Inglês

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
BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website
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%)
BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website
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
BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website
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.
BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website
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
BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website
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)