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