Carey - Organic Chemistry - chapt09

(Parte 2 de 4)

SAMPLE SOLUTION(a) The equation representing the acid–base reaction between propyne and methoxide ion is:

Alcohols are stronger acids than acetylene, and so the position of equilibrium lies to the left. Methoxide ion is not a strong enough base to remove a proton from acetylene.

Anions of acetylene and terminal alkynes are nucleophilic and react with methyl and primary alkyl halides to form carbon–carbon bonds by nucleophilic substitution. Some useful applications of this reaction will be discussed in the following section.

9.6PREPARATION OF ALKYNES BY ALKYLATION OF ACETYLENE AND TERMINAL ALKYNES

Organic synthesis makes use of two major reaction types:

1.Functional group transformations 2.Carbon–carbon bond-forming reactions

Both strategies are applied to the preparation of alkynes. In this section we shall see how to prepare alkynes while building longer carbon chains. By attaching alkyl groups to acetylene, more complex alkynes can be prepared.

CH3CPC±H

Propyne (weaker acid)

Propynide ion (stronger base)

CH3CPC

Methoxide ion (weaker base)

OCH3

Methanol (stronger acid)

Acetylene (stronger acid)

Amide ion (stronger base)

NH2

Acetylide ion (weaker base)

Ammonia (weaker acid)

346 CHAPTER NINE Alkynes

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Reactions that attach alkyl groups to molecular fragments are called alkylationreactions. One way in which alkynes are prepared is by alkylation of acetylene.

Alkylation of acetylene involves a sequence of two separate operations. In the first one, acetylene is converted to its conjugate base by treatment with sodium amide.

Next, an alkyl halide (the alkylating agent) is added to the solution of sodium acetylide. Acetylide ion acts as a nucleophile, displacing halide from carbon and forming a new carbon–carbon bond. Substitution occurs by an SN2 mechanism.

The synthetic sequence is usually carried out in liquid ammonia as the solvent. Alternatively, diethyl ether or tetrahydrofuran may be used.

An analogous sequence using terminal alkynes as starting materials yields alkynes of the type RCPCR .

Dialkylation of acetylene can be achieved by carrying out the sequence twice.

As in other nucleophilic substitution reactions, alkyl p-toluenesulfonates may be used in place of alkyl halides.

PROBLEM 9.5Outline efficient syntheses of each of the following alkynes from acetylene and any necessary organic or inorganic reagents:

(a) 1-Heptyne (b) 2-Heptyne (c) 3-Heptyne

SAMPLE SOLUTION(a) An examination of the structural formula of 1-heptyne reveals it to have a pentyl group attached to an acetylene unit. Alkylation of acetylene, by way of its anion, with a pentyl halide is a suitable synthetic route to 1-heptyne.

1. NaNH2, NH3 2. CH3CH2Br 1. NaNH2, NH3

2. CH3Br2-Pentyne (81%)

CCH2CH3CH3CAcetylene CHHC 1-Butyne

CCH2CH3HC

Sodium acetylide CNaHC 1-Bromobutane

CH3CH2CH2CH2Br NH3 1-Hexyne (70–7%) CHCH3CH2CH2CH2C

Alkyne CRHC

Sodium acetylide

Alkyl halide

Sodium halide

Acetylene CHHC Sodium acetylide

NH3

Acetylene H C

Monosubstituted or terminal alkyne

Disubstituted derivative of acetylene

NaNH2

NH3

CH3Br4-Methyl-1-pentyne CH(CH3)2CHCH2C 5-Methyl-2-hexyne (81%)

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The major limitation to this reaction is that synthetically acceptable yields are obtained only with methyl halides and primary alkyl halides. Acetylide anions are very basic, much more basic than hydroxide, for example, and react with secondary and tertiary alkyl halides by elimination.

The desired SN2 substitution pathway is observed only with methyl and primary alkyl halides.

PROBLEM 9.6Which of the alkynes of molecular formula C5H8can be prepared in good yield by alkylation or dialkylation of acetylene? Explain why the prepa-

Asecond strategy for alkyne synthesis, involving functional group transformation reactions, is described in the following section.

9.7PREPARATION OF ALKYNES BY ELIMINATION REACTIONS

Just as it is possible to prepare alkenes by dehydrohalogenation of alkyl halides, so may alkynes be prepared by a double dehydrohalogenationof dihaloalkanes. The dihalide may be a geminal dihalide,one in which both halogens are on the same carbon, or it may be a vicinal dihalide,one in which the halogens are on adjacent carbons.

Double dehydrohalogenation of a geminal dihalide

Double dehydrohalogenation of a vicinal dihalide

The most frequent applications of these procedures are in the preparation of terminal alkynes. Since the terminal alkyne product is acidic enough to transfer a proton to amide anion, one equivalent of base in addition to the two equivalents required for double

Vicinal dihalide

AmmoniaSodium amide

2NaNH2 2NaX Sodium halideAlkyne

Geminal dihalide

AmmoniaSodium amide

2NaNH2 2NaX Sodium halideAlkyne

E2HC C

Acetylide

CH3 CH2 C Br tert-Butyl bromide HC CHAcetylene

CH2 CH3

CH3 C2-Methylpropene

HCPCHAcetylene HCPCNaSodium acetylide

HCPCCH2CH2CH2CH2CH3 1-Heptyne

NaNH2 NH3 CH3CH2CH2CH2CH2Br

348 CHAPTER NINE Alkynes

BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website dehydrohalogenation is needed. Adding water or acid after the reaction is complete converts the sodium salt to the corresponding alkyne.

Double dehydrohalogenation of a geminal dihalide

Double dehydrohalogenation of a vicinal dihalide

Double dehydrohalogenation to form terminal alkynes may also be carried out by heating geminal and vicinal dihalides with potassium tert-butoxide in dimethyl sulfoxide.

PROBLEM 9.7Give the structures of three isomeric dibromides that could be used as starting materials for the preparation of 3,3-dimethyl-1-butyne.

Since vicinal dihalides are prepared by addition of chlorine or bromine to alkenes

(Section 6.14), alkenes, especially terminal alkenes, can serve as starting materials for the preparation of alkynes as shown in the following example:

PROBLEM 9.8Show, by writing an appropriate series of equations, how you could prepare propyne from each of the following compounds as starting materials. You may use any necessary organic or inorganic reagents.

(a) 2-Propanol (d) 1,1-Dichloroethane (b) 1-Propanol (e) Ethyl alcohol (c) Isopropyl bromide

SAMPLE SOLUTION(a) Since we know that we can convert propene to propyne by the sequence of reactions

all that remains to completely describe the synthesis is to show the preparation of propene from 2-propanol. Acid-catalyzed dehydration is suitable.

(CH3)2CHOH2-Propanol

CH3CHœCH2 Propene

CH3CHœCH2Propene

CH3CHCH2Br W

1,2-Dibromopropane

CH3CPCH Propyne

Br2

3-Methyl-1-butyne (52%) CH(CH3)2CHC1,2-Dibromo-3-methylbutane

Br 3-Methyl-1-butene

3NaNH2 NH3

H2O1-Decyne (54%)

Sodium salt of alkyne product (not isolated) CNaCH3(CH2)7C1,2-Dibromodecane

CH3(CH2)7CHCH2Br Br

3,3-Dimethyl- 1-butyne (56–60%)

CH(CH3)3CC

1,1-Dichloro-3,3- dimethylbutane

Sodium salt of alkyne product (not isolated)

CNa(CH3)3CC

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9.8REACTIONS OF ALKYNES

We have already discussed one important chemical property of alkynes, the acidity of acetylene and terminal alkynes. In the remaining sections of this chapter several other reactions of alkynes will be explored. Most of them will be similar to reactions of alkenes. Like alkenes, alkynes undergo addition reactions. We’l begin with a reaction familiar to us from our study of alkenes, namely, catalytic hydrogenation.

9.9HYDROGENATION OF ALKYNES

The conditions for hydrogenation of alkynes are similar to those employed for alkenes. In the presence of finely divided platinum, palladium, nickel, or rhodium, two molar equivalents of hydrogen add to the triple bond of an alkyne to yield an alkane.

PROBLEM 9.9Write a series of equations showing how you could prepare octane from acetylene and any necessary organic and inorganic reagents.

Substituents affect the heats of hydrogenation of alkynes in the same way they affect alkenes. Alkyl groups release electrons to sp-hybridized carbon, stabilizing the alkyne and decreasing the heat of hydrogenation.

Alkenes are intermediates in the hydrogenation of alkynes to alkanes.

The heat of hydrogenation of an alkyne is greater than twice the heat of hydrogenation of the derived alkene. The first hydrogenation step of an alkyne is therefore more exothermic than the second.

Noting that alkenes are intermediates in the hydrogenation of alkynes leads us to consider the possibility of halting hydrogenation at the alkene stage. If partial hydrogenation of an alkyne could be achieved, it would provide a useful synthesis of alkenes. In practice it is a simple matter to convert alkynes to alkenes by hydrogenation in the presence of specially developed catalysts. The one most frequently used is the Lindlar catalyst,a palladium on calcium carbonate combination to which lead acetate and quinoline have been added. Lead acetate and quinoline partially deactivate (“poison”) the catalyst, making it a poor catalyst for alkene hydrogenation while retaining its ability to catalyze the addition of hydrogen to alkynes.

Alkane

H° (hydrogenation) 1-Butyne 292 kJ/mol

(69.9 kcal/mol)

CHCH3CH2C

2-Butyne 275 kJ/mol (65.6 kcal/mol)

Alkane

Hydrogen 2H2 4-Methyl-1-hexyne

CHCH3CH2CHCH2C

CH3 3-Methylhexane (7%)

CH3CH2CHCH2CH2CH3 CH3

350 CHAPTER NINE Alkynes

The high energy of acetylene is released when it is mixed with oxygen and burned in an oxyacetylene torch.The temperature of the flame (about 3000°C) exceeds that of any other hydrocarbon fuel and is higher than the melting point of iron (1535°C).

The structure of quinoline is shown on page 430.

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In subsequent equations, we will not specify the components of the Lindlar palladium catalyst in detail but will simply write “Lindlar Pd” over the reaction arrow.

Hydrogenation of alkynes to alkenes yields the cis (or Z) alkene by syn addition to the triple bond.

PROBLEM 9.10Oleic acid and stearic acid are naturally occurring compounds, which can be isolated from various fats and oils. In the laboratory, each can be prepared by hydrogenation of a compound known as stearolic acid,which has the

over platinum. What are the structures of oleic acid and stearic acid?

Auseful alternative to catalytic partial hydrogenation for converting alkynes to alkenes is reduction by a Group I metal (lithium, sodium, or potassium) in liquid ammonia. The unique feature of metal–ammonia reduction is that it converts alkynes to trans (or E) alkenes whereas catalytic hydrogenation yields cis (or Z) alkenes. Thus, from the same alkyne one can prepare either a cis or a trans alkene by choosing the appropriate reaction conditions.

PROBLEM 9.11Sodium–ammonia reduction of stearolic acid (see Problem 9.10) yields a compound known as elaidic acid.What is the structure of elaidic acid?

PROBLEM 9.12Suggest efficient syntheses of (E)-and (Z)-2-heptene from propyne and any necessary organic or inorganic reagents.

The stereochemistry of metal–ammonia reduction of alkynes differs from that of catalytic hydrogenation because the mechanisms of the two reactions are different. The mechanism of hydrogenation of alkynes is similar to that of catalytic hydrogenation of alkenes (Sections 6.1 and 6.3). Amechanism for metal–ammonia reduction of alkynes is outlined in Figure 9.4.

Na NH3

C3-Hexyne

1-Ethynylcyclohexanol H2 Hydrogen

Pd/CaCO3 lead acetate, quinoline

CH2C OH

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The mechanism includes two single-electron transfers (steps 1 and 3) and two proton transfers (steps 2 and 4). Experimental evidence indicates that step 2 is ratedetermining, and it is believed that the observed trans stereochemistry reflects the distribution of the two stereoisomeric alkenyl radical intermediates formed in this step.

The more stable (E)-alkenyl radical, in which the alkyl groups R and R are trans to each other, is formed faster than its Zstereoisomer. Steps 3 and 4, which follow, are fast, and the product distribution is determined by the E–Zratio of radicals produced in step 2.

9.11ADDITION OF HYDROGEN HALIDES TO ALKYNES

Alkynes react with many of the same electrophilic reagents that add to the carbon–carbon double bond of alkenes. Hydrogen halides, for example, add to alkynes to form alkenyl halides.

RCœCR H±NH2 ±£ RCœCHR 

NH2

352 CHAPTER NINE Alkynes

Overall Reaction:

Step 1: Electron transfer from sodium to the alkyne. The product is an anion radical.

Alkyne Sodium Anion radicalSodium ion

Step 2: The anion radical is a strong base and abstracts a proton from ammonia.

Anionradical Alkenyl radical

Amide ion Ammonia

Step 3: Electron transfer to the alkenyl radical.

Alkenylradical

SodiumSodium ion Alkenyl anion

Step 4: Proton transfer from ammonia converts the alkenyl anion to an alkene.

AmmoniaAlkenyl anionAlkeneAmide ion

Trans alkeneSodium amide

FIGURE 9.4 Mechanism of the sodium–ammonia reduction of an alkyne. BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website

The regioselectivity of addition follows Markovnikov’s rule. Aproton adds to the carbon that has the greater number of hydrogens, and halide adds to the carbon with the fewer hydrogens.

When formulating a mechanism for the reaction of alkynes with hydrogen halides, we could propose a process analogous to that of electrophilic addition to alkenes in which the first step is formation of a carbocation and is rate-determining. The second step according to such a mechanism would be nucleophilic capture of the carbocation by a halide ion.

Evidence from a variety of sources, however, indicates that alkenyl cations (also called vinylic cations) are much less stable than simple alkyl cations, and their involvement in these additions has been questioned. For example, although electrophilic addition of hydrogen halides to alkynes occurs more slowly than the corresponding additions to alkenes, the difference is not nearly as great as the difference in carbocation stabilities would suggest.

Furthermore, kinetic studies reveal that electrophilic addition of hydrogen halides to alkynes follows a rate law that is third-order overall and second-order in hydrogen halide.

This third-order rate dependence suggests a termolecular transition state, one that involves two molecules of the hydrogen halide. Figure 9.5 depicts such a termolecular process using curved arrow notation to show the flow of electrons, and dashed-line notation to

RC CHAlkyne slow fast

Hydrogen halide HXAlkenyl cation

Alkenyl halide

RC CH2 X

1-Hexyne

CHCH3CH2CH2CH2C Hydrogen bromide HBr 2-Bromo-1-hexene (60%)

CH2Br

Alkyne CR RC Hydrogen halide

Alkenyl halide X

FIGURE 9.5 (a), Curved arrow notation and (b) transition-state representation for electrophilic addition of a hydrogen halide HX to an alkyne.

BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website indicate the bonds being made and broken at the transition state. This mechanism, called

AdE3for addition-electrophilic-termolecular,avoids the formation of a very unstable alkenyl cation intermediate by invoking nucleophilic participation by the halogen at an early stage. Nevertheless, since Markovnikov’s rule is observed, it seems likely that some degree of positive character develops at carbon and controls the regioselectivity of addition.

In the presence of excess hydrogen halide, geminal dihalides are formed by sequential addition of two molecules of hydrogen halide to the carbon–carbon triple bond.

(Parte 2 de 4)