Butane, anti conformation bond rotation

Figure 4-18 The Potential Energy for Rotation of n-Butane About its Central Bond Axis. The anti conformer in the center is slightly lower in energy than the two gauche conformers.

Example The reaction coordinate for rotation about the central carbon-carbon bond in -butane has several stationary points. A, C, E, and G are minima and B, D, and Fare maxima. Only the structures at the minima represent stable species and of these, the anti conformation is more stable than the gauche. 

Chemists routinely manipulate physical models in an attempt to ascertain what actually occurs during a conformational change. A successful example of this is in showing first-time students of organic chemistry that interconversion between anti and gauche conformers of w-butane involves a simple rotation about the central carbon-carbon bond (see discussion in Chapter 1). Much less satisfactory is the attempt to show the interconversion of chair forms of cyclohexane. Here, computer animations provide a better alternative. 

Use SpartanView to step through the sequence of structures showing bond rotation in butane, and identify the gauche and anti conformers. What changes in the C2-C3 bond distance and C-C-C bond angles accompany bond rotation What do you think causes these changes ... 

In the vast majority of appropriately constructed molecules, for example derivatives of butane, the anti conformation, e.g. 12, in which the groups X are each flanked by two hydrogens, is the most stable however, there are a few exceptions. The anti conformation of butane is more stable than the cis by ca. 18.8 kJ mol1. An energy profile of rotation around the central C-C bond of butane is given in Figure 1.7. 

Figure 3.7 shows a potential energy curve for rotation about the C2—C3 bond of butane in the gas phase. Although the gauche and anti conformations are... 

Stereoisomers also include conformational isomers, in which different isomers are generated through rotations about bonds. Conformational isomers are often called con-formers. Eclipsed and staggered ethane are typical examples. Note that a conformational isomer need not be an energy minimum— the eclipsed conformation of ethane is an energy maximum, for example. Conformational isomers can be either enantiomeric or diastereomeric.The two gauche forms of butane are conformational enantiomers, but the gauche and the anti forms of butane are conformational diastereomers (Rg. 4.57). 

It is of interest that A//° (gas phase) for dibromoethane is greater than that for butane (3.7-4.1 kJ mol ) even though bromine and methyl are usually considered to be of comparable size. This suggests that a factor other than steric repulsion affects the situation the obvious candidate is dipole-dipole repulsion in the gauche conformers of the dibromide. This contributor to the overall repulsion should be diminished in more polar solvents, both because of an increase in the effective dielectric constant and a concomitant decrease in coulom-bic repulsion, and because of more effective solvation of the higher-dipole confotmer in more polar solvents. Indeed, A// for 1,2-dibromoethane diminishes to 3.6 kJ mol in the pure liquid (dielectric constant e = 4.8) and to 2.8 kJ mol in acetonitrile (e = 37.5). (It must be noted, however, that because of differential volatility of the gauche and anti conformers of (nonpolar) butane, the conformational enthalpy difference A//° between these conformers also diminishes in the liquid phase, to 2.3-2.4 kJ mol .) We indicated earlier that ethane has a threefold potential (called V3). In the 1,2-dihaloethane, there is a superposed onefold potential (Vi) since the optimum (anti) orientation of the C-X dipoles is achieved only once in the course of a 360° rotation about the C-C bond. 

Rotation around the C2—C3 bond of butane starting from a methyl—methyl eclipsed conformation gives two nonequivalent eclipsed and two nonequivalent staggered conformations. The gauche conformation is 3.8 kj mole higher in energy than the anti conformation. 

Prepare models of the four isomeric butenes, C4H8. Note that the restricted rotation about the double bond is responsible for the cis-trans stereoisomerism. Verify this by observing that breaking the n bond of cw-2-butene allows rotation and thus conversion to rran5 -2-butene. Is any of the four isomeric butenes chiral (nonsuperposable with its mirror image) Indicate pairs of butene isomers that are structural (constitutional) isomers. Indicate pairs that are diastereomers. How does the distance between the Cl and C4 atoms in mw5 -2-butene compare with that of the anti conformation of butane Compare the Cl to C4 distance in cw-2-butene with that in the conformation of butane in which the methyls are eclipsed. 

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