CHAPTER THREE Conformations of Alkanes and Cycloalkanes plane and the C-C-H plane. The torsion angle is easily seen in a Newman projection of ethane as the angle between C-H bonds of adjacent carbons H Torsion angle=60° Eclipsed bonds are characterized by a torsion angle of 0. when the torsion angle is approximately 60, we say that the spatial relationship is gauche; and when it is 180 we say that it is anti. Staggered conformations have only gauche or anti relationships between bonds on adjacent atoms Of the two conformations of ethane, the staggered is more stable than the eclipsed The measured difference in potential energy between them is 12 kJ/mol(2.9 kcal/mol) A simple explanation has echoes of VSEPR(Section 1.10). The staggered conformation allows the electron pairs in the C-H bonds of one carbon to be farther away from the electron pairs in the C-H bonds of the other than the eclipsed conformation allows Electron-pair repulsions on adjacent carbons govern the relative stability of staggered and eclipsed conformations in much the same way that electron-pair repulsions influence the bond angles at a central atom. he destabilization that comes from eclipsed bonds on adjacent atoms is called torsional strain. Torsional strain is one of several structural features resulting from its three-dimensional makeup that destabilize a molecule. The total strain of all of the spa Steric is derived from the tially dependent features is often called steric strain. Because three pairs of eclipsed Greek word stere bonds produce 12 kJ/mol (2.9 kcal/mol) of torsional strain in ethane, it is reasonable to "solid" and refers to the assign an"energy cost"of 4 kJ/mol (1 kcal/mol) to each pair of eclipsed bonds three-dimensional or spatial aspects of chemistry. In principle there are an infinite number of conformations of ethane, differing by only tiny increments in their torsion angles. Not only is the staggered conformation more stable than the eclipsed, it is the most stable of all of the conformations; the eclipsed is the least stable. Figure 3. 4 shows how the potential energy of ethane changes for a 360 rotation about the carbon-carbon bond. Three equivalent eclipsed conformations and The animation on the three equivalent staggered conformations occur during the 360 rotation; the eclipsed earning By Modeling CD shows conformations appear at the highest points on the curve(potential energy maxima), the rotation about the c- taggered ones at the lowest (potential energy minima) TPROBLEM 3.2 Find the conformations in Figure 3.4 in which the red circles arc L(a) gauche and(b)anti Diagrams such as Figure 3. 4 can be quite helpful for understanding how the pot tial energy of a system changes during a process. The process can be a simple one such as the one described here--rotation around a carbon-carbon bond. Or it might be more complicated-a chemical reaction, for example. We will see applications of potentia energy diagrams to a variety of processes throughout the text. Let's focus our attention on a portion of Figure 3. 4. The region that lies between a torsion angle of 60o and 180o tracks the conversion of one staggered conformation of Back Forward Main MenuToc Study Guide ToC Student o MHHE Websiteplane and the C±C±H plane. The torsion angle is easily seen in a Newman projection of ethane as the angle between C±H bonds of adjacent carbons. Eclipsed bonds are characterized by a torsion angle of 0°. When the torsion angle is approximately 60°, we say that the spatial relationship is gauche; and when it is 180° we say that it is anti. Staggered conformations have only gauche or anti relationships between bonds on adjacent atoms. Of the two conformations of ethane, the staggered is more stable than the eclipsed. The measured difference in potential energy between them is 12 kJ/mol (2.9 kcal/mol). A simple explanation has echoes of VSEPR (Section 1.10). The staggered conformation allows the electron pairs in the C±H bonds of one carbon to be farther away from the electron pairs in the C±H bonds of the other than the eclipsed conformation allows. Electron-pair repulsions on adjacent carbons govern the relative stability of staggered and eclipsed conformations in much the same way that electron-pair repulsions influence the bond angles at a central atom. The destabilization that comes from eclipsed bonds on adjacent atoms is called torsional strain. Torsional strain is one of several structural features resulting from its three-dimensional makeup that destabilize a molecule. The total strain of all of the spa￾tially dependent features is often called steric strain. Because three pairs of eclipsed bonds produce 12 kJ/mol (2.9 kcal/mol) of torsional strain in ethane, it is reasonable to assign an “energy cost” of 4 kJ/mol (1 kcal/mol) to each pair of eclipsed bonds. In principle there are an infinite number of conformations of ethane, differing by only tiny increments in their torsion angles. Not only is the staggered conformation more stable than the eclipsed, it is the most stable of all of the conformations; the eclipsed is the least stable. Figure 3.4 shows how the potential energy of ethane changes for a 360° rotation about the carbon–carbon bond. Three equivalent eclipsed conformations and three equivalent staggered conformations occur during the 360° rotation; the eclipsed conformations appear at the highest points on the curve (potential energy maxima), the staggered ones at the lowest (potential energy minima). PROBLEM 3.2 Find the conformations in Figure 3.4 in which the red circles are (a) gauche and (b) anti. Diagrams such as Figure 3.4 can be quite helpful for understanding how the poten￾tial energy of a system changes during a process. The process can be a simple one such as the one described here—rotation around a carbon–carbon bond. Or it might be more complicated—a chemical reaction, for example. We will see applications of potential energy diagrams to a variety of processes throughout the text. Let’s focus our attention on a portion of Figure 3.4. The region that lies between a torsion angle of 60° and 180° tracks the conversion of one staggered conformation of Torsion angle 180° Anti H H 180° Torsion angle 60° Gauche H H 60° Torsion angle 0° Eclipsed HH 0° 92 CHAPTER THREE Conformations of Alkanes and Cycloalkanes Steric is derived from the Greek word stereos for “solid” and refers to the three-dimensional or spatial aspects of chemistry. The animation on the Learning By Modeling CD shows rotation about the C±C bond in ethane. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website