Dipole moment of 1,2-dichloroethane

The dipole moment of 1,2-dichloroethane is 1.12 D at room temperature. Show how one could use this dipole moment to calculate the proportions of the possible conformations that are expected to be present. 

Evidence for such internal rotation comes from infrared spectra [3] or from dipole moments of substituted hydrocarbons [4]. For example, if the average value of 0 = 0, then the dipole moment of 1,2 dichloroethane would be zero. At finite values of 0, the dipole moment is finite and can be calculated from the non-vanishing components given by... 

From studies of the dipole moment of 1,2-dichloroethane in the gas phase at room temperature (25°C), it is estimated that the ratio of molecules in the anti conformation to gauche conformation is 7.6 to 1. Calculate the difference in Gibbs free energy between these two conformations. 

Draw the two staggered conformations of 1,2-dichloroethane. Which of the conformations has a dipole moment The dipole moment of 1,2 dichloroethane is 1.1 D. Does this fact provide any information about the composition of the mixture of conformations ... 

Isomerizations are very simple model reactions that are among the most common chemical processes. Solvent elfects on these reactions have been studied extensively in the bulk phase. The study of solvent effects near the interface is especially interesting, since the gauche-trans equilibrium can involve two species with significantly different dipole moments consequently, a substantial interfacial effect can be expected. The isomerization reaction of 1,2-dichloroethane near the water/vacuum interface [249], and of 1,2-dichloroethane and of dialanine near the water/hexane interface [249, 250] has been studied. An increase of the gauche —> trans isomerization rate is observed near the interface (for a review see Ref. 251). 

Here, p and are the matrix representation of the dipole moment operator and its transpose. This calculation can be implemented by available programs, such as Gaussian-92, described by Foresman and Frisch (1993), or Jaguar, described by Bochevarov et al. (2013). Foresman and Frisch give as an example (p. 193) the calculation of At/ for the gauche-anti rotation of 1,2-dichloroethane in the gas phase and in solution in cyclohexane (e =2), using the 6-31 h- G(d) basis at both the HF and MP2 levels. Their results are summarized in Table 5.1. 

The considered above electrostatic models of ion interaction are, undoubtedly, simplified. Each ion is surrounded by the solvate shell, whose character and sizes are determined by the ion, its charge and radius, and sizes of solvent molecules and such their parameters as the dipole moment of their polar groups, structure and sizes of the molecule. The solvent, its solvating ability, and the influence on the ion interaction are not reduced to the medium with the dielectric constant e only. Similarly, the interaction of ions is not restricted by the formation of only the ion atmosphere ion pairs, triples, and associates of several ions appear in the solution. Ion pairs, which can be separated by the solvate shell or be in contact to form contact pairs, also differ in structure. As a whole, the situation is more complex and diverse than its description by the classical theory of interaction of spherical charges in the liquid medium of dielectrics. The solvating ability of the solvent is determined only in part by its dielectric constant. For aprotic solvents, the ability of their heteroatoms to be donors of a free pair of electrons for cations is very significant. The donating ability of Ihe solvent is characterized by its donor number DN, which for the solvent is equal to the enthalpy of its interaction with SbCls in a solution of 1,2-dichloroethane... 

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). 

The mobile phases used in reversed-phase chromatography are based on a polar solvent, typically water, to which a less polar solvent such as acetonitrile or methanol is added. Solvent selectivity is controlled by the nature of the added solvent in the same way as was described for normal-phase chromatography solvents with large dipole moments, such as methylene chloride and 1,2-dichloroethane, interact preferentially with solutes that have large dipole moments, such as nitro- compounds, nitriles, amines, and sulfoxides. Solvents that are good proton donors, such as chloroform, m-cresol, and water, interact preferentially with basic solutes such as amines... 

Sn(CH3)3l dissolved in nitrobenzene as a function of concentration of various EPD solvents added (35). In noncoordinating or weakly coordinating solvents, such as hexane, earbon tetrachloride, 1,2-dichloroethane, nitrobenzene, or nitromethane, Sn(CH3)3l is present in an unionized state (tetrahedral molecules). Addition of a stronger EPD solvent to this solution provokes ionization, presumably with formation of trigonal bipju amidal cations [Sn(CH3)3 (EPD)2J. Table II reveals that the molar conductivities at a given mole ratio EPD Sn(CH3)3l are (with the exception of pyridine) in accordance with the relative solvent donicities. No relationship appears to exist between conductivities and the dipole moments or the dielectric constants of the solvents. 

Type II is determined by the negative deviations from isotherm [9.26]. This type corresponds to systems with weak heteromolecular interactions between components, but with strong homomolecular association of one of the components. The system formic acid-anisole is an example of this kind of isotherm. Furthermore, these isotherms are characteristic when non-associating components in pure state form heteromolecular associates with lower dipole moment, DM, then DM of both components. The average DM for such kind of interaction in mixed systems is lower than correspondent additive value for non-interacting system. The system 1,2-dichloroethane - n-butylbromide can be referenced as an example of this kind of mixed binary solvent. 

The nonsteric interactions in ipc depend on the chemical structure of the analyte, and also on nature of stationary and mobile phases. In normal- or reversed-phase hplc, neutral solutes are separated on the basis of their polarity. In the former case, polar stationary phases are employed (eg, bare sihca with polar silanol groups) and less polar mobile phases based on nonpolar hydrocarbons are used for elution of the analytes. Solvent selectivity is controlled by adding a small amoimt of a more polar solvent, such as 2-propanol or acetonitrile or other additives with large dipole moments (methylene chloride and 1,2-dichloroethane), proton donors (chloroform, ethyl acetate, and water), or proton acceptors (alcohols, ethers, and amines). Correspondingly, the more polar the solute, the greater is its retention on the column, yet increasing the polarity of the mobile phase results in decreased solute retention. 

A breakdown of part of the calculation for 1,2-dichloroethane is given in Table 9. The potential selected to give best fit to calorimetric and dipole moment data is... 

The compound 1,2-dichloroethane (C2H4CI2) is nonpolar, while cw-dichloroethylene (C2H2CI2) has a dipole moment The reason for the difference is that groups connected by a single bond can rotate with respect to each other, but no rotation occurs when a double bond connects the groups. On the basis of bonding considerations, explain why rotation occurs in 1,2-dichloroethane but not in cis-dichloroethylene. 

This fact has led to the introduction of the principle of free rotation . The expression free rotation does not fully capture the facts. There is no doubt that in this case certain positions of the substituents axe also favoured. This follows in particular from the most recent investigations on the spatial structure of the compounds with the help of the determination of dipole moments and with the help of the scattering of X-rays on the vapour as in the new method of Debye. As Prof. Debye communicated to me, recently X-ray investigations on 1,2-dichloroethane yielded the fact that one certain position of the Cl-atoms is favoured, namely the one in which the two Cl-atoms are in the trans-position. In this case no temperature effect could be found between 80° and 220°. 

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