14_BRCLoudon_pgs4-2.qxd 11/26/08 9:04 AM Page 644 14 14 The Chemistry of Alkynes An alkyne is a hydrocarbon containing a carbon–carbon triple bond; the simplest member of this family is acetylene, H C'C H. The chemistry of the carbon–carbon triple bond is similar in many respects toL that ofL the carbon–carbon double bond; indeed, alkynes and alkenes undergo many of the same addition reactions. Alkynes also have some unique chem- istry, most of it associated with the bond between hydrogen and the triply bonded carbon, the 'C H bond. L 14.1 NOMENCLATURE OF ALKYNES In common nomenclature, simple alkynes are named as derivatives of the parent compound acetylene: H3CCC' H L L methylacetylene H3CCC' CH3 dimethylacetyleneL L CH3CH2 CC' CH3 ethylmethylacetyleneL L Certain compounds are named as derivatives of the propargyl group, HC'C CH2 , in the common system. The propargyl group is the triple-bond analog of the allyl group.L L HC' C CH2 Cl H2CA CH CH2 Cl L L LL propargyl chloride allyl chloride 644 14_BRCLoudon_pgs4-2.qxd 11/26/08 9:04 AM Page 645 14.1 NOMENCLATURE OF ALKYNES 645 We might expect the substitutive nomenclature of alkynes to be much like that of alkenes, and it is. The suffix ane in the name of the corresponding alkane is replaced by the suffix yne, and the triple bond is given the lowest possible number. H3CCC' H CH3CH2CH2CH2 CC' CH3 H3C CH2 C ' CH L L L L L L L propyne 2-heptyne 1-butyne H3C CH C ' C CH3 HC' C CH2 CH2 C' C CH3 L L L L 1,5-heptadiyneLL L "CH3 4-methyl-2-pentyne Substituent groups that contain a triple bond (called alkynyl groups) are named by replac- ing the final e in the name of the corresponding alkyne with the suffix yl. (This is exactly anal- ogous to the nomenclature of substituent groups containing double bonds; see Sec. 4.2A.) The alkynyl group is numbered from its point of attachment to the main chain: HC' C HC' C CH2 L LL ethynyl group 2-propynyl group (ethyne yl) + OH 1 " 2 ` 12 3 CH C ' CH $ 2 3-(2-propynyl)cyclohexanol position of triple bond within the substituent position of the 2-propynyl group on the ring As with alkenes, groups that can be cited as principal groups, such as the OH group in the following example (as well as in the previous one), are given numerical precedenceL over the triple bond. (See Appendix I for a summary of nomenclature rules.) OH HC' C CH2 "CH CH3 5 43L L 2L 1 4-pentyn-2-ol OH group receives numerical priority When a molecule contains both double and triple bonds, the bond that has the lower num- ber at first point of difference receives numerical precedence. However, if this rule is ambigu- ous, a double bond receives numerical precedence over a triple bond. 14_BRCLoudon_pgs4-2.qxd 11/26/08 9:04 AM Page 646 646 CHAPTER 14 • THE CHEMISTRY OF ALKYNES 123 45 54 3 2 1 12345 HC C CH CHCH3 CH3CC CH CH2 H2C CHCH2C CH 1-pentyn-3-ene 1-penten-3-yne 1-penten-4-yne precedence is given to the bond that has precedence is given to the double bond lower number at first point of difference when numbering is ambiguous PROBLEMS 14.1 Draw a Lewis structure for each of the following alkynes. (a) isopropylacetylene (b) cyclononyne (c) 4-methyl-1-pentyne (d) 1-ethynylcyclohexanol (e) 2-butoxy-3-heptyne (f) 1,3-hexadiyne 14.2 Provide the substitutive name for each of the following compounds. Also provide common names for (a) and (b). (a) CH3CH2CH2CH2C'CH (b) CH3CH2CH2CH2C'CCH2CH2CH2CH3 (c) OH (d) HC ' CCHCH2CH2CH3 H3CC"C C ' CH3 H2C H LL L "CH3 CC H CH2OCH3 (e) OH HC C CH CH CH2 14.2 STRUCTURE AND BONDING IN ALKYNES Because each carbon of acetylene is connected to two groups—a hydrogen and another car- bon—the H C'C bond angle in acetylene is 180 (Sec. 1.3B); thus, the acetylene molecule is linear. L ° 1.20 Å HHCC'' LL 180° 1.06 Å The C'C bond, with a bond length of 1.20 Å, is shorter than the CAC and C C bonds, which have bond lengths of 1.33 Å and 1.54 Å, respectively. L Because of the 180 bond angles at the carbon–carbon triple bond, cis–trans isomerism cannot occur in alkynes.° Thus, although 2-butene exists as cis and trans stereoisomers, 2-bu- tyne does not. Another consequence of this linear geometry is that cycloalkynes smaller than cyclooctyne cannot be isolated under ordinary conditions (see Problem 14.3). The hybrid orbital model for bonding provides a useful description of bonding in alkynes. We learned in Secs. 1.9B and 4.1A that carbon hybridization and geometry are correlated: tetrahedral carbon is sp3-hybridized, and trigonal planar carbon is sp2-hybridized. The linear geometry found in alkynes is characterized by a third type of carbon hybridization, called sp hybridization. Imagine that the 2s orbital and one 2p orbital (say, the 2px orbital) on carbon mix to form two new hybrid orbitals. Because these two new orbitals are each one part s and one part p, they are called sp hybrid orbitals. Two of the 2p orbitals (2py and 2pz) are not in- cluded in the hybridization..