294 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS decomposes to give H•.S and an aidehyde. It is this mechanism which satisfactorily accounts for the results obtained by Sch/3berl without requir- ing the postulation of a sulfenic acid. We shall now consider the spectral evidence for the dissociation of the • hydrogens, which is the basis for our mechanism for the attack of alkali on the disulfide bond. III. STRUCTURE OF THE DISULFIDE BONY ANY ITS SPECTRAL MANIFESTATIONS Many of the unique properties associated with sulfur, either in the ele- mental state or combined with carbon in organic compounds may be inter- preted in terms of the orbital distribution of its •r electrons. These "un- committed electrons" are readily polarizable and are easily unpaired when the molecule in which they are contained is introduced into an environ- ment of free electrons or radicals, or when it is activated either thermally or photochemically. Thus, much of the spectral data as well as the chemical reactivity of sulfur compounds, as we shall indicate later, may be correlated on the basis of a consideration of these •r electrons. The angle which the S--S bond makes with an adjacent carbon atom is equal to 104 ø , as has been calculated from measurements made on such compounds as p,p'-dibromodiphenyl disulfide, dimethyl trisulfide, and ele- mental sulfur (34). The dihedral angle C--S--S--C as calculated from measurements taken on N,N'-diglycyl cystine has been shown to be 101 ø (35). This combined evidence suggests that a non-planar, almost right- angle skew distribution of valences about the S--S bond, is energetically preferred. Pauling has shown on the basis of theoretical calculations that repulsion of the •r electrons on the sulfur atoms should lead to a chain struc- ture for these molecules with dihedral angles C--S--S--C about 90-100 ø (36). He thus explained the stability of S8 as compared to S6 and S•0 and other puckered rings by noting that in S8 the dihedral angle appears to be closest to the optimum value as judged by the literature values available to date. In compounds in which the bond angles of the disulfide bond deviate from those stated above, it is expected that the S--S linkage will be in a strained condition. These deviations may arise from such factors as steric hindrance of bulky groups situated on either side of the disulfide linkage, ionic interaction of charged groups within the molecule, a tendency on the part of the molecule to gain in total resonance stability, or as a consequence of electronic and angular distortions resulting from the formation of planar rings containing the S--S bond. However, in all cases of structures which have imposed strain there is a general approach to co-planarity of the C--S--S--C unit at the expense of the dihedral which concomitantly ap- proaches zero. The resultant distortion of electronic orbitals should, as one

PROGRESS IN THE CHEMISTRY OF DISULFIDES 295 might predict, manifest itself in a greater ease of electronic perturbility. Thus one would expect that electromagnetic waves of longer wavelength, that is, light of lesser energy will be required to excite the • electrons to an activated state. This is what Calvin (37) essentially observed in his com- parative study of the ultraviolet spectra of cyclic disulfide compounds as is shown in Table 1. Qualitative chemical evidence of strain associated with disulfide ring size was obtained by Affleck and Dougherty (38), who noted that tl•e speed of polymerization of cyclic disulfides in the presence of A1Cla decreased with an increase in the number of atoms in the ring in the range of 5 to 7 in which they made their observations. Birch (39) had similarly noted that the 5-membered ring was only stable at 0øC. in the dark--in the presence of light it underwent immediate polymerization. Thus we may regard Calvin's data as indicating that the S--S linkage in cyclic disulfides is strainless when it absorbs at the lower wavelength (2500 J-.) and the strain is greater the greater is the wavelength of the ab- sorption maximum. From the above, it would appear that the relative strain existing in any disulfide might readily be determined by an observation of its ultraviolet spectrum. This, however, is not the case. The conclusions thus far stated apply only to cyclic disulfides of the type examined by Calvin. Many disul- fides fail to display any specific ultraviolet absorption and are described as being "highly transparent" in this region. Furthermore some disulfide-con- taining compounds such as /-cystine have ultraviolet spectra which are highly sensitive to changes in pH. This is probably the reason why there are at least a dozen publications which contest the precise ultraviolet ab- sorption of/-cystine. In view of all these seemingly inconsistent and uncorrelated data we undertook an intensive study of the ultraviolet spectra of disulfide com- pounds with the view to answering the following questions: 1. Why is it that not all disulfide-containing compounds show absorp- tion maximum in the ultraviolet ? 2. What is the precise nature of the chromophoric group responsible for the ultraviolet absorption ? TABLE 1--ULTImAVIOLET ABSORPTION SPECTRA O1• CYCLIC DISULlrlDES (37) No. of Atoms No. of S Wavelength, -•., Compound in Ring Atoms Ultraviolet Max. 6,8 Thioctic acid 5 2 3340 Trimethylene disulfide 5 2 3340 5,8 Thioctic acid 6 2 2860 Tetramethylene disulfide 6 2 2865 4,8 Thioeric acid 7 2 2580 n-Propyl disulfide Linear compound 2 2500