Alkyl aryl sulphoxides are reduced by dialkyl sulphides in 2 : 1 methanol-water in the presence of ~M -H C I . The kinetics of the reaction between p-chlorophenyl methyl sulphoxide and dibutyl sulphide and the basic properties of the sulphoxide have been investigated in detail. Reaction orders, acid catalysis, and comparison of the reduction rate with that of the loss of optical activity by the (+)-R-sulphoxide indicate a predominant role for the chlorosulphonium cation. A mechanism is suggested in which the changes of the oxidation states occur through the reaction between the sulphide and this intermediate.THE chemistry of sulphoxides has been the subject of a number of mechanistic studies in these laboratories. We were attracted by a reaction of some theoretical and synthetic interest, namely oxygen transfer from sulphoxides to organic sulphides. This reaction has been reported to occur 3-s particularly well in the presence of HCl,9-11 other acids such as H,SO, or HClO, being much less effective catalysts ; the reaction is strongly dependent on the structure of the reactants. The only related mechanistic study so far publisheds deals with intramolecular oxygen migration from o-alkylthiophenyl aryl sulphoxides to a sulphide sulphur in 70--80y0 H,S04. In the light of our earlier hypothesis,2 our interest was stimulated by the idea that catalysis by HC1 of the oxygen exchange reaction is mechanistically related to the effective oxidising and halogenating abilities of the sulphoxide-hydrogen halide system.l2-I5We present here a kinetic study of the HCI catalysed reaction between 9-chlorophenyl methyl sulphoxide (Ia) and dibutyl sulphide (IId), an equilibrium which because it is almost completely shifted towards the products and is virtually irreversible under suitable concentration conditions, offers considerable advantages for kinetic investigation. RESULTSProducts.-Sulphides and sulphoxides were identified and determined by g.1.c.; other products beside those of Taken from the doctoral Thesis of A. Box7i0, University' of Padua, 1973. 2 Some of the results have been presented by G. Modena, 5th

In wäßrigem Methanol stellt sich in Gegenwart von HCl zwischen den Sulfoxiden (I) und (V) und den Sulfiden (II) und (IV) ein Gleichgewicht ein, das weit auf der Seite der Produkte liegt.