It is to be expected that the alkyl halides will dissociate on absorption of light into alkyl residues and excited halogen atoms. Their absorption spectra are continuous, as is also the case with phenyl iodide, which liberates iodine under identical conditions. The C-I and C-Br linkages are certainly homopolar in the gas state. On Franck’s theory the long-wave limit of the absorption continuum should correspond to the heat of binding of C-I together with the excitation energy of the halogen atom. We would expect the alkyl halides to resemble the halogen hydrides in their behaviour, although with HI and HBr the long-wave limits are not very happily situated. The limit for HI is at 3320 Å., corresponding to a heat of dissociation of 65,000 calories (the thermochemical heat is 69,000 calories) and the limit for HBr at 2640 Å. (97,000 calories, the thermochemical being 85,000). It is doubtful if these disagreements are entirely due to inaccuracy in the thermochemical data. The determination of these absorption limits seems to be a difficult matter. However, in the case of some of the alkyl halides we have been successful in obtaining very satisfactory agreement between the two sets of data, and have thought it worth while to give a brief account of our experimental method, and to reproduce at least one of our photometric records.
Absorption spectra of the alkyl halides: energies of the C-I and C-Br bonds