Dichloroethane, rotational

The first example involves calculating the potential of mean force for the rotation of the C-C bond in 1,2-dichloroethane (DCE) dissolved in water. In the second... 

The concept of atropisomerism developed to a considerable extent following other developments in chemistry, especially those in spectroscopy. Early work by Kohlrausch (4) and Mizushima (3), based on Raman spectra and dipole moment studies, established that rotational isomers—rotamers—must exist in 1,2-dichloroethane. Pitzer established that there are three energy minima when ethane is rotated about its C—C axis (6). Rotamers about single bonds have been found in a wide variety of organic compounds since then, mainly as a result of the application of vibrational spectroscopy to organic molecules (7). 

Medium Component. The elementary spectra for the medium component were derived by Bergmann and Nawotki12,13 on the basis of a spectrum calculated by Gutowsky and Pake52) for paired protons such as in 1,2-dichloroethane with hindered rotation around the C—C bond. 

As seen above, infrared absorption showed intramolecular hydrogen bonding to be extensive for 4 in 1,2-dichloroethane. Thus, the low molecular rotation of 13° (Table II) in this solvent is consistent with a high population of 4a. To keep the relative amounts of 4b and 4c constant, a plausible equilibrium for 4 in 1,2-dichloroethane is,... 

Figure 1. Variation in molecular rotation with increasing concentrations of dimethyl sulfoxide in 1,2-dichloroethane (1) l,5-anhydro-2,3-dideoxy-6-0-methyl-D-erythro-hexitol (8) (2) l,5-anhydro-2,3-dideoxy-D-erythro-hexitol (5) (3) ln,2s-hydroxy-methylcyclohexanol (4).

The decrease in rotation observed on changing the solvent to dimethyl sulfoxide is probably reflecting a decreased population of 5 c. The molecular rotation of 42° for 5 in 1,2-dichloroethane suggests a change in the equilibrium toward a situation such as,... 

All optical rotations were measured at the D-line of sodium with a Perkin-Elmer polarimeter (model 141) using a thermostatted 10-cm polarimeter tube dried by a stream of filtered, dried air. Precaution was necessary to exclude water from the solutions in 1,2-dichloroethane and dimethyl sulfoxide. In this connection all glassware was dried at 120 °C and stoppered while hot. All transfers of solvents or solutions were made with syringes using serum caps to exclude moist atmosphere. The compounds which could not be prepared in crystalline condition and recrystallized to purity were obtained from a pure crystalline derivative... 

Li, J.C.M., Pitzer, K.S. (1956) The thermodynamic properties of 1,1-dichloroethane heat capacities from 14 to 294 K, heats of fusion and vaporization, vapor pressure and entropy of the ideal gas. The barrier to internal rotation. J. Am. Chem. Soc. 78, 1077-1080. 

Free energies of activation for rotation about the C—N bond have been determined by analysis of NMR lineshapes by Drakenberg and Forsen57) to be 87 kJ mol-1 (35% NMA in 1,2-dichloroethane) and 89 kJ mol-1 (20% NMA in H20) at 60 °C. The difference between these values is probably not significant. Unfortunately, at these concentrations in these solvents, solute-solute and solvent-solute interactions... 

Figure 2-28. The l,2-dibromo-l,2-dichloroethane molecule. Its center of symmetry is the midpoint of the C-C bond. An inversion is equivalent to the consecutive application of twofold rotation and reflection.

Rotational isomerism relative to a single bond is illustrated by ethane and 1,2-dichloroethane, both depicted in Figure 3-4. First, take the ethane molecule, H3C-CH3. During a complete rotation of one methyl group around the C-C bond relative to the other methyl group,... 

Figure 3-4. Potential energy functions for rotation about a single bond, cp is the angle of rotation, (a) Ethane, H3C-CH3. There are two different symmetrical forms. Both the staggered form with Did symmetry and the eclipsed form with D2h symmetry occur three times in a complete rotational circuit (b) 1,2-dichloroethane, CIH2C-CH2CI. There is no other symmetrical form in the region between the two symmetrical staggered forms shown. The eclipsed form with C2v symmetry and the staggered form with C2h symmetry occur once, while the staggered form with C2 symmetry occurs twice in a complete rotational circuit.

In the older organic chemistry it was assumed that a free rotation was possible about a single C—C bond because, for example, there exist no isolatable isomers of 1,2 dichloroethane. Accurate measurements of the specific heat of ethane at low temperatures and likewise the difference between determinations and calculations of equilibria of hydrocarbons have, however, shown that there is no question of a free rotation rotation is indeed possible but there is a potential barrier of about 3 kcal/mole which has to be surmounted. The state of lowest energy is that in which the two methyl groups, or the methyl and the CH2 group, are alternate. In 1, 2 dichloroethane etc. there appear to be two positions of (relatively) minimum energy, the trans position and an oblique position which lies 1.2 kcal higher than the former. 

FromA = 3.4x 10 s, weobtainTc= 1.38 x 10 sandrjr = 13.8cPA. Assuming an effective radius between 2.5 and 3.0 A that seems geometrically reasonable, the effective viscosity would range between 0.50 and 0.89 cR For 1,2-dichloroethane, the solvent used in the experiments rj = 0.464 cP at 25 °C. Since, however, the radical is linked to the complex by a tetramethylene chain one might well expect some increase of the effective viscosity for rotational diffusion. 

As demonstrated by the spectra of 1,2-dichloroethane shown in Fig. 4.1-11C, two halogen atoms in 1,2-position also show in-phase as well as out-of-phase vibrations. The antiperiplanar conformation is subject to the exclusion rule op. Raman 750 cm Oa . IR 708 cm. In the case of the synclinal conformation, on the other hand, the in-phase vibration at 645 cm is stronger both in the IR and in the Raman spectrum than the out-of-phase vibration at 677 cm. Freely rotating 1,2-dihalogen compounds therefore show four different C-Cl stretching vibrations (Fig. 4.1-11C). 

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