CONFORMATIONS OF ALKANES AND CYCLOALKANES 3.2 Red circles gauche: 60 and 300. Red circles anti: 180. Gauche and anti relationships occur only in staggered conformations; therefore, ignore the eclipsed conformations(0%, 1200, 2400, 3600) 3.3 All the staggered conformations of propane are equivalent to one another, and all its eclipsed con- formations are equivalent to one another. The energy diagram resembles that of ethane in that it is a symmetrical one. H H H D H H, C H H H 240 300 Torsion angle(degrees) The activation energy for bond rotation in propane is expected to be somewhat higher than that in ethane because of van der Waals strain between the methyl group and a hydrogen in the eclipsed nformation. This strain is, however, less than the van der Waals strain between the methyl groups of butane, which makes the activation energy for bond rotation less for propane than for 3.4(b) To be gauche, substituents X and a must be related by a 60 torsion angle. If A is axial as specified in the problem, X must therefore be equatorial. gauche (c) For substituent X at C-1 to be anti to C-3, it must be equatorial (d) When X is axial at C-1, it is gauche to C-3. 3.5(b) According to the numbering scheme given in the problem, a methyl group is axial when it is up"at C-1 but is equatorial when it is up at C-4. Since substituents are more stable when they Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website3.2 Red circles gauche: 60° and 300°. Red circles anti: 180°. Gauche and anti relationships occur only in staggered conformations; therefore, ignore the eclipsed conformations (0°, 120°, 240°, 360°). 3.3 All the staggered conformations of propane are equivalent to one another, and all its eclipsed con￾formations are equivalent to one another. The energy diagram resembles that of ethane in that it is a symmetrical one. The activation energy for bond rotation in propane is expected to be somewhat higher than that in ethane because of van der Waals strain between the methyl group and a hydrogen in the eclipsed conformation. This strain is, however, less than the van der Waals strain between the methyl groups of butane, which makes the activation energy for bond rotation less for propane than for butane. 3.4 (b) To be gauche, substituents X and A must be related by a 60° torsion angle. If A is axial as specified in the problem, X must therefore be equatorial. (c) For substituent X at C-1 to be anti to C-3, it must be equatorial. (d) When X is axial at C-1, it is gauche to C-3. 3.5 (b) According to the numbering scheme given in the problem, a methyl group is axial when it is “up” at C-1 but is equatorial when it is up at C-4. Since substituents are more stable when they 3 X A 3 X A X and A are gauche. X A Potential energy 0 60 120 180 240 300 360 Torsion angle (degrees) H3C H H H H H H H H H H H3C H CH3 H H H H H3C H H H H H H H H H H H3C H H H CH3 H H H H H H CH3 H CONFORMATIONS OF ALKANES AND CYCLOALKANES 47 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website