292 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS other than a direct attack on disulfide. What then is this other reaction which involves OH- and subsequently results in disulfide cleavage? We believe it is the following: H H I I - (1) OH- q- R--C--S--S--C--R 2 ' R--C--S--S--CH_o--R + HOH I I I H H H (2) R--C--S--SCH2R • • R--C=•--SCH2R H (3) R--C=S + - S--CHaR [ H H20 (4) R--C=S RCHO + H.,S I H The attack of base upon a hydrogen atom joined to a carbon atom which is situated • to the site of cleavage is essentially a • elimination reaction (28). The analogy to a/• elimination is quite valid in the sense that in this case also the group that cleaves in departing takes with it an electron pair while the entity which remains contains a double bond. In a typical f$ elimination cleavage occurs between carbon and sulfur or carbon and nitro- gen in this instance the cleavage is occurring between the sulfur-sulfur bond. It is well recognized that hydrogens on a carbon atom attached to a sulfide or sulfone have a tendency to be acidic (29). This ionizing tendency or hyperconjugation effect is greatly enhanced by the resonance stabiliza- tion of the anion thus formed through expansion of the sulfur octet to a valency of 10. Specific examples of this effect have been noted in the case of methyl vinyl sulfide, whereas the high Q value has been interpreted as signifying an apparent tendency on the part of the R--•H--S--CH,• radical to expand the sulfur shell in order to achieve a resonance stabiliza- tion (30): R--•H--S--CH, • R--CH--•--CHs Another example of this is the base catalyzed condensation of a diketone with thiodiglycolic acid to give substituted thiophenes (31). Chemical evidence in favor of the • elimination mechanism is the fact that only primary and secondary disulfides are observed to give off hydro gen sulfide on attack by alkali. This corresponds with the fact that only primary and secondary disulfides possess hydrogen capable of being at- tacked. Sch6berl, in addition, had noted that tertiary disulfides such as tetramethyl dithiodiglycolic acid are completely stable to alkali. To ac- count for these differences in terms of his sulfenic acid hypothesis, Sch6berl

PROGRESS IN THE CHEMISTRY OF DISULFIDES 293 was compelled to postulate that the evolution of H.•S was the exclusive property of primary and secondary sulfenic acids and not that of tertiary sulfenic acids. primary disulfide R--•--S L R J., I secondary disulfide R--C--S R ,_, tertiary disulfide L The conclusions that can be arrived at regarding the mechanism of di- sulfide cleavage, aside from thermal or photochemical dissociation of S--S into thiyl radicals which are capable of initiating polymerization, are that there are two distinct mechanisms capable of operating, one ionic and one radical in nature: (1) Radical Mechanism This reaction proceeds via a direct attack upon the S--S linkage, i.e., a typical radical displacement reaction similar to those observed by Tobolsky (32) and Stockmayer (33) to occur in the case of reaction of free radicals with both linear and cyclic disulfides. Included in this category are such supposed ionic reagents as: sulfite, cyanide, sulfide, and mercaptide. In addition such radical reagents as an active metal with an acid, sodium metal in liquid ammonia, or radicals resulting from growing polymer chains, or arising from the decomposition of radical catalysts, are also capableof under- going radical displacement reactions on the disulfide bond. (2) Ionic Mechanism This mechanism which involves an indirect attack upon the S--S link- age results from a direct nucleophilic displacement on hydrogen by base, or it may be viewed simply as an ionization of an acidic hydrogen in the pres- ence of base to form an anion which is resonance stabilized, one form of which involves an expansion of the sulfur shell to 10 electrons. --•H--S--S • This anion then proceeds to react via a B elimination to yield a mercaptan and a thioaldehyde. The latter compound in the presence of water readily