informational Analysis of Butane called van der Waals strain, or steric hindrance and contributes to the total steric strain. In the case of butane, van der Waals strain makes the gauche conformation approx- imately 3.2 kJ/mol(0. 8 kcal/mol) less stable than the anti. Figure 3. 7 illustrates the potential energy relationships among the various confor- mations of butane. The staggered conformations are more stable than the eclipsed. At any instant, almost all the molecules exist in staggered conformations, and more are present in the anti conformation than in the gauche. The point of maximum potential energy lies some 25 kJ/mol(6. 1 kcal/mol) above the anti conformation. The total strain in this structure is approximately equally divided between the torsional strain associated with three pairs of eclipsed bonds(12 kJ/mol: 2.9 kcal/mol) and the van der Waals strain between the methyl groups PROBLEM 3.3 Sketch a potential energy diagram for rotation around a car bon-carbon bond in propane. Clearly identify each potential energy maximum and minimum with a structural formula that shows the conformation of propane It that point. Does your diagram more closely resemble that of ethane or of butane? Would you expect the activation energy for bond rotation in propane to be more than or less than that of ethane? of butane? 大丈 15金 FIGURE 3.7 Potential energy diagram for rotation around the central carbon-carbon bond n butane Back Forward Main MenuToc Study Guide ToC Student o MHHE Websitecalled van der Waals strain, or steric hindrance and contributes to the total steric strain. In the case of butane, van der Waals strain makes the gauche conformation approx￾imately 3.2 kJ/mol (0.8 kcal/mol) less stable than the anti. Figure 3.7 illustrates the potential energy relationships among the various confor￾mations of butane. The staggered conformations are more stable than the eclipsed. At any instant, almost all the molecules exist in staggered conformations, and more are present in the anti conformation than in the gauche. The point of maximum potential energy lies some 25 kJ/mol (6.1 kcal/mol) above the anti conformation. The total strain in this structure is approximately equally divided between the torsional strain associated with three pairs of eclipsed bonds (12 kJ/mol; 2.9 kcal/mol) and the van der Waals strain between the methyl groups. PROBLEM 3.3 Sketch a potential energy diagram for rotation around a car￾bon–carbon bond in propane. Clearly identify each potential energy maximum and minimum with a structural formula that shows the conformation of propane at that point. Does your diagram more closely resemble that of ethane or of butane? Would you expect the activation energy for bond rotation in propane to be more than or less than that of ethane? Of butane? 3.2 Conformational Analysis of Butane 95 Potential energy, kcal/mol 3 kJ/mol 6 5 4 3 2 1 0 Potential energy, kJ/mol 25 20 15 10 5 0 0 60 120 180 240 300 360 Torsion angle,
14 kJ/mol FIGURE 3.7 Potential energy diagram for rotation around the central carbon–carbon bond in butane. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website