Carey - Organic Chemistry - chapt09

Carey - Organic Chemistry - chapt09

(Parte 1 de 4)

CHAPTER 9 ALKYNES

Hydrocarbons that contain a carbon–carbon triple bond are called alkynes.Noncyclic alkynes have the molecular formula CnH2n 2. Acetylene(HCPCH) is the simplest alkyne. We call compounds that have their triple bond at the end of a carbon chain (RCPCH) monosubstituted,or terminal, alkynes.Disubstituted alkynes (RCPCR ) are said to have internaltriple bonds. You will see in this chapter that a carbon–carbon triple bond is a functional group, reacting with many of the same reagents that react with the double bonds of alkenes.

The most distinctive aspect of the chemistry of acetylene and terminal alkynes is their acidity. As a class, compounds of the type RCPCH are the most acidic of all simple hydrocarbons. The structural reasons for this property, as well as the ways in which it is used to advantage in chemical synthesis, are important elements of this chapter.

9.1SOURCES OF ALKYNES

Acetylene was first characterized by the French chemist P. E. M. Berthelot in 1862 and did not command much attention until its large-scale preparation from calcium carbide in the last decade of the nineteenth century stimulated interest in industrial applications. In the first stage of that synthesis, limestone and coke, a material rich in elemental carbon obtained from coal, are heated in an electric furnace to form calcium carbide.

Calcium carbide is the calcium salt of the doubly negative carbide ion (). Car-bide dianion is strongly basic and reacts with water to form acetylene: CPC

Calcium oxide (from limestone)

Carbon (from coke)

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PROBLEM 9.1Use curved arrows to show how calcium carbide reacts with water to give acetylene.

Beginning in the middle of the twentieth century, alternative methods of acetylene production became practical. One of these is based on the dehydrogenation of ethylene.

The reaction is endothermic, and the equilibrium favors ethylene at low temperatures but shifts to favor acetylene above 1150°C. Indeed, at very high temperatures most hydrocarbons, even methane, are converted to acetylene. Acetylene has value not only by itself but is also the starting material from which higher alkynes are prepared.

Natural products that contain carbon–carbon triple bonds are numerous. Two examples are tariric acid,from the seed fat of a Guatemalan plant, and cicutoxin,a poisonous substance isolated from water hemlock.

Diacetylene (HCPC±CPCH) has been identified as a component of the hydrocarbon-rich atmospheres of Uranus, Neptune, and Pluto. It is also present in the atmospheres of Titan and Triton, satellites of Saturn and Neptune, respectively.

9.2 NOMENCLATURE

In naming alkynes the usual IUPAC rules for hydrocarbons are followed, and the suffix -aneis replaced by -yne.Both acetylene and ethyne are acceptable IUPAC names for HCPCH. The position of the triple bond along the chain is specified by number in a manner analogous to alkene nomenclature.

When the ±CPCH group is named as a substituent, it is designated as an ethynyl group.

Propyne

HCPCCH3 1-Butyne

HCPCCH2CH3 2-Butyne CH3CPCCH3 4,4-Dimethyl-2-pentyne

Cicutoxin

HOCH2CH2CH2CPC±CPCCHœCHCHœCHCHœCHCHCH2CH2CH3 W

Ethylene

CH2œCH2 Hydrogen H2HCPCHAcetylene heat

Water 2H2O Ca(OH)2Calcium hydroxide

AcetyleneCalcium carbide

340 CHAPTER NINE Alkynes

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9.3PHYSICAL PROPERTIES OF ALKYNES

Alkynes resemble alkanes and alkenes in their physical properties. They share with these other hydrocarbons the properties of low density and low water-solubility. They are slightly more polar and generally have slightly higher boiling points than the corresponding alkanes and alkenes.

9.4STRUCTURE AND BONDING IN ALKYNES: spHYBRIDIZATION

Acetylene is linear, with a carbon–carbon bond distance of 120 pm and carbon–hydrogen bond distances of 106 pm.

Linear geometries characterize the H±CPC±C and C±CPC±C units of terminal and internal triple bonds, respectively as well. This linear geometry is responsible for the relatively small number of known cycloalkynes.Figure 9.1 shows a molecular model for cyclononyne in which the bending of the C±CPC±C unit is clearly evident. Angle strain destabilizes cycloalkynes to the extent that cyclononyne is the smallest one that is stable enough to be stored for long periods. The next smaller one, cyclooctyne, has been isolated, but is relatively reactive and polymerizes on standing.

In spite of the fact that few cycloalkynes occur naturally, they gained recent attention when it was discovered that some of them hold promise as anticancer drugs. (See the boxed essay Natural and “Designed” Enediyne Antibioticsfollowing this section.)

An sphybridization model for the carbon–carbon triple bond was developed in

Section 1.18 and is reviewed for acetylene in Figure 9.2. Figure 9.3 maps the electrostatic potential in ethylene and acetylene and shows how the second bond in acetylene causes a band of high electron density to encircle the molecule.

120 pm 106 pm106 pm

9.4Structure and Bonding in Alkynes: spHybridization341

FIGURE 9.1 Molecular model of cyclononyne, showing bending of bond angles associated with triply bonded carbons. This model represents the structure obtained when the strain energy is minimized according to molecular mechanics and closely matches the structure determined experimentally. Notice too the degree to which the staggering of bonds on adjacent atoms governs the overall shape of the ring.

Examples of physical properties of alkynes are given in Appendix 1.

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At this point, it’s useful to compare some structural features of alkanes, alkenes, and alkynes. Table 9.1 gives some of the most fundamental ones. To summarize, as we progress through the series in the order ethane →ethylene →acetylene:

All of these trends can be accommodated by the orbital hybridization model. The bond angles are characteristic for the sp3, sp2, and sphybridization states of carbon and don’t require additional comment. The bond distances, bond strengths, and acidities are related to the scharacter in the orbitals used for bonding. sCharacter is a simple concept, being nothing more than the percentage of the hybrid orbital contributed by an sorbital. Thus, an sp3orbital has one quarter scharacter and three quarters p,an sp2orbital has one third sand two thirds p,and an sporbital one half sand one half p.We then use this information to analyze how various qualities of the hybrid orbital reflect those of its s and pcontributors.

Take C±H bond distance and bond strength, for example. Recalling that an electron in a 2sorbital is, on average, closer to the nucleus and more strongly held than an

Ethylene Acetylene

FIGURE 9.2 The carbon atoms of acetylene are connected by a triple bond. Both carbon atoms are sp-hybridized, and each is bonded to a hydrogen by an sp–1s bond. The component of the triple bond arises by sp–spoverlap. Each carbon has two porbitals, the axes of which are perpendicular to each other. One bond is formed by overlap of the porbitals shown in (b), the other by overlap of the porbitals shown in (c). Each bond contains two electrons.

FIGURE 9.3 Electrostatic potential maps of ethylene and acetylene. The region of highest negative charge (red) is associated with the bonds and lies between the two carbons in both. This electron-rich region is above and below the plane of the molecule in ethylene. Because acetylene has two bonds, its band of high electron density encircles the molecule.

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electron in a 2porbital, it follows that an electron in an orbital with more scharacter will be closer to the nucleus and more strongly held than an electron in an orbital with less scharacter. Thus, when an sporbital of carbon overlaps with a hydrogen 1s orbital to give a C±H bond, the electrons are held more strongly and the bond is stronger and shorter than electrons in a bond between hydrogen and sp2-hybridized carbon. Similar reasoning holds for the shorter C±C bond distance of acetylene compared to ethylene, although here the additional bond in acetylene is also a factor.

The pattern is repeated in higher alkynes as shown when comparing propyne and propene. The bonds to the sp-hybridized carbons of propyne are shorter than the corresponding bonds to the sp2hybridized carbons of propene.

An easy way to keep track of the effect of the scharacter of carbon is to associate it with electronegativity. As the scharacter of carbon increases, so does that carbon’s apparent electronegativity (the electrons in the bond involving that orbital are closer to carbon). The hydrogens in C±H bonds behave as if they are attached to an increasingly more electronegative carbon in the series ethane →ethylene →acetylene.

PROBLEM 9.3How do bond distances and bond strengths change with electronegativity in the series NH3, H2O, and HF?

The property that most separates acetylene from ethane and ethylene is its acidity.

It, too, can be explained on the basis of the greater electronegativity of sp-hybridized carbon compared with sp3and sp2 .

106 pm146 pm 121 pm

C CH3Propyne

C CH3H

H 134 pm151 pm108 pm

Propene

9.4Structure and Bonding in Alkynes: spHybridization343

TABLE 9.1Structural Features of Ethane, Ethylene, and Acetylene

Feature

Systematic name Molecular formula

Approximate acidity as measured by Ka (pKa)

Structural formula

Acetylene

Ethylene

Ethane

How do the bond distances of molecular models of propene and propyne compare with the experimental values?

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9.5ACIDITY OF ACETYLENE AND TERMINAL ALKYNES

The C±H bonds of hydrocarbons show little tendency to ionize, and alkanes, alkenes, and alkynes are all very weak acids. The ionization constant Kafor methane, for example, is too small to be measured directly but is estimated to be about 10 60(pKa60).

Methide anion (a carbanion)

344 CHAPTER NINE Alkynes NATURAL AND “DESIGNED” ENEDIYNE ANTIBIOTICS

Beginning in the 1980s, research directed toward the isolation of new drugs derived from natural sources identified a family of tumor-inhibitory antibiotic substances characterized by novel structures containing a CPC±CœC±CPC unit as part of a 9- or 10-membered ring. With one double bond and two triple bonds (-ene di- -yne), these compounds soon became known as enediyneantibiotics. The simplest member of the class is dynemicin A*;most of the other enediynes have even more complicated structures.

Enediynes hold substantial promise as anticancer drugs because of their potency and selectivity. Not only do they inhibit cell growth, they have a greater tendency to kill cancer cells than they do normal cells. The mechanism by which enediynes act involves novel chemistry unique to the CPC±CœC±CPC unit, which leads to a species that cleaves DNA and halts tumor growth. The history of drug development has long been based on naturally occurring substances. Often, however, compounds that might be effective drugs are produced by plants and microorganisms in such small amounts that their isolation from natural sources is not practical. If the structure is relatively simple, chemical synthesis provides an alternative source of the drug, making it more available at a lower price. Equally important, chemical synthesis, modification, or both can improve the effectiveness of a drug. Building on the enediyne core of dynemicin A, for example, Professor Kyriacos C. Nicolaou and his associates at the Scripps Research Institute and the University of California at San Diego have prepared a simpler analog that is both more potent and more selective than dynemicin A. It is a “designed enediyne” in that its structure was conceived on the basis of chemical reasoning so as to carry out its biochemical task. The designed enediyne offers the additional advantage of being more amenable to large-scale synthesis.

CH3C C

OCH3

Dynemicin A“Designed” enediyne

*Learning By Modelingcontains a model of dynemicin A, which shows that the CPC±CœC±CPC unit can be incorporated into the molecule without much angle strain.

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The conjugate base of a hydrocarbon is called a carbanion.It is an anion in which the negative charge is borne by carbon. Since it is derived from a very weak acid, a car- banion such as :CH3is an exceptionally strong base. In general, the ability of an atom to bear a negative charge is related to its elec- tronegativity. Both the electronegativity of an atom X and the acidity of H±X increase across a row in the periodic table.

Using the relationship from the preceding section that the effective electronegativity of carbon in a C±H bond increases with its scharacter (sp3 sp2 sp), the order of hydrocarbon acidity behaves much like the preceding methane, ammonia, water, hydrogen fluoride series.

The acidity increases as carbon becomes more electronegative. Ionization of acetylene gives an anion in which the unshared electron pair occupies an orbital with 50% s character.

In the corresponding ionizations of ethylene and ethane, the unshared pair occupies an orbital with 3% (sp2) and 25% (sp3) scharacter, respectively. Terminal alkynes (RCPCH) resemble acetylene in acidity.

Although acetylene and terminal alkynes are far stronger acids than other hydrocarbons, we must remember that they are, nevertheless, very weak acids—much weaker than water and alcohols, for example. Hydroxide ion is too weak a base to convert acetylene to its anion in meaningful amounts. The position of the equilibrium described by the following equation lies overwhelmingly to the left:

Because acetylene is a far weaker acid than water and alcohols, these substances are not suitable solvents for reactions involving acetylide ions. Acetylide is instantly converted to acetylene by proton transfer from compounds that contain hydroxyl groups.

Acetylene (weaker acid)

Hydroxide ion (weaker base)

Acetylide ion (stronger base)

Water (stronger acid)

Acetylene Proton

CH3CH3

Ethane pKa 62 (weakest acid)

Acetylene 10 26 26 (strongest acid)

CH4

Methane pKa 60 (weakest acid)

NH3

Hydrogen fluoride 3.5 10 4 3.2 (strongest acid)

The electrostatic poten- tial map of (CH3)3CCPCH on Learning By Modelingclearly shows the greater positive character of the acetylenic hydrogen relative to the methyl hydrogens.

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Amideion is a much stronger base than acetylide ion and converts acetylene to its conjugate base quantitatively.

Solutions of sodium acetylide (HCPCNa) may be prepared by adding sodium amide

(NaNH2) to acetylene in liquid ammonia as the solvent. Terminal alkynes react similarly to give species of the type RCPCNa.

PROBLEM 9.4Complete each of the following equations to show the conjugate acid and the conjugate base formed by proton transfer between the indicated species. Use curved arrows to show the flow of electrons, and specify whether the position of equilibrium lies to the side of reactants or products.

(b) (c)

(d)

(Parte 1 de 4)

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