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.

    34/450 (page 18)
    In order to calculate the molecular refractivity of the sub¬ stance diallyl, CH2 : CH . CH2. CH2. CH : CH2, on this basis, all that is necessary is to multiply the appropriate constants by the proper numbers and add up the resulting figures, as shown below:— [El]«. 6 carbon atoms. 2*365 x 6 = 14*190 10 hydrogen atoms .... 1*103 x 10 = 11*03 2 ethylenic linkages .... 1*836 x 2 = 3*672 Calculated for molecular refraction . . 28*892 Observed value1.28*77 It will be seen that in this case the calculated and the observed results differ only by a little over one-third per cent. Now, since the substance 2, 4-hexadiene— CH3. CH : CH. CH : CH . CH3 contains exactly the same number of carbon atoms, hydrogen atoms and double bonds as diallyl does, we should find its cal¬ culated refractivity to be 28*892 also. When the observed value2 is determined, however, it is almost a whole unit greater than this, viz., 29*87. The only difference between diallyl and 2, 4-hexadiene lies in the fact that the latter compound con¬ tains a conjugated system which is absent from the structure of diallyl; and it is to the presence of this conjugated system that the optical anomaly must be ascribed, since there is no other possible origin. Another example may be chosen from the carboxylic acids, so as to include all the constants given above ; and in this case three isomeric substances 3 can be compared. CH3. CH2. CH2. CH : CH . COOH 3156) CH3 . CH2. CH : CH . CH2. COOH 30*99 \ Observed values. CH3 . CH : CH . CH2. CH2. COOH 30*98J Calculated value . . . 30*88 It can be seen that in the first compound there is a con¬ jugated system comprising the ethylenic linkage and the carbonyl group of the carboxyl radicle ; whereas no such con¬ jugation is present in either of the other two molecules. The 1 Bruhl, Ber., 1907, 40, 881. 2Bruhl, loc. cit. 3Eykman, Chem. Centralblatt, 1907, II. 1205.