3. 1 Conformational Analysis of Ethane 55:5 FIGURE 3. 4 Potential energy diagram for rotation about the carbon -carbon bond in ethane. two of the hydrogens are shown in re and four in green so as 2.9 kcal/m 12 k/mol ei dicate more clearly the bond Torsion angl ethane to the next one. Both staggered conformations are equivalent and equal in energy, out for one staggered conformation to get to the next, it must first pass through an eclipsed conformation and needs to gain 12 kJ/mol (2.9 kcal/mol) of energy to reach it. This amount of energy is the activation energy(Eact) for the process. Molecules must become energized in order to undergo a chemical reaction or, as in this case, to undergo rotation around a carbon-carbon bond. Kinetic(thermal)energy is absorbed by a mole cule from collisions with other molecules and is transformed into potential energy. When instant can relr rgy exceeds Eact, the unstable arrangement of atoms that exists at that the potential er lisions with other molecules or with the walls of a container. The point of maximum potential energy encountered by the reactants as they proceed to products is called the transition state. The eclipsed conformation is the transition state for the conversion of The structure that exists at one staggered conformation of ethane to another the transition state is some. Rotation around carbon-carbon bonds is one of the fastest processes in chemistry. times referred to as the tran- Among the ways that we can describe the rate of a process is by its half-life, which is activated compler the length of time it takes for one half of the molecules to react It takes less than 10-6 econds for half of the molecules in a sample of ethane to go from one staggered con- formation to another at 25C. At any instant, almost all of the molecules are in staggered onformations; hardly any are in eclipsed conformations As with all chemical processes, the rate of rotation about the carbon-carbon bond increases with temperature. The reason for this can be seen by inspecting Figure 3.5, where it can be seen that most of the molecules in a sample have energies that are clus- tered around some average value; some have less energy, a few have more. Only mole- cules with a potential energy greater than Eact, however, are able to go over the transi- tion state and proceed on to products. The number of these molecules is given by the shaded areas under the curve in Figure 3. 5. The energy distribution curve flattens out at higher temperatures, and a greater proportion of molecules have energies in excess of Eact at T2(higher) than at TI(lower). The effect of temperature is quite pronounced; an increase of only 10oC produces a two-to threefold increase in the rate of a typical chem Back Forward Main MenuToc Study Guide ToC Student o MHHE Websiteethane to the next one. Both staggered conformations are equivalent and equal in energy, but for one staggered conformation to get to the next, it must first pass through an eclipsed conformation and needs to gain 12 kJ/mol (2.9 kcal/mol) of energy to reach it. This amount of energy is the activation energy (Eact) for the process. Molecules must become energized in order to undergo a chemical reaction or, as in this case, to undergo rotation around a carbon–carbon bond. Kinetic (thermal) energy is absorbed by a mole￾cule from collisions with other molecules and is transformed into potential energy. When the potential energy exceeds Eact, the unstable arrangement of atoms that exists at that instant can relax to a more stable structure, giving off its excess potential energy in col￾lisions with other molecules or with the walls of a container. The point of maximum potential energy encountered by the reactants as they proceed to products is called the transition state. The eclipsed conformation is the transition state for the conversion of one staggered conformation of ethane to another. Rotation around carbon–carbon bonds is one of the fastest processes in chemistry. Among the ways that we can describe the rate of a process is by its half-life, which is the length of time it takes for one half of the molecules to react. It takes less than 106 seconds for half of the molecules in a sample of ethane to go from one staggered con￾formation to another at 25°C. At any instant, almost all of the molecules are in staggered conformations; hardly any are in eclipsed conformations. As with all chemical processes, the rate of rotation about the carbon–carbon bond increases with temperature. The reason for this can be seen by inspecting Figure 3.5, where it can be seen that most of the molecules in a sample have energies that are clus￾tered around some average value; some have less energy, a few have more. Only mole￾cules with a potential energy greater than Eact, however, are able to go over the transi￾tion state and proceed on to products. The number of these molecules is given by the shaded areas under the curve in Figure 3.5. The energy distribution curve flattens out at higher temperatures, and a greater proportion of molecules have energies in excess of Eact at T2 (higher) than at T1 (lower). The effect of temperature is quite pronounced; an increase of only 10°C produces a two- to threefold increase in the rate of a typical chem￾ical process. 3.1 Conformational Analysis of Ethane 93 Potential energy, kcal/mol Potential energy, kJ/mol 0 60 120 180 240 300 360 Torsion angle,
3 2 1 0 12 8 4 0 2.9 kcal/mol 12 kJ/mol The structure that exists at the transition state is some￾times referred to as the tran￾sition structure or the activated complex. FIGURE 3.4 Potential energy diagram for rotation about the carbon–carbon bond in ethane. Two of the hydrogens are shown in red and four in green so as to in￾dicate more clearly the bond rotation. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website