1.2- Dibromoethane, dipole moment

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 ... 

Problem 4.43 1,2-Dibromoethane has a zero dipole moment, whereas ethylene glycol, CH,OHCH,OH, has a measurable dipole moment. Explain. 

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. 

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. 

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