chapter7alkeneskynes

© 2006 Thomson Higher Education

Chapter 7 Reactions of Alkenes and Alkynes

Alkene Addition Reactions

• Addition of a halogen to give 1,2-dihalide

• Addition of a hypohalous acid to give halohydrin

• Addition of water to give alcohol

• Addition of hydrogen to give alkane

• Addition of single oxygen to give three-membered cyclic ether: epoxide

• Addition of two hydroxyl groups to give 1,2-diol

Alkene addition reactions

7.1 Preparations of Alkenes: A Preview of Elimination Reactions

Preparation of alkenes: elimination reactions

Precursors to alkenes • Biological systems – usually alcohols• Laboratory – either alcohols or alkyl halides

Alkenes and alcohols are chemically related through addition and elimination reactions

• Alkenes add H2O to form alcohols

• Alcohols eliminate water to form alkenes

Preparations of Alkenes: A Preview of Elimination Reactions

Dehydrohalogenation • Loss of HX from alkyl halide• Usually occurs by reaction of an alkyl halide with a

strong base


Dehydration • Loss of water from an alcohol• Usually occurs by treatment of an alcohol with a strong

acid


In biological pathways dehydrations normally take place on substrates in which –OH is positioned two carbons away from a carbonyl group

7.2 Halogenation of Alkenes

Halogenation • Addition reaction of alkenes• Addition of Br2 and Cl2 to alkenes to yield 1,2-

dihalides

Halogenation of Alkenes

Halogenation of cycloalkenes• Only trans-stereoisomer of dihalide product is

formed• Reaction occurs with anti stereochemistry – the

two halogen atoms come from opposite faces of double-bond, one from top face and one form bottom face


Reaction occurs through an intermediate bromonium ion (R2Br+), formed by interaction of the alkene with

Br2 and simultaneous loss of Br-


Alkene halogenation reaction• Common laboratory reaction• Limited primarily to marine organisms in nature

• Carried out by enzymes called haloperoxidases that

oxidize Br- or Cl- ions to a biological equivalent of Br+ or Cl+

7.3 Halohydrins from Alkenes

Halohydrin Formation (electrophilic addition) • Reaction of alkenes with hypohalous acids HO-Cl or

HO-Br to yield 1,2-halo alcohols called halohydrins

• In marine organisms halohydrin formation is carried

out by haloperoxidases that oxidize Br- or Cl- ions to corresponding HOBr or HOCl bonded to a metal atom in the enzyme for subsequent addition to the double bond of substrate

Halohydrins from Alkenes

• X2 reacts with alkene to give cyclic halonium ion intermediate

• Intermediate halonium ion is intercepted by water nucleophile

• Oxygen loses proton to give the neutral halohydrin product

7.4 Hydration of Alkenes

Alkenes undergo an acid catalyzed addition reaction with water to yield alcohols

• Not of much use in the laboratory because of the high temperatures often required

Hydration of Alkenes

Acid-catalyzed hydration of isolated double bonds• Uncommon in biological pathways

Acid-catalyzed hydration of double bond adjacent to carbonyl group

• More common in biological pathways• Adjacent carbonyl group required for elimination of

water• Not an electrophilic addition mechanism


Laboratory hydrations of alkenes• Oxymercuration

• Electrophilic addition of Hg2+ to alkene on treatment with mercury(II) acetate [(CH3CO2)2Hg, or Hg(OAc) 2] in aqueous tetrahydrofuran (THF) solvent

• Reaction yields an alcohol• Product corresponds to Markovnikov regiochemistry

(more highly substituted alcohol formed)


• Hydroboration/oxidation • Addition of a B-H bond of borane, BH3, to an

alkene• Occurs in single step• No carbocation intermediate• Reaction yields an alcohol• Syn stereochemistry

• Both C-H and C-B bonds form at the same time and from the same face of the double-bond

• Product has non-Markovnikov regiochemistry


Alkene Hydroboration

Worked Example 7.1Predicting the Products of a Hydration Reaction

What products would you obtain from reaction of 2-methylpent-2-ene with:

(a) BH3, followed by H2O2,OH-

(b) Hg(OAc)2, followed by NaBH4


Strategy• Determine type of reaction being carried out

• Two methods of hydration• Hydroboration/oxidation

• Occurs with syn stereochemistry• Gives non-Markovnikov alcohol

• Oxymercuration • Gives the Markovnikov alcohol


Solution

Worked Example 7.2Synthesizing an Alcohol

How might you prepare the following alcohol?


Strategy• To synthesize a specific target molecule work

backwards• Look at target molecule• Identify functional group(s)• Devise a method for preparing functional group


Solution

Note: 4-methylhex-2-ene has a disubstituted double and would probably give a mixture of two alcohol products with either hydration method

7.5 Reduction of Alkenes

Hydrogenation • Addition reaction process by which alkenes are

reduced to alkanes

Reduction• Increases electron density on carbon by

• Forming C-H • Breaking C-O, C-N, or C-X bond

Common catalysts for alkene hydrogenation:• Platinum – PtO2 (Adams’ Catalyst)• Palladium – very fine powder supported on inert

material such as charcoal (Pd/C)

Reduction of Alkenes

Catalytic hydrogenation• A heterogeneous process that takes place on the

surface of insoluble catalyst particles• Occurs with syn stereochemistry

• Both hydrogens add to the double bond from the same side


Steps of Catalytic hydrogenation:

1. Adsorption of H2 onto catalyst surface

2. Complexation between catalyst and alkene occurs as a vacant orbital on metal interacts with filled p orbitals

3. Hydrogen added to double bond

4. Saturated product diffuses away from catalyst


Hydrogenation• Unsaturated vegetable oils reduced to

produce saturated fats used in margarine and cooking products• Vegetable oils

• Triesters of glycerol, HOCH2CH(OH)CH2OH, with three long-chain carboxylic acids called fatty acids

• Fatty acids• Polyunsaturated carboxylic acids containing

long hydrocarbon chains• Double bonds have cis stereochemistry


Catalytic hydrogenation of polyunsaturated fats


Catalytic hydrogenation of polyunsaturated fats• Complete hydrogenation leads to saturated fatty acids• Incomplete hydrogenation results in isomerized trans fats that

release trans fatty acids upon digestion, increasing blood cholesterol levels

Biological hydrogenation (reduction) of isolated double bonds• Double bond must be adjacent to a carbonyl group

• The reduction of isolated double bonds is rare in biological pathways

• Process occurs in two steps1. NADPH (coenzyme reduced nicotinamide adenine

dinucleotide phosphate) adds hydride ion (H:-) to double bond to produce an ion

2. Protonation of an anion by acid HA leading to an overall addition of H2


Biological reduction of double bond in trans-Crotonyl ACP leads to the formation of Butyryl ACP

7.6 Oxidation of Alkenes: Epoxidation

Oxidation • A reaction that results in a loss of electron

density by carbon

Oxidation• Decreases electron density on carbon by

• Breaking C-H bond• Forming C-O, C-N, or C-X bond

Note: oxidation often adds oxygen; reduction often adds hydrogen

Oxidation of Alkenes: Epoxidation

Alkenes on treatment with a peroxyacid, RCO3H, are oxidized to give epoxides

Epoxide (oxiranes) • Cyclic ethers with an oxygen atom in a three-

membered ring


Synthesis of epoxides from alkenes• Peroxyacid transfers oxygen to alkene • Syn stereochemistry

• Both C-O bonds form on the same face of the double

• One step mechanism• No intermediates


Synthesis of epoxides from halohydrins• Preparation of halohydrin through electrophilic

addition of HO-X to alkene• Treatment of halohydrin with base deprotonates OH

• O- nucleophile reacts with C-Cl electrophile substituting C-O bond for C-Cl bond

• Cl- eliminated yielding the epoxide


Epoxides in biological pathways:

• Epoxides prepared from alkenes as intermediates

• Peroxyacids are not involved

• Conversion of squalene into 2,3- oxidosqualene; a key step in the biosynthesis of steroids

7.7 Oxidation of Alkenes: Hydroxylation

Hydroxylation• The addition of an –OH group to each of the two double-

bond carbons• Two step process:

1. Epoxidation 2. Hydration

• Epoxides undergo an acid-catalyzed reaction with water to give corresponding 1,2-dialcohol, or diol

Oxidation of Alkenes: Hydroxylation

Acid catalyzed epoxide-opening takes place by:1. Protonation of the epoxide increasing the electrophilicity of

carbon

2. Nucleophilic addition of water followed by deprotonation• Trans-1,2-diol formed

Oxidation of Alkenes: Hydroxylation

Epoxides in biological pathways

• Epoxide hydrolyses are common • Pathways animals use to

detoxify harmful substances

• Benzo[a]pyrene • Carcinogenic substance

found in cigarette smoke, chimney soot, and barbecued meat

• Detoxified by conversion to a diol epoxide

Oxidation of Alkenes: HydroxylationHydroxylation in the Laboratory • Carried out directly by oxidation of an alkene with

osmium tetroxide, OsO4

• Syn stereochemistry• No carbocation intermediate• Occurs through cyclic osmate intermediate

7.8 Radical Addition to Alkenes: Alkene Polymers

Radicals add to alkene double bonds• Radicals remove one electron from double bond• One electron left behind yielding a new radical

Polymer • A large molecule built up by repetitive bonding

together of many smaller molecules called monomers• Cellulose (glucose polymer)

Radical Addition to Alkenes: Alkene Polymers• Proteins (amino acid polymers)

• Nucleic acid (nucleotide polymer)

Radical Addition to Alkenes: Alkene Polymers

Simplest polymerization • Result when an alkene is treated with a small

amount of a radical as an initiator

Radical Addition to Alkenes: Alkene PolymersInitiation 1. Small amount of benzoyl peroxide catalyst is heated

breaking weak O-O bonds and yielding radicals2. Benzoyloxy radical adds to C=C bond of ethylene

forming a carbon radical3. a) One electron from C=C bond pairs up with electron of

benzoyloxy radical to form C-O bondb) Other electron remains on carbon (a carbon-centered radical)


Propagation • Polymerization occurs when the carbon radical adds to

another ethylene molecule to yield another radical

Termination• Chain process ends by a reaction that consumes a radical

• Combination of two growing chains

2-R–CH2CH2 → R–CH2CH2CH2CH2–R

Radical Addition to Alkenes: Alkene PolymersVinyl monomers• Substituted ethylene• Undergo polymerization to yield polymer with substituted

groups regularly spaced in alternating carbon atom long chain• Polypropylene

Styrene


Polymerization of unsymmetrically substituted vinyl monomers

Propylene or Styrene• Radical addition steps can take place at either end of

the double bond to yield:• A primary radical intermediate (RCH2

.)

• A secondary radical (R2CH.)

• Similar to electrophilic addition reaction• More highly substituted, secondary radical is formed


Radical addition• Difficult to control• Limited use in the laboratory• Reaction intermediate is not

quenched so reaction continues

Electrophilic addition• Reaction occurs once• Intermediate is then quenched

and reaction stops.

Radical vs. Electrophilic Addition Reactions

Worked Example 7.3 Predicting the Structure of a Polymer

Show the structure of poly(vinyl chloride), a polymer made from H2C=CHCl, by drawing several repeating units


Strategy

Mentally break the carbon-carbon double bond in the monomer unit, and form single bonds by connecting numerous units together


Solution

The general structure of poly(vinyl chloride) is

7.9 Biological Additions of Radicals to Alkenes

Biological Reactions• Only one substrate molecule at a time is present

in the active site of the enzyme where the reaction occurs (necessary reactant groups nearby)

• More controlled • More common than laboratory radical reactions

Biological Additions of Radicals to Alkenes

Step 1 – formation of a carbon radical at C13

Step 2 – C13 radical reacts with O2 at C11 through resonance form

Step 3 – Oxygen radical reacts with C8-C9 double bond forming carbon radical at C8

Step 4 – C8 radical adds to C12-C13 double bond forming carbon radical at C13

Step 5 – resonance form of C13 carbon radical adds at C15 to a second O2 molecule

Step 6 – Reduction of O-O bond gives prostaglandin H2

7.10 Conjugated Dienes

Sites of unsaturation• Many compounds have numerous sites of unsaturation• If sites are well separated in molecule they react

independently • If sites are close together they may interact with one

another

Conjugated double bonds• Double bonds that alternate with single bonds

Conjugated Dienes

Heats of HydrogenationConjugated dienes are more stable than nonconjugated

dienes

Conjugated Dienes

Buta-1,3-diene is approximately 16 kJ/mol (3.8 kcal/mol) more stable than expected

Conjugated Dienes

Explanations for conjugated diene stability1) Valence Bond Theory

• Stability due to orbital hybridization• Alkanes

• C-C single bonds • σ overlap of sp3 orbitals on both carbons

• Conjugated dienes• σ overlap of sp2 orbitals (shorter and stronger)

Conjugated Dienes

2. Molecular Orbital Theory • Interaction between the p orbitals of the two

double bonds• Two p orbitals combine to form two p molecular

orbitals• Both electrons occupy the low-energy bonding

orbital leading to a net lowering of energy and formation of a stable bond

Conjugated Dienes

• Four adjacent p atomic orbitals of a conjugated diene

Four molecular orbitals of buta-1,3-diene

7.11 Reaction of Conjugated Dienes

Conjugated dienes• Undergo electrophilic addition reactions readily• Mixture of products obtained• Addition of HBr to buta-1,3-diene yields mixture

of two addition products

Reaction of Conjugated Dienes

Allylic carbocation is an intermediate• When buta-1,3-diene reacts with H+ electrophile two

carbocation intermediates are possible:

1. A primary carbocation

2. A secondary allylic carbocation (stabilized by resonance between two forms)

• Secondary allylic carbocation is more stable and forms faster than the nonallylic carbocation

Reaction of Conjugated Dienes

Allylic carbocation reacts with Br- to complete the electrophilic addition

• Reaction can occur at C1 or C3• Both carbons share positive charge• Mixture of 1,2- and 1,4-addition products results

Worked Example 7.4Predicting the Products of Electrophilic Addition to a Conjugated Diene

Give the structures of the likely products from reaction of 1 equivalent of HCl with 2-methylcyclohexa-1,3-diene. Show both 1,2- and 1,4- adducts.


Strategy Electrophilic addition of HCl to a conjugated

diene involves the formation of allylic carbocation intermediates

First – • Protonate the two ends of the diene • Draw resonance forms of the two allylic

carbocations that resultSecond – • Allow each resonance form to react with Cl- to

generate four possible products


Solution

7.12 Reaction of Alkynes

Alkyne Addition Reactions • Alkynes behave similarly to alkenes• Alkynes are less reactive than alkenes• Various reactions can often be stopped at the

monoaddition stage if one molar equivalent of reagent is used

Reaction of Alkynes

Reaction of Alkynes

Reaction of Alkynes

Alkyne acidity• Terminal alkynes (RC≡CH) are relatively acidic• RC≡CH treated with a strong base NaNH2

• Terminal hydrogen is removed forming and acetylide anion

Reaction of Alkynes

Bronsted-Lowry Acid• A substance that donates H+ Acidity order:• Established by measuring acid dissociation constants and

expressing the results as pKa valuesLow pKa = strong acidHigh pKa = weak acid

• Amide ion (NH2-), the conjugated base of ammonia (pKa = 35),

is often used to deprotonate terminal alkynes

Reaction of Alkynes

Terminal alkynes more acidic than alkenes or alkanes• Acetylide ions are more stable than vinylic (alkenyl) or alkyl

ions• Difference in acidities due to hybridization of negatively

charged carbon atom• Acetylide anion has sp-hybridized carbon

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