Carey - Organic Chemistry - sgchapt09

Carey - Organic Chemistry - sgchapt09

(Parte 1 de 2)

CHAPTER 9 ALKYNES

9.1The reaction is an acid–base process; water is the proton donor. Two separate proton-transfer steps are involved.

3-Methyl-1-butyne

CH3CHC CH CH3

1-Pentyne CH3CH2CH2C CH 2-Pentyne

CH Acetylene HO Hydroxide ion

Acetylide ionWater H O

Carbide ionCHAcetylide ion HO

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210 ALKYNES 9.3The bonds become shorter and stronger in the series as the electronegativity increases.

Electronegativity:N (3.0)O (3.5)F (4.0) Bond distance (pm):N@H (101)O@H (95)F@H (92) Bond dissociation energy (kJ/mol):N@H (435)O@H (497)F@H (568) Bond dissociation energy (kcal/mol):N@H (104)O@H (119)F@H (136)

The position of equilibrium lies to the right. Ethyl anion is a very powerful base and deprotonates acetylene quantitatively. (c)Amide ion is not a strong enough base to remove a proton from ethylene. The equilibrium lies to the left.

It does not matter whether the methyl group or the butyl group is introduced first; the order of steps shown in this synthetic scheme may be inverted. (c)An ethyl group and a propyl group need to be introduced as substituents on a @C>C@unit. As in part (b), it does not matter which of the two is introduced first.

1. NaNH2, NH3 2. CH3CH2CH2Br

Acetylene 1-Pentyne 3-Heptyne

CHHC1. NaNH2, NH3 2. CH3CH2BrCH3CH2CH2C CH CH3CH2CH2C CCH2CH3

1. NaNH2, NH3 2. CH3Br

Acetylene Propyne 2-Heptyne

CHHC1. NaNH2, NH3 2. CH3CH2CH2CH2BrCH3C CH CH3C CCH2CH2CH2CH3

Amide ion (stronger base)

NH2

Ammonia (weaker acid)

NH3

2-Butyn-1-olate anion (weaker base)

2-Butyn-1-ol (stronger acid)

HCH3C CCH2O

Ammonia (stronger acid)

(pKa 36)

NH3

Vinyl anion (stronger base)

Amide ion (weaker base)

NH2

Ethylene (weaker acid)

HCH2 CH

CH3CH3

Ethane (weaker acid)

Acetylide ion (weaker base)

Ethyl anion (stronger base)

CH2CH3

Acetylene (stronger acid)

(pKa 26)

(pKa 62)

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9.6Both 1-pentyne and 2-pentyne can be prepared by alkylating acetylene. All the alkylation steps involve nucleophilic substitution of a methyl or primary alkyl halide.

Athird isomer, 3-methyl-1-butyne, cannot be prepared by alkylation of acetylene, because it requires a secondary alkyl halide as the alkylating agent. The reaction that takes place is elimination, not substitution.

9.7Each of the dibromides shown yields 3,3-dimethyl-1-butyne when subjected to double dehydrohalogenation with strong base.

After propene is available, it is converted to 1,2-dibromopropane and then to propyne as described in the sample solution for part (a). (c)Treat isopropyl bromide with a base to effect dehydrohalogenation.

Next, convert propene to propyne as in parts (a) and (b). (d)The starting material contains only two carbon atoms, and so an alkylation step is needed at some point. Propyne arises by alkylation of acetylene, and so the last step in the synthesis is

The designated starting material, 1,1-dichloroethane, is a geminal dihalide and can be used to prepare acetylene by a double dehydrohalogenation.

2. H2OCH3CHCl21,1-Dichloroethane HC CH Acetylene

2. CH3BrHC CHAcetylene CH3CC H Propyne

(CH3)2CHBr NaOCH2CH3 PropeneIsopropyl bromide CH3CH CH2

CH3CH2CH2OH H2SO4 heat

Propene1-Propanol

CH3CH CH2

2,2-Dibromo-3,3- dimethylbutane

1,1-Dibromo-3,3- dimethylbutane

Br (CH3)3CCHCH2Br

1,2-Dibromo-3,3- dimethylbutane

(CH3)3CC CH 3,3-Dimethyl-1-butyne

E2CH3CHCH3

Isopropyl bromide

HC CHAcetylene

CH2 CHCH3PropeneHC Acetylide ion

1. NaNH2, NH3 2. CH3CH2Br

Acetylene 1-Butyne 2-Pentyne

CHHC1. NaNH2, NH3 2. CH3BrCH3CH2C CH CH3CH2C CCH3

Acetylene 1-Pentyne

CHHCCHCH3CH2CH2C1. NaNH2, NH3 2. CH3CH2CH2Br

ALKYNES 211

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(e)The first task is to convert ethyl alcohol to acetylene. Once acetylene is prepared it can be alkylated with a methyl halide.

9.9The first task is to assemble a carbon chain containing eight carbons. Acetylene has two carbon atoms and can be alkylated via its sodium salt to 1-octyne. Hydrogenation over platinum converts 1-octyne to octane.

Alternatively, two successive alkylations of acetylene with CH3CH2CH2Br could be carried out to give 4-octyne (CH3CH2CH2C>CCH2CH2CH3), which could then be hydrogenated to octane.

9.10Hydrogenation over Lindlar palladium converts an alkyne to a cis alkene. Oleic acid therefore has the structure indicated in the following equation:

Hydrogenation of alkynes over platinum leads to alkanes. 9.11Alkynes are converted to trans alkenes on reduction with sodium in liquid ammonia.

9.12The proper double-bond stereochemistry may be achieved by using 2-heptyne as a reactant in the final step. Lithium–ammonia reduction of 2-heptyne gives the trans alkene; hydrogenation over Lindlar palladium gives the cis isomer. The first task is therefore the alkylation of propyne to 2-heptyne.

1. NaNH2, NH3

2. CH3CH2CH2CH2Br 2-Heptyne CCH2CH2CH2CH3CH3CCHCH3C Propyne

H3C H

CH2CH2CH2CH3 C

H (Z)-2-Heptene

Li, NH3

1. Na, NH3

Oleic acid

NaNH2

BrCH2(CH2)4CH3HC CHAcetylene

Sodium acetylide HC CCH2(CH2)4CH31-Octyne

1. NaNH, NH 2. HO 1. NaNH, NH

2. CHBr H SOheat

BrCH3CH2OHEthyl alcohol

BrCH2CH2Br1,2-Dibromoethane HC CHAcetylene

CH3CC H

PropyneEthylene H2CC H2

212 ALKYNES

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(c)Since 1,1-dichloroethane can be prepared by adding 2 mol of hydrogen chloride to acetylene as shown in the sample solution to part (a), first convert 1,1-dibromoethane to acetylene by dehydrohalogenation.

9.14The enol arises by addition of water to the triple bond.

The mechanism described in the textbook Figure 9.6 is adapted to the case of 2-butyne hydration as shown:

9.15Hydration of 1-octyne gives 2-octanone according to the equation that immediately precedes this problem in the text. Prepare 1-octyne as described in the solution to Problem 9.9, and then carry out its hydration in the presence of mercury(I) sulfate and sulfuric acid.

Hydration of 4-octyne gives 4-octanone. Prepare 4-octyne as described in the solution to Problem 9.9.

fragments CH3(CH2)4CO2H and HO2CCH2CH2CO2H account for only 10 of the original 16 carbons. The full complement of carbons can be accommodated by assuming that two molecules of

CH3(CH2)4CO2H are formed, along with one molecule of HO2CCH2CH2CO2H. The starting alkyne is therefore deduced from the ozonolysis data to be as shown:

Carbocation 2-Butanone

Water

Hydronium ion

OH

Water

Carbocation

CCH3CH3CH2 OH CH3CH2CCH3

Hydroniumion 2-Buten-2-ol

CCH3CH3CH OH OH

2-Butyne 2-Butanone

2-Buten-2-ol (enol form)

CHCH3CH3C OH

CHHCAcetylene CH3CHCl21,1-Dichloroethane

CH3CHBr2 1,1-Dibromoethane

2HCl1. NaNH2, NH3 2. H2O

CHClH2C Vinyl chloride

CH3CHCl2 1,1-Dichloroethane

HCl

ALKYNES 213

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9.17Three isomers have unbranched carbon chains: Next consider all the alkynes with a single methyl branch:

One isomer has two methyl branches. None is possible with an ethyl branch.

2,5-Dimethyl-3-hexyne is CH3CHC CH3 CCHCH3 CH3

1-Octyne is HCCCH2CH2CH2CH2CH2CH3

CH3CC CH3

CH3

CCCH3 is 2,2,5,5-tetramethyl-3-hexyne CH3

CH3CH2CH2CH2CHCH2CH2CH2CH2CH3 is 4-butyl-2-nonyne C CCH3

(Parent chain must contain the triple bond.)

113 is cyclotridecyne

CH3C H3C CCHCHCH3 is 4,5-dimethyl-2-hexyne CH3

CH3CC CH3

CH3 CH

3,3-Dimethyl-1-butyne

CH3CHC

CH3 CCH34-Methyl-2-pentyne

CH3CH2CHC

CH3 CH3-Methyl-1-pentyne

4-Methyl-1-pentyne

CH3CH2CH2CH2CC H CH3CH2CH2C CCH3 CH3CH2C CCH2CH3 1-Hexyne 2-Hexyne 3-Hexyne

214 ALKYNES

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9.22The carbon skeleton of the unknown acetylenic amino acid must be the same as that of homoleucine.

The structure of homoleucine is such that there is only one possible location for a carbon–carbon triple bond in an acetylenic precursor.

CH2CH3CH2CH2CH2CH 1-Hexene CH3CH2CH2CH2CH2CH2I 1-Iodohexane

CHHCAcetylene CHCH3CH2CH2CH2C 1-Hexyne

CH3CH2CH2CH2Br

1-Hexene 1,2-Dibromohexane

CHCH3CH2CH2CH2CCH3CH2CH2CH2CH CH2 1-Hexyne

Br2 CCl4 1. NaNH2, NH3 2. H2OCH3CH2CH2CH2CHCH2Br

1,1-Dichlorohexane

CHCH3CH2CH2CH2C 1-Hexyne

H2Pt or or

3-Ethyl-1-hexyne 4-Ethyl-1-hexyne 4-Ethyl-2-hexyne 3-Ethylhexane

3-Ethyl-3-methyl-1-pentyne is CH3CH2CC CH2CH3

CH3 CH

CEthynylcyclohexane is CH ALKYNES 215

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The desired intermediate, 1-butyne, is available by halogenation followed by dehydrohalogenation of 1-butene.

Reaction of the anion of 1-butyne with ethyl bromide completes the synthesis.

Loss of bromine from C-2 gives (E)- and (Z)-1-bromo-1-decene.

1-Hexyne 1-Hexene

CH3CH2CH2CH2C 1-Hexyne

CH H2 Lindlar Pd CH3CH2CH2CH2CH 1-Hexene CH2

CH3CH2CH2CH2C 1-Hexyne

CH 2H2 Pt CH3CH2CH2CH2CH2CH3 Hexane

1,2-Dibromodecane

KOH ethanol–water C

BrH H

C HBr H

2-Bromo-1-decene

Br Br BrCH2CH(CH2)7CH3

1,2-Dibromodecane

HC HCCH Acetylene

NaNH2

HC 1-Butyne CCH2CH3

1. NaNH2, NH3

2. H2O CH3CH2C CH1-Butyne

CH3CH2CH2CHCl2 1,1-Dichlorobutane

CH3CH2CCH3CH2C CH1-Butyne

NaNH2

3-Hexyne CH3CH2C CCH2CH3

1-Butyne

CH3CH2CH CH2 1-Butene

216 ALKYNES

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3-Hexyne

CH3CH2 CH2CH3

H (Z)-3-Hexene

3-Hexyne Pt Hexane

CH3CH2CH2CH2CC H 1-Hexyne

1. O3

2. H2OPentanoic acid CH3CH2CH2CH2COH

Carbonic acid HOCOH

CH3CH2CH2CH2CC H 1-Hexyne

2-Hexanone

CH3CH2CH2CH2CCH3

CH3CH2CH2CH2CC H 1-Hexyne

Cl2 (2 mol)

1,1,2,2-Tetrachlorohexane

CH3CH2CH2CH2CCHCl2 Cl

CH3CH2CH2CH2CC H

1-Hexyne

Cl2(1 mol)C

Cl CH3CH2CH2CH2 H

(E)-1,2-Dichloro-1-hexene

CH3CH2CH2CH2CC H

1-Hexyne 2,2-Dichlorohexane

CH3CH2CH2CH2CCH3 Cl

HCl (2 mol)

CH3CH2CH2CH2CC H

1-Hexyne 2-Chloro-1-hexene

CH3CH2CH2CH2CC H2 Cl

HCl (1 mol)

Sodium 1-hexynide tert-Butyl bromide

CH3CH2CH2CH2C(CH3)3CBrNaCCH3CH2CH2CH2CCH
CH3CH2CH2CH2CCH3CH2CH2CH2BrNaC

CH3CH2CH2CH2C CCH2CH2CH2CH3 5-DecyneSodium 1-hexynide 1-Bromobutane

CH3CH2CH2CH2CC H NH3 NaNH2

1-Hexyne Sodium 1-hexynide CH3CH2CH2CH2C Na C

ALKYNES 217

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9.28The two carbons of the triple bond are similarly but not identically substituted in 2-heptyne,

CH3C>CCH2CH2CH2CH3. Two regioisomeric enols are formed, each of which gives a different ketone.

CH3C CCH2CH2CH2CH32-Heptyne

CH3COH CHCH2CH2CH2CH3

2-Hepten-2-ol

2-Heptanone

CH3CH OH CCH2CH2CH2CH3

2-Hepten-3-ol

3-Heptanone

3-Hexyne 1. O3

2. H2OPropanoic acid

3-Hexyne

3-Hexanone

3-Hexyne

Cl2(2 mol)CH3CH2C CCH2CH3 3,3,4,4-Tetrachlorohexane

CH3CH2CCl CCH2CH3

Cl Cl

3-Hexyne

Cl2(1 mol)CH3CH2C CH2CH3 C Cl

CH3CH2

CH2CH3

(E)-3,4-Dichloro-3-hexene

3-Hexyne

HCl(2 mol)CH3CH2C CCH2CH3 3,3-Dichlorohexane

CH3CH2CCH2CH2CH3 Cl

3-Hexyne

HCl(1 mol)CH3CH2C CH2CH3 C Cl

CH3CH2

CH2CH3

(Z)-3-Chloro-3-hexene

3-Hexyne

CH3CH2

CH2CH3

(E)-3-Hexene

218 ALKYNES

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9.29 The alkane formed by hydrogenation of (S)-3-methyl-1-pentyne is achiral; it cannot be optically active.

The product of hydrogenation of (S)-4-methyl-1-hexyne is optically active because a stereogenic center is present in the starting material and is carried through to the product.

Both (S)-3-methyl-1-pentyne and (S)-4-methyl-1-hexyne yield optically active products when their triple bonds are reduced to double bonds.

(c)The starting material is a geminal dichloride. Potassium tert-butoxide in dimethyl sulfoxide is a sufficiently strong base to convert it to an alkyne.

(d)Alkyl p-toluenesulfonates react similarly to alkyl halides in nucleophilic substitution reactions. The alkynide nucleophile displaces the p-toluenesulfonate leaving group from ethyl p-toluenesulfonate.

Phenylacetylide ion

Ethyl p-toluenesulfonate

CH3 1-Phenyl-1-butyne

1,1-Dichloro-1- cyclopropylethane

CHC Ethynylcyclopropane

BrCH2CHCH2CH2CHCH2Br 1,2,5,6-Tetrabromohexane

Br Br

1,5-Hexadiyne

ClCH2CH2CH2CH2CH2CH2I NaC CH ClCH2CH2CH2CH2CH2CH2C CH

Sodiumacetylide 1-Chloro-6-iodohexane 8-Chloro-1-octyne

CH3CH2 CH

(S)-4-Methyl-1-hexyne

(S)-3-Methylhexane C C

CH3

3-Methylpentane (does not have a stereogenic center; optically inactive)

C H CH3CH2

(S)-3-Methyl-1-pentyne C

ALKYNES 219

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(e) Bothcarbonsofa unitareconvertedtocarboxylgroups onozonolysis. (f)Ozonolysis cleaves the carbon–carbon triple bond.

(h)Sodium-in-ammonia reduction of an alkyne yields a trans alkene. The stereochemistry of a double bond that is already present in the molecule is not altered during the process.

8-Chlorooctyl tetrahydropyranyl ether

CCH2CH2CH2CH3NaC

Sodium 1-hexynide O O(CH2)8C CCH2CH2CH2CH3 9-Tetradecyn-1-yl tetrahydropyranyl ether

H H CH2CH2OH

1. Na, NH3 2. H2O

CH3CHCH2CC CH CH3 CH3

3,5-Dimethyl-1-hexyn-3-ol

3-Hydroxy-3,5-dimethyl-2-hexanone

Carbonic acid1-Ethynylcyclohexanol

1-Hydroxycyclohexanecarboxylic acid

Cyclodecyne

Decanedioic acid

(CO2H)C

220 ALKYNES

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9.31Ketones such as 2-heptanone may be readily prepared by hydration of terminal alkynes. Thus, if we had 1-heptyne, it could be converted to 2-heptanone.

Acetylene, as we have seen in earlier problems, can be converted to 1-heptyne by alkylation.

9.32Apply the technique of reasoning backward to gain a clue to how to attack this synthesis problem.

Areasonable final step is the formation of the Zdouble bond by hydrogenation of an alkyne over Lindlar palladium.

The necessary alkyne 9-tricosyne can be prepared by a double alkylation of acetylene.

It does not matter which alkyl group is introduced first. The alkyl halides are prepared from the corresponding alcohols.

1-Tridecanol CH3(CH2)12Br1-Bromotridecane

HBr or PBr3

CH3(CH2)7OH1-Octanol CH3(CH2)7Br1-Bromooctane

HBr or PBr3

9-Tricosyne

CHCH3(CH2)7C 1-Decyne

CHHC Acetylene

H2Lindlar PdCC

HCC(CH2)4CH3CH3CH2CH2CH2CH2BrHCCNa

NaNH2

HCCHHCCNa

NH3

2-Heptanone

HC C(CH2)4CH3 1-Heptyne

O O(CH2)8C CCH2CH2CH2CH3 9-Tetradecyn-1-yl tetrahydropyranyl ether

CH2CH2CH2CH3

(Z)-9-Tetradecen-1-yl tetrahydropyranyl ether

ALKYNES 221

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The designated starting material, 1,1-dibromopropane, is converted to propyne by a double dehydrohalogenation.

(b)As in part (a), first convert the designated starting material to propyne, and then add hydrogen bromide.

1. NaNH2, NH3 2. CH3CH2CH2CH2Br HC CCH2CH2CH2CH3

1-Hexyne

HC CH Acetylene

HC CCH2CH31-Butyne 2HI

Hydrogeniodide 2,2-Diiodobutane

NaNH2 NH3 CH3CH2Br

HCCNaSodium acetylide

HCCCH2CH31- Butyne HC CH Acetylene

Propyne

CH3CCH1. NaNH2, NH3

2. H2O1,2-Dichloropropane

CH3CHCH2Cl

Cl 1,1,2,2-Tetrachloropropane

CH3CCHCl2 Cl

Cl 2Cl2

Propyne

CH3CC H 1,2-Dibromopropane

1. NaNH2, NH3 2. H2OCH3CHCH2Br

2,2-Dibromopropane

CH3CCH3 Br

Br 2HBr

Propyne CH3CC H1,1-Dibromopropane

CH3CH2CHBr2 1. NaNH2, NH3

2HBr

Hydrogenbromide

Propyne CH3CC H 2,2-Dibromopropane

CH3CCH3 Br

2 ALKYNES

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The alkylating agent, 1-bromobutane, is prepared from 1-butene by free-radical (anti- Markovnikov) addition of hydrogen bromide.

Once 1-hexyne is prepared, it can be converted to 1-hexene by hydrogenation over Lindlar palladium or by sodium–ammonia reduction.

(f)Dialkylation of acetylene with 1-bromobutane, prepared in part (f), gives the necessary tencarbon chain.

Hydrogenation of 5-decyne yields decane.

Hydrogenation over the Lindlar catalyst converts the carbon–carbon triple bond to a cis double bond.

1-(1-Propynyl)cyclohexene (Z)-1-(1-Propenyl)cyclohexene

2. CH3Br

1-Ethynylcyclohexene 1-(1-Propynyl)cyclohexene

CH CH Br Br

1,2-Dibromocyclopentadecane (CH2)13

Cyclopentadecyne

5-Decyne 2H2

1. NaNH2, NH3 2. CH3CH2CH2CH2Br CHCH3CH2CH2CH2C

1-Hexyne

CH3CH2CH2CH2C CCH2CH2CH2CH3 5-Decyne

HC CH Acetylene

(Parte 1 de 2)

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