Cresol, dipole moment

Fig. 37. Molecular weight dependence of dipole moment (n2 112 for PBLA and PCBL in some helicogenic solvents. The line PBLA yields 4.6 D for ph and the line PCBL yields 5.4 D for nh. Data for PBLA ( ) Erenrich and Scheraga (115), (3) Saruta et al. (116) data for PCBL (O) in m-cresol, ( ) in m-cresol-EDC mixtures (117)...

The dipole moment is a property of the molecule that results from charge separations like those discussed above. However, it is not possible to measure the dipole moment of an individual bond within a molecule we can measure only the total moment of the molecule, which is the vectorial sum of the individual bond moments.32 These individual moments are roughly the same from molecule to molecule,33 but this constancy is by no means universal. Thus, from the dipole moments of toluene and nitrobenzene (Figure 1.10)34 we should expect the moment of p-nitrotoluene to be about 4.36 D. The actual value 4.39 D is reasonable. However, the moment of p-cresol (1.57 D) is quite far from the predicted value of 1.11 D. In some cases, molecules may have substantial individual bond moments but no total moments at all because the individual moments are canceled out by the overall symmetry of the molecule. Some examples are CC14, tr[Pg.16]

The mobile phases used in normal-phase chromatography are based on nonpolar hydrocarbons, such as hexane, heptane, or octane, to which is added a small amount of a more polar solvent, such as 2-propanol.5 Solvent selectivity is controlled by the nature of the added solvent. Additives with large dipole moments, such as methylene chloride and 1,2-dichlor-oethane, interact preferentially with solutes that have large dipole moments, such as nitro- compounds, nitriles, amines, and sulfoxides. Good proton donors such as chloroform, m-cresol, and water interact preferentially with basic solutes such as amines and sulfoxides, whereas good proton acceptors such as alcohols, ethers, and amines tend to interact best with hydroxylated molecules such as acids and phenols. A variety of solvents used as mobile phases in normal-phase chromatography are listed in Table 2.2, some of which may need to be stabilized by addition of an antioxidant, such as 3-5% ethanol, because of the propensity for peroxide formation. 

The values for toluene, nitrobenzene, and p-nitrotoluene are from MacClellan, A.L., Tables of Experimental Dipole Moments, Vol. 1, W.H. Freeman, San Francisco, 1963 Vol. 2, Rahara Enterprises, El Cerrito, CA, 1974. The values for phenol and p-cresol were determined by Goode, E.V. Ibbitson, D.A. J. Chem. Soc. 1960, 4265. 

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