William Brown Thomas Poon www.wiley.com/college/brown Chapter
Five Reactions of Alkenes and Alkynes
Slide 2
Characteristic Reactions of Alkenes.
Slide 3
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.
Slide 4
Energy Diagram 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: Reaction coordinate: A measure of
the progress of a reaction. Plotted on the x-axis in an energy
diagram reaction. Heat of reaction H: Heat of reaction H:The
difference in energy between reactants and products. Exothermic:
Exothermic: The products of a reaction are lower in energy than the
reactants; heat is released. Endothermic: Endothermic: The products
of a reaction are higher in energy than the reactants; heat is
absorbed.
Slide 5
Energy Diagram Figure 5.1 An energy diagram for a one-step
reaction between C and A-B to give C-A and B.
Slide 6
Energy Diagram Transition state: Transition state: An unstable
species of maximum energy formed during the course of a reaction; a
maximum on an energy diagram. Activation energy E a : Activation
energy E a : The difference in energy between the reactants and the
transition state. E a E a determines the rate of reaction. If E a
is large If E a is large very few molecular collisions occur with
sufficient energy to reach the transition state, and the reaction
is slow. If E a is small If E a is small many collisions generate
sufficient energy to reach the transition state, and the reaction
is fast.
Slide 7
Energy Diagram Figure 5.2 An energy diagram for a two-step
reaction involving formation of an intermediate.
Slide 8
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 doesnt mean that the
mechanism is correct, only that it is the best explanation we are
able to devise.
Slide 9
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.
Slide 10
Electrophilic Additions to Alkenes Addition of hydrogen halides
(HCl, HBr, HI) Hydrohalogenation Addition of water (H 2 O/H 2 SO 4
) Acid-Catalyzed hydration Addition of halogens (Cl 2, Br 2 )
Halogenation
Slide 11
Addition of HX Carried out with the pure reagents or in a polar
solvent such as acetic acid. Addition is regioselective.
Regioselective reaction: Regioselective reaction: A reaction in
which one direction of bond-forming or bond-breaking occurs in
preference to all other directions. Markovnikovs rule: Markovnikovs
rule: In additions of HX to a double bond, H adds to the carbon
with the greater number of hydrogens bonded to it.
Slide 12
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.
Slide 13
HCl + 2-Butene Figure 5.4 An energy diagram for the two-step
addition of HCl to 2-butene. The reaction is exothermic.
Slide 14
Carbocations Carbocation: Carbocation: A species containing a
carbon atom that has only six electrons in its valence shell and
bears a positive charge. Carbocations are: Carbocations are:
Classified as 1, 2, or 3 depending on the number of carbons bonded
to the carbon bearing the positive charge. Electrophile:
Electrophile: that is, they are electron-loving. Lewis acid Lewis
acid; that is, they are electron-pair acceptors.
Slide 15
Carbocations Bond angles about the positively charged carbon
are approximately 120. Carbon uses sp 2 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.
Slide 16
Carbocations 3carbocation is more stable than a 2 carbocation,
and requires a lower activation energy for its formation.
2carbocation is, in turn, more stable than a 1 carbocation, and
requires a lower activation energy for its formation. Methyl and
1carbocations are so unstable that they are never observed in
solution.
Slide 17
Relative Stability of Carbocations Inductive effect: 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.
Slide 18
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.
Slide 19
Addition of H 2 O to an Alkene hydration. Addition of H 2 O to
an alkene is called hydration. regioselective: 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 Markovnikovs
rule.
Slide 20
Step 1: Proton transfer to the alkene gives a carbocation. Step
2: A Lewis acid-base reaction gives an oxonium ion. Step 3: Proton
transfer to solvent gives the alcohol.
Slide 21
Addition of Cl 2 and Br 2 Carried out with either the pure
reagents or in an inert solvent such as CH 2 Cl 2.
Slide 22
Addition of Cl 2 and Br 2 Addition is stereoselective.
Stereoselective reaction: 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 anti stereoselectivity.
Slide 23
Addition of Cl 2 and Br 2 Step 1: Formation of a bromonium ion
intermediate, Step 2: Halide ion opens the three-membered
ring.
Slide 24
Addition of Cl 2 and Br 2 Anti coplanar addition to a
cyclohexene corresponds to trans-diaxial addition.
Slide 25
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:
Rearrangement: A reaction in which the product(s) have a different
connectivity of atoms than that in the starting material.
Slide 26
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. 1,2-shift The key step in this type of rearrangement is
called a 1,2-shift.
Slide 27
Step 1: Proton transfer from the HCl to the alkene to give a 2
carbocation intermediate. Step 2: Migration of a methyl group with
its bonding electrons from the adjacent carbon gives a more stable
3carbocation. Carbocation Rearrangements
Slide 28
Step 3: Reaction of the 3carbocation (an electrophile and a
Lewis acid) with chloride ion (a nucleophile and a Lewis base)
gives the rearranged product.
Slide 29
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.
Slide 30
Carbocations-Summary The carbon bearing a positive charge is sp
2 hybridized with bond angles of 120 The order of carbocation
stability is 3>2>1. Carbocations are stabilized by the
electron- withdrawing 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 1,2- shift, when the rearranged carbocation is
more stable than the original carbocation. The most commonly
observed pattern is from 2 to 3.
Slide 31
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, H 2
O). 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).
Slide 32
Hydroboration-Oxidation The result of hydroboration followed by
oxidation of an alkene is hydration of the carbon-carbon double
bond. 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.
Slide 33
Hydroboration-Oxidation Hydroboration is the addition of BH 3
to an alkene to form a trialkylborane. Borane is most commonly used
as a solution of BH 3 in tetrahydrofuran (THF).
Slide 34
Hydroboration-Oxidation Hydroboration is both regioselective
and syn stereoselective. Regioselectivity: Regioselectivity: -H
adds to the more substituted carbon and boron adds to the less
substituted carbon of the double bond. Stereoselectivity syn
stereoselectivity Stereoselectivity: Boron and -H add to the same
face of the double bond (syn stereoselectivity).
Slide 35
Hydroboration-Oxidation regioselectivity Chemists account for
the regioselectivity by proposing the formation of a cyclic
four-center transition state. syn stereoselectivity 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.
Slide 36
Hydroboration-Oxidation Trialkylboranes are rarely isolated.
Treatment with alkaline hydrogen peroxide (H 2 O 2 /NaOH), oxidizes
a trialkylborane to an alcohol and sodium borate.
Slide 37
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.
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.
Slide 38
Ozonolysis of an Alkene Ozone (18 valence electrons) is
strongly electrophilic. molozonide Initial reaction gives an
intermediate called a molozonide.
Slide 39
Ozonolysis of an Alkene ozonide The intermediate molozonide
rearranges to an ozonide. Treatment of the ozonide with
dimethylsulfide gives the final products.
Slide 40
Reduction of Alkenes Alkenes react with H 2 in the presence of
a transition metal catalyst to give alkanes. The most commonly used
catalysts are Pd, Pt, and Ni. catalytic reduction or catalytic
hydrogenation. The reaction is called catalytic reduction or
catalytic hydrogenation.
Slide 41
Reduction of Alkenes The most common pattern is syn addition of
hydrogens; both hydrogens add to the same face of the double bond.
syn stereoselectivity. Catalytic reduction is syn
stereoselectivity.
Slide 42
Catalytic Reduction of an Alkene Figure 5.6 Syn addition of H 2
to an alkene involving a transition metal catalyst. (a) H 2 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.
Slide 43
Heats of Hydrogenation Table 5.2 Heats of Hydrogenation for
Several Alkenes
Slide 44
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.
Slide 45
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
Slide 46
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.
Slide 47
The Acidity of Terminal Alkynes Treatment of a 1-alkyne with a
very strong base such as sodium amide, NaNH 2, converts the alkyne
to an acetylide anion. Note that hydroxide ion is not a strong
enough base to form an acetylide anion.
Slide 48
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 carbon- carbon bond. 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. alkylation Because an alkyl group is
added to the original alkyne, this reaction is called
alkylation.
Slide 49
Acetylide Anions in Synthesis The importance of alkylation of
acetylide anions is that it can be used to create larger carbon
skeletons. For reasons we will discuss fully in Chapter 7, this
type of alkylation is successful only for methyl and primary alkyl
halides (CH 3 X and RCH 2 X).
Slide 50
Acid-Catalyzed Hydration of Alkynes In the presence of
concentrated sulfuric acid and Hg(II) salts, alkynes undergo
hydration in accordance with Markovnikovs rule. enol The initial
product is an enol, a compound containing an hydroxyl group bonded
to a carbon-carbon double bond. keto-enol tautomerism The enol is
in equilibrium with a more stable keto form. This equilibration is
called keto-enol tautomerism.
Slide 51
Reduction of Alkynes Treatment of an alkyne with H 2 in the
presence of a transition metal catalyst results in addition of two
moles of H 2 and conversion of the alkyne to an alkane. By the
proper choice of catalyst it is possible to stop the reaction at
the addition of one mole of H 2. The most commonly used catalyst
for this purpose is the Lindlar catalyst.
Slide 52
Reduction of Alkynes syn stereoselective Reduction using the
Lindlar catalyst is syn stereoselective. anti stereoselective
Reduction of an alkyne using an alkali metal (Li, Na, or K) in
liquid ammonia is anti stereoselective and gives a trans
alkene.
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