Applications and Results of Polarization Measurements

Molar refraction has long been an important aid for determining constitution in organic chemistry. Since there seems to be a growing 

On the other hand, in aromatic compounds, particularly in benzene, a decrease is apparent in the molar refraction. This depression is specific to the aromatic character and is consistent with the fact that, on account of the ring arrangement of the three double bonds within the nuclear framework, a definite distribution of negative charge density occurs. The same consequence was deduced from quantum-mechanical considerations by the far-reaching studies of Hiickel, Pauling and Slater. It 

Ki and K2 are present in the same proportion, while Di, D2 and Ds are likewise equal in amount. In this collection of the five forms the Kekul properties emerge as unquestionably stronger than the Dewar. This conception is also supported by all other methods of attack—x-ray interference, electron interference and Raman effect. The further discovery that cumulative double bonds give smaller excentrics of refraction and dispersion accords with the quantum mechanical idea of the nature of the double bond.  

Finally, attention may be drawn to some interesting experiments on the variation in optical behavior of isomeric compounds. Generally it is found that highly branched aliphatic molecules, as well as molecules whose form permits dense packing, exhibit higher molecular refractions. As examples may be cited mns-a-jS-dibromoethylene (MRd = 26.16) and the cis isomer MRj = 25.64) as well as p-dibromobenzene MRd = 

More details are given in the two mentioned books of Heilman and Pauling, in the comprehensive paper of E. Hiickel and furthermore K. W. F. Kohlrausch, Phys, Z., 37, 58 (1936) J. H. Hibben, Chem, Rev., 18, 7 (1936) L. Simons, J. Applied Phya., 9, 781 (1938). 

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