《有机化学》课程教学资源（教材文献，英文版）CHAPTER 9 ALKYNES

chAPTER 9 ALKYNES H ydrocarbons that contain a carbon-carbon triple bond are called alkynes. Non- cyclic alkynes have the molecular formula C,H2n-2 Acetylene(HC=CH) is th simplest alkyne. We call compounds that have their triple bond at the end of carbon chain(RC=CH) monosubstituted, or terminal, alkynes. Disubstituted alkyne (RCECR') are said to have internal triple bonds. You will see in this chapter that a car- bon-carbon triple bond is a functional group, reacting with many of the same reagents that react with the double bonds of alkenes their. The most distinctive aspect of the chemistry of acetylene and terminal alkynes is acidity. As a class, compounds of the type RC=CH are the most acidic of all sim- ple 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 9.1 SOURCES 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 car- on obtained from coal are heated in an electric furnace to form calcium carbide 800-2100° Cac Calcium oxide Carbon Calcium carbide Carbon monoxide (from limestone) (from coke) Calcium carbide is the calcium salt of the doubly negative carbide ion (CEC ) Car- bide dianion is strongly basic and reacts with water to form acetylene 339 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

339 CHAPTER 9 ALKYNES Hydrocarbons that contain a carbon–carbon triple bond are called alkynes. Non￾cyclic alkynes have the molecular formula CnH2n2. 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 internal triple bonds. You will see in this chapter that a car￾bon–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 sim￾ple 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. 9.1 SOURCES 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 car￾bon 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) CaO Carbon (from coke) 3C Carbon monoxide CO 1800–2100°C CaC2 Calcium carbide   Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

CHAPTER NINE Alkynes +2H2O Ca(OH)2+HC≡CH Calcium carbide Water Calcium hydroxide Acetylene PROBLEM 9.1 Use curved arrows to show how calcium carbide reacts with water Beginning in the middle of the twentieth century, alternative methods of acetylene luction became practical. One of these is based on the dehydrogenation of ethylene CH,=CH, HC≡CH+H Ethylene The reaction is endothermic, and the equilibrium favors ethylene at low temperatures but shifts to favor acetylene above 1150C. Indeed, at very high temperatures most hydro- carbons, 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 exam- ples are tariric acid, from the seed fat of a Guatemalan plant, and cicutoxin, a poiso- CH3(CH)IoC=C(CH2)4COH HOCH, CH,CH,=C-C=CCH=CHCH=CHCH=CHCHCH,CH, CH3 OH Diacetylene(HC=C-C=CH) has been identified as a component of the hydro- carbon-rich atmospheres of Uranus, Neptune, and Pluto. It is also present in the atmo- spheres of Titan and Triton, satellites of Saturn and Neptune, respectively 9.2 NOMENCLATURE ning alkynes the usual IUPAC rules for hydrocarbons are followed, and the suffix -ane is replaced by -yne. Both acetylene and ethyne are acceptable IUPAC names for HCECH. The position of the triple bond along the chain is specified by number in a manner analogous to alkene nomenclature C≡CCH HC≡CCH2CH3CH3C≡CCH3(CH3)3CC≡CCH 1-Butyne 2-Butyne 4, 4-Dimethyl-2-pentyne PROBLEM 9.2 Write structural formulas and give the IUPAC names for all the alkynes of molecular formula CsHg When the -C=CH group is named as a substituent, it is designated as an ethy Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

PROBLEM 9.1 Use 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 hydro￾carbons, 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 exam￾ples are tariric acid, from the seed fat of a Guatemalan plant, and cicutoxin, a poiso￾nous substance isolated from water hemlock. Diacetylene (HCPC±CPCH) has been identified as a component of the hydro￾carbon-rich atmospheres of Uranus, Neptune, and Pluto. It is also present in the atmo￾spheres 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 -ane is 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. PROBLEM 9.2 Write structural formulas and give the IUPAC names for all the alkynes of molecular formula C5H8. 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 (CH3)3CCPCCH3 Tariric acid CH3(CH2)10CPC(CH2)4COH O X Cicutoxin HOCH2CH2CH2CPC±CPCCHœCHCHœCHCHœCHCHCH2CH2CH3 W OH Ethylene CH2œCH2 Hydrogen HCPCH H2 Acetylene  heat   Water 2H2O Ca(OH)2 Calcium hydroxide HCPCH Calcium carbide Acetylene Ca2 C Ω C 2 340 CHAPTER NINE Alkynes Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

9.4 Structure and Bonding in Alkynes: sp Hybridization 9.3 PHYSICAL PROPERTIES OF ALKYNES Alkynes resemble alkanes and alkenes in their physical properties. They share with these Examples of physical proper- other hydrocarbons the properties of low density and low water-solubility. They are ties of alkynes are given in slightly more polar and generally have slightly higher boiling points than the corre- Appendix sponding alkanes and alkenes 9.4 STRUCTURE AND BONDING IN ALKYNES: sp HYBRIDIZATION Acetylene is linear, with a carbon-carbon bond distance of 120 pm and carbon-hydro- gen bond distances of 106 pn HC≡C-H Linear geometries characterize the H-C=C-C and C-C=C-C units of ter- minal 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-C=C-C unit is clearly evi- dent. Angle strain destabilizes cycloalkynes to the extent that cyclononyne is the small est one that is stable enough to be stored for long periods. The next smaller one, cyclooc tyne, has been isolated, but is relatively reactive and polymerizes on standing In spite of the fact that few cycloalkynes occur naturally, they gained recent atten- tion when it was discovered that some of them hold promise as anticancer drugs. (See the boxed essay Natural and"Designed"Enediyne Antibiotics following this section. An sp hybridization model for the carbon-carbon triple bond was develope Section 1. 18 and is reviewed for acetylene in Figure 9. 2. Figure 9.3 maps the electro- static potential in ethylene and acetylene and shows how the second T bond in acety lene causes a band of high electron density to encircle the molecule IGURE 9.1 Molecular model of cyclononyne, showing bending of bond angles th triply bonded carbons. This model represents the structure obtained when the stra is minimized according to molecular mechanics and closely matches the structure dete perimentally Notice too the degree to which the staggering of bonds on adjacent atom the overall shape of the ring Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

9.3 PHYSICAL 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 corre￾sponding alkanes and alkenes. 9.4 STRUCTURE AND BONDING IN ALKYNES: sp HYBRIDIZATION Acetylene is linear, with a carbon–carbon bond distance of 120 pm and carbon–hydro￾gen bond distances of 106 pm. Linear geometries characterize the H±CPC±C and C±CPC±C units of ter￾minal 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 evi￾dent. Angle strain destabilizes cycloalkynes to the extent that cyclononyne is the small￾est one that is stable enough to be stored for long periods. The next smaller one, cyclooc￾tyne, has been isolated, but is relatively reactive and polymerizes on standing. In spite of the fact that few cycloalkynes occur naturally, they gained recent atten￾tion when it was discovered that some of them hold promise as anticancer drugs. (See the boxed essay Natural and “Designed” Enediyne Antibiotics following this section.) An sp hybridization 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 electro￾static potential in ethylene and acetylene and shows how the second bond in acety￾lene causes a band of high electron density to encircle the molecule. H C C H 120 pm 106 pm 106 pm 180° 180° 9.4 Structure and Bonding in Alkynes: sp Hybridization 341 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 ex￾perimentally. Notice too the degree to which the staggering of bonds on adjacent atoms governs the overall shape of the ring. Examples of physical proper￾ties of alkynes are given in Appendix 1. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

CHAPTER NINE Alkynes FIGURE 9.2 The carbon atoms of acetylene are connected by a o+T+ triple bond. Both carbon atoms are sp-hybridized, and each is bonded to a hydrogen by an sp-1s o bond. The component of the triple bond arises by sp-sp overlap. Each carbon has two p orbitals, the axes f which are perpendicular to each other. One t bond is formed by overlap of the p orbitals shown in(b), the other by overlap of the p orbitals shown in(c). Each t bond contains two FIGURE 9.3 Electro- static potential maps of eth ylene and acetylene. The re (red) is associated with the Tr bonds and lies be. tween the two carbons in both. This electron-rich re- plane of the molecule in eth ylene. Because acetylene ha two bonds, its band of high electron density encircles the 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: 1. The geometry at carbon changes from tetrahedral trigonal planar -linear 2. The C-C and C-H bonds become shorter and stronger 3. The acidity of the C-H bonds increases. All of these trends can be accommodated by the orbital hybridization model. The bond angles are characteristic for the sp, sp", and sp hybridization states of carbon and dont require additional comment. The bond distances, bond strengths, and acidities are related to the s character in the orbitals used for bonding. s Character is a simple concept, being nothing more than the percentage of the hybrid orbital contributed by an s orbital. Thus, an sp orbital has one quarter s character and three quarters p, an sp- orbital has one third s and two thirds P, and an sp orbital one half s and one half p. We then use this information to analyze how various qualities of the hybrid orbital reflect those of its s and p contributors Take C-H bond distance and bond strength, for example. Recalling that an elec- tron in a 2s orbital is, on average, closer to the nucleus and more strongly held than an Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

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: 1. The geometry at carbon changes from tetrahedral → trigonal planar → linear. 2. The C±C and C±H bonds become shorter and stronger. 3. The acidity of the C±H bonds increases. All of these trends can be accommodated by the orbital hybridization model. The bond angles are characteristic for the sp3 , sp2 , and sp hybridization states of carbon and don’t require additional comment. The bond distances, bond strengths, and acidities are related to the s character in the orbitals used for bonding. s Character is a simple concept, being nothing more than the percentage of the hybrid orbital contributed by an s orbital. Thus, an sp3 orbital has one quarter s character and three quarters p, an sp2 orbital has one third s and two thirds p, and an sp orbital one half s and one half p. We then use this information to analyze how various qualities of the hybrid orbital reflect those of its s and p contributors. Take C±H bond distance and bond strength, for example. Recalling that an elec￾tron in a 2s orbital is, on average, closer to the nucleus and more strongly held than an 342 CHAPTER NINE Alkynes (a) (b) (c) 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–sp overlap. Each carbon has two p orbitals, the axes of which are perpendicular to each other. One bond is formed by overlap of the p orbitals shown in (b), the other by overlap of the p orbitals shown in (c). Each bond contains two electrons. FIGURE 9.3 Electro￾static potential maps of eth￾ylene and acetylene. The re￾gion of highest negative charge (red) is associated with the bonds and lies be￾tween the two carbons in both. This electron-rich re￾gion is above and below the plane of the molecule in eth￾ylene. Because acetylene has two bonds, its band of high electron density encircles the molecule. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

9.4 Structure and Bonding in Alkynes: sp Hybridization TABLE 9.1 Structural Features of Ethane, Ethylene, and Acetylene Feature Ethane Ethylene Acetylene Systematic name Ethane Ethene Ethyne Molecular formula CH CH Structural formula H-C≡c-H C-C bond distance, pm 153 134 120 C-H bond distance, pm 111 106 C bond angles 111.0° 1214° 180 C-C bond dissociation energy kJ/mol (kcal/mol) 368(88) 611(146) 820(196) C-H bond dissociation energy, kJ/mol (kcal/mol) 452(108) 536(128) Hybridization of carbon character in C-H bonds Approximate acidity as measured by Ka(pka) 10-45(45) electron in a 2p orbital, it follows that an electron in an orbital with more s character will be closer to the nucleus and more strongly held than an electron in an orbital with less s character. Thus, when an sp orbital of carbon overlaps with a hydrogen ls orbital to give a C-H o bond, the electrons are held more strongly and the bond is stronger nd shorter than electrons in a bond between hydrogen and sp-hybridized carbon. Sim- ilar reasoning holds for the shorter C-C bond distance of acetylene compared to eth ylene, although here the additional T 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 corre- ponding bonds to the sp- hybridized carbons of propene propene and propyne compare H with the experimental values? Propyne ate // An easy way to keep track of the effect of the s character of carbon is to associ- ate it with electronegativity. As the s character of carbon increases, so does that carbon's apparent electronegativity( the electrons in the bond involving that orbital are closer to arbon). 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 9.3 How do bond distances and bond strengths change with elec tronegativity 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 cor red with sp Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

electron in a 2p orbital, it follows that an electron in an orbital with more s character will be closer to the nucleus and more strongly held than an electron in an orbital with less s character. Thus, when an sp orbital 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. Sim￾ilar reasoning holds for the shorter C±C bond distance of acetylene compared to eth￾ylene, 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 corre￾sponding bonds to the sp2 hybridized carbons of propene. An easy way to keep track of the effect of the s character of carbon is to associ￾ate it with electronegativity. As the s character 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 9.3 How do bond distances and bond strengths change with elec￾tronegativity 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 sp3 and sp2 . H 106 pm 146 pm 121 pm C C CH3 Propyne C H CH3 H H 134 pm 151 pm 108 pm C Propene 9.4 Structure and Bonding in Alkynes: sp Hybridization 343 TABLE 9.1 Structural Features of Ethane, Ethylene, and Acetylene Feature Systematic name Molecular formula C±C bond distance, pm C±H bond distance, pm H±C±C bond angles C±C bond dissociation energy, kJ/mol (kcal/mol) C±H bond dissociation energy, kJ/mol (kcal/mol) Hybridization of carbon s character in C±H bonds Approximate acidity as measured by Ka (pKa) Structural formula Ethyne C2H2 120 106 180° 820 (196) 536 (128) sp 50% 1026 (26) Acetylene H C C H Ethene C2H4 134 110 121.4° 611 (146) 452 (108) sp2 33% 1045 (45) Ethylene C H H H H C Ethane C2H6 153 111 111.0° 368 (88) 410 (98) sp3 25% 1062 (62) Ethane C H H H H H H C How do the bond dis￾tances of molecular models of propene and propyne compare with the experimental values? Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website