CHAPTER THREE Conformations of Alkanes and Cycloalkanes FIGURE 3.5 Distribution of molecular energies. (a) The r of molecules with temperature(T)) energy greater than Eact at area.( b)At some high temperature T2, the shape of the energy distribution emperature(T2) curve is different, and more molecules have energies in excess of eact. Energy 3.2 CONFORMATIONAL ANALYSIS OF BUTANE The next alkane that we examine is butane. In particular, we consider conforma- tions related by rotation about the bond between the middle two carbons (CH3CH2-CH2CH3) Unlike ethane, in which the staggered conformations are equiva lent, two different staggered conformations occur in butane, shown in Figure 3.6. The methyl groups are gauche to each other in one, anti in the other. Both conformations ar staggered, so are free of torsional strain, but two of the methyl hydrogens of the gauche conformation lie within 210 pm of each other. This distance is less than the sum of their an der Waals radii(240 pm), and there is a repulsive force between them. The desta- bilization of a molecule that results when two of its atoms are too close to each other is FIGURE 3. 6 The gauche and anti conforma and as Newman projections right). The gauche confor- mation is less stable than the anti because of the van de Waals strain between the methyl groups. Back Forward Main MenuToc Study Guide ToC Student o MHHE Website3.2 CONFORMATIONAL ANALYSIS OF BUTANE The next alkane that we examine is butane. In particular, we consider conforma￾tions related by rotation about the bond between the middle two carbons (CH3CH2±CH2CH3). Unlike ethane, in which the staggered conformations are equiva￾lent, two different staggered conformations occur in butane, shown in Figure 3.6. The methyl groups are gauche to each other in one, anti in the other. Both conformations are staggered, so are free of torsional strain, but two of the methyl hydrogens of the gauche conformation lie within 210 pm of each other. This distance is less than the sum of their van der Waals radii (240 pm), and there is a repulsive force between them. The desta￾bilization of a molecule that results when two of its atoms are too close to each other is 94 CHAPTER THREE Conformations of Alkanes and Cycloalkanes Fraction of molecules having a particular energy Low temperature (T1) High temperature (T2) Eact Energy FIGURE 3.5 Distribution of molecular energies. (a) The number of molecules with energy greater than Eact at temperature T1 is shown as the darker-green shaded area. (b) At some higher temperature T2, the shape of the energy distribution curve is different, and more molecules have energies in excess of Eact. CH3 CH3 H H H H CH3 CH3 H H H H FIGURE 3.6 The gauche and anti conforma￾tions of butane shown as ball-and-spoke models (left) and as Newman projections (right). The gauche confor￾mation is less stable than the anti because of the van der Waals strain between the methyl groups. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website