Reactions
of Alkenes
Unit 7
7-1
Characteristic Reactions
D escrip tive N ame(s )
React ion
C C
+
HCl
( HX)
C C
+
H2 O
C C
+
Br2
( X2 )
C C
+
Br2
( X2 )
H
C C
Cl (X)
H
C C
OH
(X) Br
C C
Br (X)
H2 O
HO
C C
Br (X)
Hydrochlorin ation
(hydrohalogen ation )
Hydration
Bromin ation
(h alogenation)
Bromo(halo)hydrin
formation
7-2
Characteristic Reactions
C C
+ Hg(OAc) 2
C C
+
BH3
C C
+
OsO4
C C
+
H2
H2 O
HgOAc
Oxymercuration
C C
HO
C C
H BH2
Hydrob oration
C C
HO OH
D iol formation
(oxidation)
C C
H H
Hydrogenation
(reduction )
7-3
Reaction Mechanisms
A
reaction mechanism describes how a reaction
occurs
• which bonds are broken and which new ones are
formed
• the order and relative rates of the various bondbreaking and bond-forming steps
• if in solution, the role of the solvent
• if there is a catalyst, the role of a catalyst
• the position of all atoms and energy of the entire
system during the reaction
7-4
Developing a Reaction Mechanism

How it is done
• design experiments to reveal details of a particular chemical
reaction
• propose a set or sets of steps that might account for the overall
transformation
• a mechanism becomes established when it is shown to be
consistent with every test that can be devised
• this does mean that the mechanism is correct, only that it is the
best explanation we are able to devise
7-5
Why Mechanisms?
• they are the framework within which to organize
descriptive chemistry
• they provide an intellectual satisfaction derived from
constructing models that accurately reflect the
behavior of chemical systems
• they are tools with which to search for new information
and new understanding
7-6
Electrophilic Additions
•
•
•
•
•
hydrohalogenation using HCl, HBr, HI
hydration using H2O in the presence of H2SO4
halogenation using Cl2, Br2
halohydrination using HOCl, HOBr
oxymercuration using Hg(OAc)2, H2O followed by
reduction
7-7
Addition of HX
 Carried
out with pure reagents or in a polar
solvent such as acetic acid
Br H
CH3 CH=CH2
Propene
+ HBr
or
 It
H
Br
CH3 CH-CH2 + CH3 CH-CH2
2-Bromoprop ane 1-Bromopropane
(not obs erved)
is a two-step mechanism:
• The first step is the slow, rate-determining step.
• The second step is fast.
7-8
Addition of HX
 Addition
is regioselective
• regioselective reaction: an addition or substitution
reaction in which one of two or more possible
products is formed in preference to all others that
might be formed
• Markovnikov’s rule: in the addition of HX, H2O, or ROH
to an alkene, H adds to the carbon of the double bond
having the greater number of hydrogens
7-9
Addition of H2O
• addition of water is called hydration
• acid-catalyzed hydration of an alkene is regioselective;
hydrogen adds preferentially to the less substituted
carbon of the double bond
• HOH adds in accordance with Markovnikov’s rule
CH3 CH=CH2 + H2 O
Propene
CH3
CH3 C=CH2 + H2 O
2-Methylprop ene
H2 SO4
OH H
CH3 CH-CH2
2-Propanol
CH3
CH3 C-CH2
HO H
2-Methyl-2-propanol
H2 SO4
7-10
Addition of Cl2 and Br2
• carried out with either the pure reagents or in an inert
solvent such as CH2Cl2
Br Br
CH3 CH=CHCH3
2-Buten e
+
Br2
CH2 Cl2
CH3 CH-CHCH3
2,3-D ib romob utane
• addition of bromine or chlorine to a cycloalkene gives
a trans-dihalocycloalkane
Br
+
Br2
Cyclohexen e
CH2 Cl2
Br
+
Br
Br
trans-1,2-D ibromocyclohexane
(a racemic mixture)
• addition occurs with anti stereoselectivity; halogen
atoms add from the opposite face of the double bond
• we will discuss this selectivity in detail in Section 6.7
7-11
Addition of HOCl and HOBr
 Treatment
of an alkene with Br2 or Cl2 in water
forms a halohydrin
 Halohydrin: a compound containing -OH and -X
on adjacent carbons
CH3 CH=CH2 + Cl2 + H2 O
Propene
HO Cl
CH3 CH-CH2
+ HCl
1-Chloro-2-p ropanol
(a ch loroh yd rin )
7-12
Oxymercuration/Reduction
 Oxymercuration
followed by reduction results in
hydration of a carbon-carbon double bond
• oxymercuration
OH
+ Hg(OAc) 2 + H2 O
1-Pen tene
O
+ CH3 COH
HgOAc
An organ omercury
comp ou nd
Mercu ry(II)
acetate
A cetic
acid
• reduction
OH
OH
NaBH4
HgOAc
H
2-Pen tanol
O
+ CH3 COH + Hg
Acetic acid
7-13
Hydroboration/Oxidation
 Hydroboration:
the addition of borane, BH3, to an
alkene to form a trialkylborane
H
H B
CH2 CH3
+ 3 CH2 = CH2
H
Borane
 Borane
CH3 CH2 B
CH2 CH3
Triethylborane
(a trialkylborane)
dimerizes to diborane, B2H6
2 BH 3
B2 H6
Borane
Diborane
7-14
Oxidation/Reduction
 Oxidation:
the loss of electrons
• alternatively, the loss of H, the gain of O, or both
 Reduction:
the gain of electrons
• alternatively, the gain of H, the loss of O, or both
 Recognize
using a balanced half-reaction
1. write a half-reaction showing one reactant and its
product(s)
2. complete a material balance; use H2O and H+ in acid
solution, use H2O and OH- in basic solution
3. complete a charge balance using electrons, e-
7-15
Oxidation/Reduction
• three balanced half-reactions
OH
CH 3 CH= CH2 + H2 O
Propene
CH3 CHCH3
2-Propanol
HO OH
CH 3 CH= CH2 + 2 H 2 O
Propene
CH 3 CH= CH2 + 2 H+ + 2 e Propene
CH3 CHCH 2 + 2 H+ + 2 e 1,2-Propanediol
CH3 CH 2 CH 3
Propane
7-16
Reduction of Alkenes
 Most
alkenes react with H2 in the presence of a
transition metal catalyst to give alkanes
+ H2
Cyclohexene
Pd
25°C, 3 atm
Cyclohexane
• commonly used catalysts are Pt, Pd, Ru, and Ni
• the process is called catalytic reduction or,
alternatively, catalytic hydrogenation
7-17