290 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS H20 (e) R'--S- d- R--SSR RSSR' + RSH To test the validity of this concept we sought to ascertain whether other anions such as acetate, oxalate, fluoride, azide, cyanate, and sulfate, would function similarly. In no case did we find evidence ofmercaptan formation. Hence no nucleophilic attack has occurred. It should be noted that those reagents which do successfully attack the disulfide (the reagents listed in equations a-e above), save the hydroxyl, have the property in common of being able to readily lose an electron in the presence of an electron acceptor to yield a free radical (21). Stated differently, all reagents effective in cleaving the disulfide bond are capable of readily undergoing one-electron transfer. Thus it would appear that the attacking moiety is essentially a free radical and the reaction which occurs is not ionic at all but a radical dis- placement reaction. If one reflects upon other known reagents that reduce disulfide, such as Zn and HCI or sodium metal in liquid ammonia, it would appear here as well that cleavage is occurring via the attack of a free radi- cal. Zn R--SSR + 2H. ) 2RSH HC1 R--SSR d- 2Na. Na 2RSNa NH, For the present, in writing the radical displacement reaction which is oc- curring, we prefer to be non-committal with regard to whether it is the thiyl radical, formed as a result of disulfide dissociation or as a consequence of a one-electron transfer on the part of the ion, which is attacking the di- sulfide linkage. The principal reaction products obtained by either ap- proach are identical. Speakman (22), Birch (23), Tobolsky (24), and others, have demon- strated that in the absence of oxygen, disulfide-containing compounds on exposure to either ultraviolet or diffuse daylight dissociate into thiyl radicals (RS.) which are capable of polymerizing vinyl monomers. Sch6nberg (25) has equally shown that many SS compounds will thermally dissociate into thiyl radicals. Let us for the moment visualize the consequences of the energized production of such radicals in the presence of ions capable of one- electron transfer. initiation R--S--S--R 2RS. RS. d- HSOa- ) RS- d- HSOa. propagation HSO,. d- RSSR ) RSSOaH + RS- termination RS. d- HSOa. ) RSSOaH The products resulting from these reactions are RS- and RSSOaH. Once again let us consider a possible initiation mechanism wherein the

PROGRESS IN THE CHEMISTRY OF DISULFIDES 291 ion capable of one electron transfer, in the presence of oxygen, undergoes an auto-oxidation-reduction reaction to form radicals. initiation SOa z + SOF q- 1/202 --SOa-- q- SO4= propagation SOa-. q- RSSR ) RSSOa- q- RS. RS. +SOa= ' • RS- +SOs-. termination RS- q- RS. RSSR The products resulting from these reactions are identical with those ob- tained above. Clarke (26) had noted that sulfate is formed during the re- action of a sulfite with cystine which suggests that the latter mechanism is operative. As further evidence for the one-electron transfer concept, Karchmer (27) in polarigraphic studies of mercaptans, has shown that mercaptans readily undergo the following reaction: R'SH • R'S. q- H* q- le One would predict that in the presence of an electron acceptor this step would proceed rapidly requiring almost no energy of activation. Thus in the presence of a simple disulfide, a mercaptan would be expected to rapidly undergo a radical displacement reaction resulting in a disulfide interchange equilibrium. Current work being done in our laboratory indicates that this is indeed the case, namely: (1) R'SH •" R'S. q- H + q- le (2) R'S. + R--S- .S--R • R'S--SR + RS. (3) R'S. + R'S- R'S--S--R' One-electron transfer would also account for the ease with which a mer- captan will oxidize in an alkaline medium in the presence of both oxygen and a one-electron metal catalyst such as Ag + or Cu ++ or Fe +++. For ex- ample: R'SH q- Cu** • R'S. q- H* q- Cu* Cu* + O2 + H + Cu ++ + H20• R'SH q- HaOa R'SSR' q- HaO If we accept the one-electron transfer-radical displacement theory as the mode of action of disulfide cleavage, how does one reconcile the fact that alkali or water, as Sch6berl initially showed, is capable of cleaving the S--S bond? It should be remembered that the formation of hydroxyl radical from hydroxyl ion or from water under normal reaction conditions is ener- getically impossible. What is probably occurring is that the hydroxyl or alkoxy ion is performing in the role of a base by reacting with a proton in an acid-base relationship. Thus the hydroxyl ion is involved in a reaction