Dibromoethane, conformations

Step through the sequence of structures depicting rotation about the carbon-carbon bond in the two dibromoethane isomers l,2-dibromo-l,2-diphenylethane A andfi). For each, plot energy (vertical axis) vs. BrCCBr torsion angle (horizontal axis), and identify all minimum-energy structures. Which of these are reactive conformers , that is, conformers which are set up for either syn or anti elimination of HBr Which are non-reactive conformers , that is, which do not meet the requirements for elimination Do the reactive conformers correspond only to syn elimination, only to anti elimination, or are both pathways represented Which alkene would these reactive conformers lead to Are your results consistent with the observation that each isomer of the starting material gives only one alkene Explain. 

Construct a qualitative potential-energy diagram for rotation about the C-C bond of i,2-dibromoethane. Which conformation would you expect to be more stable Label the anti and gauche conformations of 1,2-dibromoethane. 

Which conformation of 1,2-dibromoethane (Problem 3.44) would you expect to have the larger dipole moment The observed dipole moment of 1,2-dibromoethane is /x = 1.0 D. What does this tell you about the actual structure of the molecule ... 

Dibromoethane exists in the anti form, so that the C< Br dipoles cancel and the net dipole moment is zero. When the glycol exists in the gauche form, intramolecular H-bonding occurs. Intramolecular H-bonding is a stabilizing effect which cannot occur in the anti conformer. 

We so far have ignored the relationship of chemical shifts to conformational equilibria. Consider a specific example, 1,2-dibromoethane, for which there are three staggered conformations 10a, 10b, and 10c ... 

To illustrate the application of simulated aimealing to problems of physical chemistry we present an example of molecular conformation optimization calculated by John H. Hall et al. [13] at the Los Alamos National Laboratory. The purpose of Hall s exploratory study was to demonstrate the feasibility of using simulated annealing to determine minimum energy configurations of the molecules of chemical compounds such as bicyclo-HMX, Tyr-Gly-Gly, or dibromoethane. 

In the case of dibromoethane, utilization of symmetry and other properties results in a reduction of the expression for the total potential energy to a dependence on only one variable which is the torsion angle. Because of this fortuitous circumstance it is convenient to present the conformation optimization calculations and results for that compound. 

Figure 11. Initial conformation of dibromoethane stereo view of Neumann projection.

Using the models you made in exercise 1, replace a hydrogen on each carbon with a bromine. Notice in 1,2-dibromoethane that you can rotate around the carbon-carbon single bond to get all of the conformations and that they are interconvertible. However, notice that there is no rotation around the carbon-carbon double bond. If you put the two bromines on the same side you have the cis geometric isomer and if you put them on opposite sides you have trans. The cis and trans isomers are not interconvertible. 

The information drawn from polarizability and polarization studies seems, in general, to compare reasonably with that from other physical methods, if the natures of these methods are borne in mind. The techniques of X-ray, neutron and electron diffraction and microwave spectroscopy provide metrical structural details of the highest precision relating to the crystalline or gaseous states flexible molecules in solution may be expected to adopt conformations between the extremes found in the states mentioned. Thus dimethyl oxalate, diethyl terephthalate, 1,2-dibromoethane, and 1,4-dimethoxybenzene all exhibit dipole moments as solutes, but nevertheless assume trans—and therefore nonpolar—forms in their solid phases (cf. Chemical Society Tables, 1958). Accordingly it is difficult to assess the correctness of the findings summarized in Table 23 when available, results secured in other ways have been quoted in the papers cited. 

Conformational isomers, as illustrated by the two Newman projections 1 and 2 for 1,2-dibromoethane, are stereoisomers that are interconverted by rotation... 

These terms can be clarified by looking at some specific examples. In the following ro-tamers of ) cso-l,2-dichloro-l,2-dibromoethane, the only achirotopic site in rotamer A is the point of inversion in the middle of the structure. Every atom is in a locally chiral environment, and so is chirotopic. For rotamer B, all points in the mirror plane (a plane perpendicular to the page of fhe paper) are achirotopic. All other points in these conformers are chirotopic, existing at sites of no symmefry. In other words, all other points in these conformers feel a chiral environment, even though the molecule is achiral. 

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