William Brown
Thomas Poon
www.wiley.com/college/brown
Chapter Five
Reactions of Alkenes and Alkynes
Characteristic Reactions of Alkenes
.
Reaction Mechanism
• A reaction mechanism describes how a reaction
occurs.
– Which bonds are broken and which new ones are
formed.
– The order in which bond-breaking and bond-forming
steps take place.
– The role of the catalyst (if any is present).
– The energy of the entire system during the reaction.
Energy Diagram
• Energy diagram: A graph showing the changes in
energy that occur during a chemical reaction;
energy is plotted in the y-axis and progress of the
reaction is plotted in the x-axis.
• Reaction coordinate: A measure of the progress
of a reaction. Plotted on the x-axis in an energy
diagram reaction.
• Heat of reaction H:The difference in energy
between reactants and products.
– Exothermic: The products of a reaction are lower in
energy than the reactants; heat is released.
– Endothermic: The products of a reaction are higher in
energy than the reactants; heat is absorbed.
Energy Diagram
Figure 5.1 An energy diagram for a one-step reaction
between C and A-B to give C-A and B.
Energy Diagram
• Transition state: An unstable species of maximum
energy formed during the course of a reaction; a
maximum on an energy diagram.
• Activation energy Ea: The difference in energy
between the reactants and the transition state.
– Ea determines the rate of reaction.
– If Ea is large very few molecular collisions occur with
sufficient energy to reach the transition state, and the
reaction is slow.
– If Ea is small many collisions generate sufficient energy
to reach the transition state, and the reaction is fast.
Energy Diagram
Figure 5.2 An energy diagram for a two-step reaction
involving formation of an intermediate.
Developing a Reaction Mechanism
• Design experiments to reveal the 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 doesn’t mean that the mechanism is correct,
only that it is the best explanation we are able to
devise.
Why Mechanisms?
Mechanisms provide:
• A theoretical framework within which to organize
descriptive chemistry.
• An intellectual satisfaction derived from
constructing models that accurately reflect the
behavior of chemical systems.
• A tool with which to search for new information
and new understanding.
Electrophilic Additions to Alkenes
• Addition of hydrogen halides (HCl, HBr, HI)
– Hydrohalogenation
• Addition of water (H2O/H2SO4)
– Acid-Catalyzed hydration
• Addition of halogens (Cl2, Br2)
–Halogenation
Addition of HX
• Carried out with the pure reagents or in a polar
solvent such as acetic acid.
H
Cl
CH2 =CH2 + HCl
Ethylene
CH2 -CH2
Chloroeth ane
• Addition is regioselective.
– Regioselective reaction: A reaction in which one direction of
bond-forming or bond-breaking occurs in preference to all
other directions.
Cl H
H Cl
CH3 CH=CH2 + HCl
Prop ene
CH3 CH-CH2 + CH3 CH-CH2
2-Chloropropane 1-Ch loroprop ane
(not ob served )
– Markovnikov’s rule: In additions of HX to a double bond, H
adds to the carbon with the greater number of hydrogens
bonded to it.
Addition of HCl to 2-Butene
• A two-step mechanism
– Step 1: Formation of a sec-butyl cation, a 2° carbocation intermediate.
– Step 2: Reaction of the sec-butyl cation (an electrophile) with chloride ion
(a nucleophile) completes the reaction.
HCl + 2-Butene
– Figure 5.4 An energy diagram for the two-step
addition of HCl to 2-butene. The reaction is
exothermic.
Carbocations
• Carbocation: A species containing a carbon atom that
has only six electrons in its valence shell and bears a
positive charge.
• Carbocations are:
– Classified as 1°, 2°, or 3° depending on the
number of carbons bonded to the carbon bearing the
positive charge.
– Electrophile: that is, they are electron-loving.
– Lewis acid; that is, they are electron-pair acceptors.
Carbocations
– Bond angles about the positively charged carbon are
approximately 120°.
– Carbon uses sp2 hybrid orbitals to form sigma bonds
to the three attached groups.
– The unhybridized 2p orbital lies perpendicular to the
sigma bond framework and contains no electrons.
Carbocations
– 3°carbocation is more stable than a 2° carbocation,
and requires a lower activation energy for its
formation.
– 2°carbocation is, in turn, more stable than a 1°
carbocation, and requires a lower activation energy
for its formation.
– Methyl and 1°carbocations are so unstable that they
are never observed in solution.
Relative Stability of Carbocations
• Inductive effect: The polarization of the electron
density of a covalent bond as a result of the
electronegativity of a nearby atom.
– The electronegativity of a carbon atom bearing a
positive charge exerts an electron-withdrawing
inductive effect that polarizes electrons of adjacent
sigma bonds toward it.
– the positive charge of a carbocation is not localized on
the trivalent carbon, but rather is delocalized over
nearby atoms as well.
– The larger the area over which the positive charge is
delocalized, the greater the stability of the cation.
Relative Stability of
Carbocations
– Figure 5.5 Delocalization of positive charge by the
electron-withdrawing inductive effect of the
positively charged trivalent carbon according to
molecular orbital calculations.
Addition of H2O to an Alkene
• Addition of H2O to an alkene is called hydration.
– Acid-catalyzed hydration of an alkene is regioselective:
hydrogen adds preferentially to the less substituted
carbon of the double bond. Thus H-OH adds to
alkenes in accordance with Markovnikov’s rule.
CH3 CH=CH2 + H2 O
Prop ene
CH3
CH3 C=CH2 + H2 O
2-Methylprop ene
H2 SO4
H2 SO4
OH H
CH3 CH-CH2
2-Prop anol
CH3
CH3 C-CH2
HO H
2-Methyl-2-propanol
Step 1: Proton transfer to the alkene gives a carbocation.
slow , rate
determining
+
CH3 CH=CH2 + H O H
+
CH3 CHCH3
A 2o carb ocation
intermediate
H
+
O H
H
Step 2: A Lewis acid-base reaction gives an oxonium ion.
+
CH3 CHCH3 + O-H
fast
H
CH3 CHCH3
O+
H
H
An oxonium ion
Step 3: Proton transfer to solvent gives the alcohol.
CH3 CHCH3 + H-O-H
+
O
H
H
fas t
+
CH3 CHCH3 + H-O-H
O
H
H
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-Butene
+
Br2
CH2 Cl2
CH3 CH-CHCH3
2,3-Dibromobutane
Addition of Cl2 and Br2
– Addition is stereoselective.
– Stereoselective reaction: A reaction in which one
stereoisomer is formed or destroyed in preference to
all others that might be formed or destroyed.
– Addition to a cycloalkene, for example, gives only a
trans product. The reaction occurs with
• anti stereoselectivity.
Br
+ Br2
Cyclohexen e
CH2 Cl2
Br
t rans -1,2-D ib romocycloh exane
Addition of Cl2 and Br2
– Step 1: Formation of a bromonium ion intermediate,
– Step 2: Halide ion opens the three-membered ring.
Addition of Cl2 and Br2
• Anti coplanar addition to a cyclohexene
corresponds to trans-diaxial addition.
Br
+ Br2
Br
Br
tran s diaxial
(les s stable)
Br
tran s diequ atorial
(more stable)
Carbocation Rearrangements
• Product of electrophilic addition to an alkene involves rupture of a
pi bond and formation of two new sigma bonds in its place. In the
following addition, however, only 17% of the expected product is
formed.
• Rearrangement: A reaction in which the product(s) have a
different connectivity of atoms than that in the starting material.
Carbocation Rearrangements
• Typically either an alkyl group or a hydrogen
atom migrates with its bonding electrons from an
adjacent atom to an electron-deficient atom as
illustrated in the following mechanism.
• The key step in this type of rearrangement is
called a 1,2-shift.
Carbocation Rearrangements
• Step 1: Proton transfer from the HCl to the alkene
to give a 2° carbocation intermediate.
+
+
H
Cl
+
Cl
A 2° carbocation
intermediate
3,3-Dimethy-1-butene
• Step 2: Migration of a methyl group with its
bonding electrons from the adjacent carbon gives
a more stable 3°carbocation.
The two electrons in
this bond move to
the electron-deficient
carbocation.
+
A 2° carbocation
intermediate
+
A 3° carbocation
intermediate
Carbocation Rearrangements
• Step 3: Reaction of the 3°carbocation (an
electrophile and a Lewis acid) with chloride ion
(a nucleophile and a Lewis base) gives the
rearranged product.
+
Cl
+
A 3° carbocation
inte rme diate
Cl
Carbocation Rearrangements
• Rearrangements also occur in the acid-catalyzed
hydration of alkenes, especially where the
carbocation formed in the first step can rearrange
to a more stable carbocation.
This H migrates
to an adjacent
carbon
CH3
H3O+
CH3 CHCH= CH2 + H2 O
3-Methyl-1-butene
CH3
CH3 CCH2 CH 3
OH
2-Methyl-2-butanol
Carbocations-Summary
• The carbon bearing a positive charge is sp2
hybridized with bond angles of 120°
• The order of carbocation stability is 3°>2°>1°.
• Carbocations are stabilized by the electronwithdrawing inductive effect of the positively
charged carbon.
• Methyl and primary carbocations are so unstable
that they are never formed in solution.
• Carbocations may undergo rearrangement by a 1,2shift, when the rearranged carbocation is more
stable than the original carbocation. The most
commonly observed pattern is from 2° to 3°.
Carbocations-Summary
• Carbocation intermediates undergo three types
of reactions:
1. Rearrangement by a 1,2-shift to a more stable
carbocation.
2. Addition of nucleophile (e.g halide ion, H2O).
3. Loss of a proton to give an alkene (the reverse of the
first step in both the hydrohalogenation and the
acid-catalyzed hydration of an alkene).
Hydroboration-Oxidation
• The result of hydroboration followed by oxidation
of an alkene is hydration of the carbon-carbon
double bond.
1. BH3
OH
1-Hexene
2. NaOH, H2 O2
1-Hexanol
• Because -H adds to the more substituted carbon
of the double bond and -OH adds to the less
substituted carbon, we refer to the
regiochemistry of hydroboration/oxidation as
anti-Markovnikov hydration.
Hydroboration-Oxidation
• Hydroboration is the addition of BH3 to an alkene to form a
trialkylborane.
H
H B
H
CH 2 =CH 2
Boran e
CH3 CH2
CH3 CH2
H
B CH2 =CH2
H
CH3 CH2
CH2 CH 3
CH 2 =CH2
B
H
D iethylboran e
(a dialkylborane)
Ethylborane
(an alkylborane)
CH 2 CH3
B
CH 2 CH3
Trieth ylborane
(a trialkylborane)
• Borane is most commonly used as a solution of BH3 in
tetrahydrofuran (THF).
Tetrahydrofuran
(THF)
+ B2 H6
2
+ O BH3
••
O
••
••
2
BH3 •THF
Hydroboration-Oxidation
• Hydroboration is both regioselective and syn
stereoselective.
• Regioselectivity: -H adds to the more substituted
carbon and boron adds to the less substituted
carbon of the double bond.
• Stereoselectivity: Boron and -H add to the same
face of the double bond (syn stereoselectivity).
CH3
+ BH3
1-Methylcyclopentene
CH3
H
BR2
+
CH3
H
BR2
H
H
(Syn addition of BH 3)
(R = 2-methylcyclopentyl)
Hydroboration-Oxidation
• Chemists account for the regioselectivity by
proposing the formation of a cyclic four-center
transition state.
• And for the syn stereoselectivity by steric factors.
Boron, the larger part of the reagent, adds to the
less substituted carbon and hydrogen to the more
substituted carbon.
 
H B
H
CH3 CH2 CH2 CH=CH2
B
CH3 CH2 CH2 CH-CH2
Hydroboration-Oxidation
• Trialkylboranes are rarely isolated. Treatment
with alkaline hydrogen peroxide (H2O2/NaOH),
oxidizes a trialkylborane to an alcohol and sodium
borate.
(RO) 3 B
A trialkylborate
+
3NaOH
3ROH
+
Na3 BO3
Sodium
borate
Ozonolysis of an Alkene
• Ozonolysis of an alkene followed by suitable
workup, cleaves the carbon-carbon double bond
and forms two carbonyl (C=O) groups in its place.
Ozonolysis is one of the few organic reactions
that cleaves carbon-carbon double bonds.
CH3
CH3 C=CHCH2 CH3
2-Methyl-2-penten e
1 . O3
2 . ( CH3 ) 2 S
O
O
O
CH3 CCH3 + HCCH2 CH3 +
CH3 -S-CH3
Propanone
Propanal
Dimethylsu lfoxide
(a ketone)
(an ald ehyd e)
(D MSO)
Ozonolysis of an Alkene
• Ozone (18 valence electrons) is strongly
electrophilic.
O
O
O
O
O
O
O
O
O
O
O
O
Ozone is strongly electrophilic
because of the positive formal
charges on the two end oxygens.
• Initial reaction gives an intermediate called a
molozonide.
O
O
O
O
O
O
O
O
O
A molozonide
Ozonolysis of an Alkene
• The intermediate molozonide rearranges to an
ozonide.
• Treatment of the ozonide with dimethylsulfide
gives the final products.
CH3 CH=CHCH3
2-Butene
O3
O
O
O
CH3 CH-CHCH3
A mol ozo nid e
H
H3 C
H
O
C
C
O O
CH3
A n o zoni de
(CH3 ) 2 S
O
2 CH3 CH
A cetal dehy de
Reduction of Alkenes
• Alkenes react with H2 in the presence of a
transition metal catalyst to give alkanes.
– The most commonly used catalysts are Pd, Pt, and Ni.
– The reaction is called catalytic reduction or catalytic
hydrogenation.
+ H2
Cyclohexene
Pd
25°C, 3 atm
Cyclohexane
Reduction of Alkenes
– The most common pattern is syn addition of
hydrogens; both hydrogens add to the same face of
the double bond.
– Catalytic reduction is syn stereoselectivity.
CH3
+ H2
CH3
1,2-D imeth ylcycloh exene
Pt
CH3
CH3
cis-1,2-D imeth ylcyclohexane
Catalytic Reduction of an Alkene
• Figure 5.6 Syn addition of H2 to an alkene
involving a transition metal catalyst.
(a) H2 and the alkene are absorbed on the catalyst.
(b) One H is transferred forming a new C-H bond.
(c) The second H is transferred. The alkane is desorbed.
Heats of Hydrogenation
– Table 5.2 Heats of Hydrogenation for Several Alkenes
Heats of Hydrogenation
• Reduction involves net conversion of a weaker pi
bond to a stronger sigma bond.
• The greater the degree of substitution of a double
bond, the lower its heat of hydrogenation.
– The greater the degree of substitution, the more stable
the double bond.
• The heat of hydrogenation of a trans alkene is lower
than that of the isomeric cis alkene.
– A trans alkene is more stable than its isomeric cis alkene.
– The difference is due to nonbonded interaction strain in
the cis alkene.
Heats of Hydrogenation
– Figure 5.7 Heats of hydrogenation of cis-2-butene and
trans-2-butene.
– trans-2-butene is more stable than cis-2-butene by
1.0 kcal/mol
The Acidity of Terminal Alkynes
• One of the major differences between the
chemistry of alkanes, alkenes, and alkynes is that
terminal alkynes are weak acids.
• Table 5.3 Acidity of Alkanes, Alkenes, and
Alkynes.
The Acidity of Terminal Alkynes
• Treatment of a 1-alkyne with a very strong base
such as sodium amide, NaNH2, converts the
alkyne to an acetylide anion.
H-C C-H +
Acetylene
pKa 25
(Stronger
acid)
NH 2
Sodium
Amide
(Stronger
base)
H-C C:- + NH3
Aceylide
anion
(Weak er
base)
Keq = 1013
Ammonia
pKa 38
(Weaker
acid)
• Note that hydroxide ion is not a strong enough
base to form an acetylide anion.
H-C C-H +
OH
pKa 25
(Weaker (Weaker
base)
acid)
H-C C:- + H–OH
(Stronger
base)
pKa 15.7
(Stronger
acid)
Acetylide Anions in Synthesis
• An acetylide anion is both a strong base and a
nucleophile. It can donate a pair of electrons to an
electrophilic carbon atom and form a new carboncarbon bond.
H-C C:- +
H +
H C Cl
H
nucleophilic
substitution
H-C C CH3 +
Cl
• In this example, the electrophile is the partially
positive carbon of chloromethane. As the new
carbon-carbon bond is formed, the carbon-halogen
bond is broken.
• Because an alkyl group is added to the original
alkyne, this reaction is called alkylation.
Acetylide Anions in Synthesis
• The importance of alkylation of acetylide anions
is that it can be used to create larger carbon
skeletons.
1. N aN H 2
CH 3 CH 2C CH
HC CH
2. CH3 CH 2 Br
Acetylene
1-Butyne
3. N aN H 2
4. CH3 CH 2 CH 2 Br
CH 3 CH 2C CCH 2CH 2CH 3
3-Heptyne
• For reasons we will discuss fully in Chapter 7, this
type of alkylation is successful only for methyl
and primary alkyl halides (CH3X and RCH2X).
Acid-Catalyzed Hydration of Alkynes
• In the presence of concentrated sulfuric acid and
Hg(II) salts, alkynes undergo hydration in
accordance with Markovnikov’s rule.
– The initial product is an enol, a compound containing
an hydroxyl group bonded to a carbon-carbon double
bond.
– The enol is in equilibrium with a more stable keto
form. This equilibration is called keto-enol
tautomerism.
OH
O
CH3 C CH + H2 O
Propyne
H2 SO4
Hg SO4
CH3 C=CH2
Propen-2-ol
(an enol)
CH3 CCH3
Propanone
(Acetone)
Reduction of Alkynes
• Treatment of an alkyne with H2 in the presence of a
transition metal catalyst results in addition of two
moles of H2 and conversion of the alkyne to an
alkane.
CH3 C CCH3 + 2H2 Pd , Pt, or Ni
3 atm
2-Butyne
CH3 CH2 CH2 CH3
Butane
• By the proper choice of catalyst it is possible to stop
the reaction at the addition of one mole of H2. The
most commonly used catalyst for this purpose is the
Lindlar catalyst.
HC
CH -CH
H
H 3C C C CH 2CH 3
2-Pen tyne
2
Lindlar
caalys
3
2
H
H
cis -2-Pentene
3
Reduction of Alkynes
• Reduction using the Lindlar catalyst is syn
stereoselective.
• Reduction of an alkyne using an alkali metal (Li,
Na, or K) in liquid ammonia is anti stereoselective
and gives a trans alkene.
H 3C C C CH 2 CH 3
2-Pentyne
Li
N H 3(l)
H 3C
H
H
CH 2 CH 3
trans-2-Penene
Reactions of Alkenes
End Chapter 5