3.3 Conformations of Higher Alkanes The first widely used molecular mechanics pro- that molecular mechanics is no longer considered a gram was developed by Professor N L Allinger of the novelty but rather as one more tool to be used by the University of Georgia and was known in its various practicing organic chemist. They have been joined by versions as MM2, MM3, and so on. they have been re- programs that calculate the energies of conforma- fined to the extent that many structural features can tions by molecular orbital methods. The Learning by be calculated more easily and more accurately than Modeling CD that accompanies this text contains hey can be measured experimentally molecular mechanics software that lets you seek out Once requiring minicomputers and worksta. the most stable conformation of the structures you tions, many molecular mechanics programs are avail- assemble. It also contains the most stable conforma- strain energy calculations can provide is so helpful orbital calculations les as determined by molecular FIGURE 3.8 Ball-and-spoke and space-filling models of methyl-methyl eclipsed conformation of butane. 3.3 CONFORMATIONS OF HIGHER ALKANES Higher alkanes having unbranched carbon chains are like butane most stable in their all-anti conformations. The energy difference between gauche and anti conformations is similar to that of butane, and appreciable quantities of the gauche conformation are present in liquid alkanes at 25C. In depicting the conformations of higher alkanes it is often more helpful to look at them from the side rather than end-on as in a Newman projection. Viewed from this perspective, the most stable conformations of pentane and hexane have their carbon ""backbones"arranged in a zigzag fashion, as shown in Figure 3.9. All the bonds are staggered, and the chains are characterized by anti arrangements 68 Hexane FIGURE 3.9 Ball-and-spoke models of pentane and hexane in their all-anti (zigzag)con- Back Forward Main MenuToc Study Guide ToC Student o MHHE Website3.3 Conformations of Higher Alkanes 97 The first widely used molecular mechanics pro￾gram was developed by Professor N. L. Allinger of the University of Georgia and was known in its various versions as MM2, MM3, and so on. They have been re- fined to the extent that many structural features can be calculated more easily and more accurately than they can be measured experimentally. Once requiring minicomputers and worksta￾tions, many molecular mechanics programs are avail￾able for personal computers. The information that strain energy calculations can provide is so helpful that molecular mechanics is no longer considered a novelty but rather as one more tool to be used by the practicing organic chemist. They have been joined by programs that calculate the energies of conforma￾tions by molecular orbital methods. The Learning By Modeling CD that accompanies this text contains molecular mechanics software that lets you seek out the most stable conformation of the structures you assemble. It also contains the most stable conforma￾tions of some molecules as determined by molecular orbital calculations. 3.3 CONFORMATIONS OF HIGHER ALKANES Higher alkanes having unbranched carbon chains are, like butane, most stable in their all-anti conformations. The energy difference between gauche and anti conformations is similar to that of butane, and appreciable quantities of the gauche conformation are present in liquid alkanes at 25°C. In depicting the conformations of higher alkanes it is often more helpful to look at them from the side rather than end-on as in a Newman projection. Viewed from this perspective, the most stable conformations of pentane and hexane have their carbon “backbones” arranged in a zigzag fashion, as shown in Figure 3.9. All the bonds are staggered, and the chains are characterized by anti arrangements of C±C±C±C units. Pentane Hexane FIGURE 3.8 Ball-and-spoke and space-filling models of methyl-methyl eclipsed conformation of butane. FIGURE 3.9 Ball-and-spoke models of pentane and hexane in their all-anti (zigzag) con￾formations. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website