC
CH3
+
H2O
Note the orientation (regioselectivity) of elimination, to give the more highly substituted
(more stable) alkene product
Addition of Halogens to Alkenes
Alkenes react easily with Cl2 and Br2 to form 1,2-dihalides: F2 is too reactive and
I2 is inert to many alkenes. Non-aqueous solutions of Cl2 or Br2 can be used,
such as bromine in tetrachloromethane.
The major difference between halogenation and hydrohalogenation is that the
former is anti or trans stereoselective, whereas the latter is more or less nonselective, as illustrated by the two examples overleaf.
Br
+
Br2
ONLY
Br
Cyclohexene
trans-1,2-dibromocyclohexane
H
CH3
+
CH3
CH3 and
Br
HBr
H
Br
CH3
CH3
CH3
1,2-dimethylcyclohexene
cis
trans
1-bromo-1,2-dimethylcyclohexanes
Information like this led to the proposal of a bridged (“bromonium”) cation as
the intermediate, rather than the open cation intermediate that we have already
seen in the addition of hydrogen halide to an alkene.
..
: Br :
-BrC
C
+
Br
Br
+
C
C
+..
Br :
fast
C
C
bromonium ion
This mechanism readily explains the observed stereoselectivity:
In the 1970s, George Olah studied carbocations produced in non-aqueous
“super-acid” media: he provided NMR evidence for the existence of bromonium
ions in liquid sulfur dioxide.
CH3
CH3
C
C
F
+
Br
SbF5
: Br
CH3
H
SbF6CH3
CH3
SO2(l)
CH3
H
SbF5
Halohydrin Formation
If alkenes are treated with an aqueous solution of Cl2 or Br2, a halohydrin results
as the major product: as if HOCl or HOBr has addied across the double bond.
This reaction is also anti stereoselective and is believed to go via a similar
mechanism to halogenation. Like hydrohalogenation, this reaction is Markovnikov
in orientation.
..
: OH2
-H+
H
H
Br
H
Br
HO
H
H
: Br
.. :
H
Br
.. :
+
Step1: attack of Br2 on
Step 2: attack of H2O
C=C
on bromonium ion
In practice, aqueous dimethyl sulfoxide (DMSO) is often used, in order to
solubilize the alkene, and N-bromosuccinimide (NBS) is used as a (safer) supply
of bromine. E.g.
O
CH
CH2
NBr
OH
(NBS)
CH
CH2
Br
O
DMSO/H2O
Phenylethene
(styrene)
2-Bromo-1-phenyl-1-ethanol
Hydration of Alkenes
Addition of water to alkenes to give alcohols is one of the most important
reactions of alkenes. In industry, this is accomplished by the use of strong acid
catalysts and high temperatures, but this is not really of much value in the
laboratory. However, tertiary alcohols can be produced from highly substituted
alkenes using strong acids (especially H2SO4 or H3PO4) (see example below),
but, in general, much better general methods of hydration are the oxymercuration
and hydroboration/oxidation procedures, which are discussed next.
strong H2SO4
CH3
C
CH2 +
CH3
H+
CH3
+
C
H2O
OH
C
CH3
CH3
Methylpropene
CH3
-H+
CH3
CH3
2-Methyl-2-propanol
3o carbocation
Hydration of Alkenes by Oxymercuration
Treatment of alkenes with mercury (II) acetate in aqueous tetrahydrofuran (THF),
followed by reduction with sodium borohydride (NaBH4), leads to alcohols. The
Overall addition is Markovnikov in orientation (regioselectivity), as illustrated by
the example below.
H
CH3
1. Hg(OAc)2,
THF(aq)
2. NaBH4
CH3
OH
The likely mechanism for oxymercuration is shown below, where it can be seen
that Markovnikov addition is a natural consequence of such a pathway.
Hydration of Alkenes by Hydroboration/Oxidation
One of the most useful methods of preparing alcohols from alkenes is
hydroboration/oxidation, first described by H C Brown in 1959. Brown originally
used diborane (B2H6 – a reactive gas), prepared in an inert atmosphere from
NaBH4 and BF3. Nowadays, BH3 adducts are commonly used, especially
BH3.THF. See below for details.
highly reactive 3-center,
2 electron bonds
H
H
electron deficient
(electrophilic)
H
B
B
H
B2H6
2 :O:
+
(THF)
H
H
2 H3B
+
O:
diborane
("BH3.THF")
Other "BH3 adducts" include
H3B
+
N(CH3)3 , H3B
N +
and H3B
+
S(CH3)2
BH3 adducts can react three times with less substituted alkenes to give firstly
mono- and dialkylboranes and finally trialkylboranes, by successive addition of
B-H across C=C. This procedure is called hydroboration:
More highly substituted alkenes tend to produce mono- or dialkylboranes, which
themselves can be used as hydroboration reagents for less substituted alkenes.
Alkylboranes usually easily undergo an oxidation reaction with alkaline hydrogen
peroxide to give the corresponding alcohol:
H2O2/OH-
B
3 HO
+ borate salts
3
Regioselectivity (Orientation) and Stereochemistry; the Mechanism of
Hydroboration/Oxidation
One of the most useful features of the hydroboration/oxidation of alkenes to
produce alcohols is the regioselectivity: the anti-markovnikov (less highly
substituted) alcohol is produced:
Additionally, the overall stereochemistry of addition of H2O to C=C is syn.
Both these important aspects are explained by the accepted mechanism of the
reaction, as illustrated below, for 1-methylcyclopentene.
Step 1: addition of BH3 to form the alkylborane intermediate
Step 2: oxidation of the alkylborane intermediate
The second stage begins by an attack of hydroperoxide anion on the
organoborane. The resulting anion rearranges by BO migration of R to form
the alcohol, with conservation of the configuration of R. In simple terms, the
second stage leads to direct stereochemical replacement of the boron moiety by
OH, giving
H
HO
CH3
H
Class Questions
1. Predict the alcohols that would be produced from the following alkenes.
CH3
(a)
CH3
C
OH H
1. Hg(OAc)2/H2O
CH
CH2
CH3
CH3
CH2
CH3
Markovnikov addition
OH
1. BH3.THF
CH3
CH
CH3
2. NaBH4
(b)
C
H
CH3
2. H2O2/OH-
Anti-Markovnikov addition
2. What alkenes might the following be prepared from, by oxymercuration?
OH
OH
(a)
(b)
CH3CCH2CH2CH2CH3
CH3
CH3
CH2
C
CH2CH2CH2CH3
or
(CH3)2C
CHCH2CH2CH3
3. What alkenes may be used to prepare the following alcohols by
hydroboration/oxidation?
(a)
(CH3)2CHCHCH3
(b)
CH2OH
OH
(CH3)2C
CHCH3
CH2