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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). - PowerPoint PPT Presentation
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William Brown Thomas Poon
www.wiley.com/college/brown
Chapter FiveReactions 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 DiagramFigure 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 DiagramFigure 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.
• Addition is regioselective. – Regioselective reaction: A reaction in which one direction of bond-
forming or bond-breaking occurs in preference to all other directions.
– 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.
CH3CH=CH2 HCl CH3CH-CH2
HClCH3CH-CH2
ClH
1-Chloropropane (not observed)
2-ChloropropanePropene++
CH2=CH2 HCl CH2-CH2
H Cl
Ethylene+
Chloroethane
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.
CH3CH=CH2 H2O H2SO4 CH3CH-CH2
OH H
Propene 2-Propanol+
CH3C=CH2
CH3
H2OH2SO4
HCH3C-CH2
CH3
HO2-Methyl-2-propanol2-Methylpropene
+
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.
CH3CH=CH2 HOHH
CH3CHCH3
HHO+
++
intermediateA 2o carbocation
+slow, rate
determining
CH3CHCH3 O-HH O
HH
CH3CHCH3+
++
An oxonium ion
fast
OHH
CH3CHCH3 H-O-HO
H
CH3CHCH3 H-O-HH
+
++
fast+
Addition of Cl2 and Br2• Carried out with either the pure reagents or in an
inert solvent such as CH2Cl2.
CH3CH=CHCH3 Br2 CH2Cl2CH3CH-CHCH3
Br Br
2,3-Dibromobutane2-Butene+
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.
Br2 CH2Cl2Br
Brtrans-1,2-DibromocyclohexaneCyclohexene
+
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.
Br2
Br
Br
BrBr+
trans diaxial(less stable)
trans diequatorial(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.
• 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 3°carbocation.
Carbocation Rearrangements
H Cl Cl+
3,3-Dimethy-1-butene
+
+
A 2° carbocationintermediate
+
A 2° carbocationintermediate
+
A 3° carbocationintermediate
The two electrons inthis bond move to
the electron-deficient carbocation.
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° carbocationintermediate
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.
H2OCH3CHCH=CH2
CH3CH3CCH2CH3
CH3
OH3-Methyl-1-butene
+
2-Methyl-2-butanol
This H migrates to an adjacent
carbon
H3O+
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 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°.
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.
• 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.
1. BH3
2. NaOH, H2O2OH
1-Hexene 1-Hexanol
Hydroboration-Oxidation• Hydroboration is the addition of BH3 to an alkene to form a
trialkylborane.
• Borane is most commonly used as a solution of BH3 in tetrahydrofuran (THF).
22
Tetrahydrofuran (THF)
-++ •••• ••O O BH3B2H6
BH3•THF
H BH
HCH2=CH2
CH3CH2 BCH2CH3
CH2CH3
CH3CH2 BH
HCH2=CH2 CH3CH2 B
CH2CH3
HCH2=CH2
Borane Ethylborane(an alkylborane)
Diethylborane(a dialkylborane)
Triethylborane(a trialkylborane)
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).
CH3BH3
CH3HBR2
H
CH3HBR2
H (Syn addition of BH3)
(R = 2-methylcyclopentyl)
1-Methylcyclopentene
+ +
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
CH3CH2CH2CH=CH2 CH3CH2CH2CH-CH2
H B
Hydroboration-Oxidation• Trialkylboranes are rarely isolated. Treatment
with alkaline hydrogen peroxide (H2O2/NaOH), oxidizes a trialkylborane to an alcohol and sodium borate.
(RO)3B 3NaOH 3ROH Na3BO3
A trialkylborate 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.
CH3CH3C=CHCH2CH3
1. O32. (CH3)2S CH3CCH3
OHCCH2CH3
OCH3-S-CH3
O
Propanal(an aldehyde)
Propanone(a ketone)
2-Methyl-2-pentene+
Dimethylsulfoxide(DMSO)
+
Ozonolysis of an Alkene• Ozone (18 valence electrons) is strongly
electrophilic.
• Initial reaction gives an intermediate called a molozonide.
O OO
O OO
O OO
O OO
Ozone is strongly electrophilicbecause of the positive formal charges on the two end oxygens.
O OO
O OO
O OO
A molozonide
Ozonolysis of an Alkene• The intermediate molozonide rearranges to an
ozonide. • Treatment of the ozonide with dimethylsulfide
gives the final products.
CH3CH=CHCH3O3 CH3CH-CHCH3
O OO
O OC
OC
H
CH3
HH3C
(CH3)2S OCH3CH
Acetaldehyde2-Butene A molozonide An ozonide
2
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.
+ H2Pd
Cyclohexene Cyclohexane25°C, 3 atm
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
CH3
H2Pt
CH3
CH3cis-1,2-Dimethyl-
cyclohexane1,2-Dimethyl-cyclohexene
+
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.
• Note that hydroxide ion is not a strong enough base to form an acetylide anion.
NH2H-C C-H H-C C:- NH3 Keq = 1013+Ammonia
pKa 38(Weaker
acid)
AcetylenepKa 25
(Strongeracid)
+SodiumAmide
(Stronger base)
Aceylide anion
(Weaker base)
H-C C:- H–OHOH+ +pKa 15.7(Stronger
acid)
pKa 25(Weaker
acid)
H-C C-H
(Stronger base)
(Weaker base)
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.
• Because an alkyl group is added to the original alkyne, this reaction is called alkylation.
H-C C:- CH
ClHH+-
+ H-C C CH3 + Clnucleophilic substitution
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 (CH3X and RCH2X).
CH3CH2C CCH2CH2CH3
3-HeptyneHC CH
1. NaNH2
2. CH3CH2BrCH3CH2C CH
3. NaNH2
4. CH3CH2CH2BrAcetylene 1-Butyne
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.
CH3C CH H2OH2SO4HgSO4
CH3C=CH2OH
CH3CCH3O
+Propanone(Acetone)
Propen-2-ol(an enol)
Propyne
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.
• 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.
2-Butyne Butane+CH3C CCH3 Pd, Pt, or Ni2H2 CH3CH2CH2CH33 atm
C C CH2CH3H3CH2 H3C CH2-CH3
HH2-Pentyne
Lindlarcaalys cis-2-Pentene
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.
C C CH2CH3H3CLi H3C H
CH2CH3Htrans-2-Penene
NH3(l)2-Pentyne
Reactions of Alkenes
End Chapter 5