Some physico-chemical themes / by Alfred W. Stewart with five plates and thirty-seven diagrams in the text.

  • Alfred Walter Stewart
Date:
1922
Catalogue details

Licence: In copyright

Credit: Some physico-chemical themes / by Alfred W. Stewart with five plates and thirty-seven diagrams in the text. Source: Wellcome Collection.

    33/450 (page 17)
    Thus the presence of a conjugated system of double bonds appears to enhance the cohesive power of a liquid to a very marked extent, as measured by the boiling-point. 5. Refractivity. The optical properties of compounds are roughly divisible into two main classes: additive properties and constitutive properties. This division is, however, a somewhat arbitrary one; for all optical properties appear to be influenced by both the additive and constitutive factors, though the relative in¬ fluence of these differs from property to property. Thus, in the main, the refractive index of a compound is obtainable by a summation of various constants, each of which corresponds to a particular part of the molecular structure of the compound under examination; and the approximation between theory and practice is generally a very close one. The presence of isolated double or triple linkages in a sub¬ stance raises the refractivity by a fixed amount, but when a conjugated system of double bonds exists in the molecule, the ordinary method of calculation breaks down ; and the observed value of the molecular refractive power is found to diverge considerably from that calculated from the ordinary constants. This deviation is termed optical anomaly, and it may be either positive or negative in sign—i.e., the calculated value may exceed or be less than the observed value. One or two examples may be given in order to show the extent of the deviation which is produced by the presence of a conjugated system. In the first place, the constants employed are the following. They hold good for the a-line in the spectrum and are calcu¬ lated on the basis of the formula :— ?l2 - 1 m r-r-> -i wVT2 * d - [Kl]- Here [RL] represents the molecular refraction ; n is the observed refractive index; m is the molecular weight, and d the density of the substance under examination. 1 carbon atom .... . 2-365 1 hydrogen atom . 1-103 1 ketonic oxygen atom . 2-328 1 hydroxylic oxygen atom . . 1-506 1 ethylenic linkage . 1-836 2