Mechanism of formation of benzothiazole-2-thiol Neil S. Isaacs,* Fyaz Ismail, Mark J. Hilton and Mark Coulson Department of Chemistry, University of Reading, Whiteknights P.O., Reading RG6 6AD, UK Received 22 May 1997 ABSTRACT: The formation of benzothiazole-2-thiol from aniline, carbon disulphide and sulphur at 230°C was shown to occur by a sequence of three principal steps. Labelling experiments confirmed that both sulphur atoms originate from carbon disulphide. An initial polar reaction to form thiocarbanilide via phenylcarbamic acid and a tetrahedral intermediate is followed by radical cyclization of these to benzothiazole-2-thiol and 2-phenylamino- benzothiazole; the latter is converted into the desired product by a polar displacement of aniline by H 2 S. Mechanisms for the formation of minor byproducts are also considered.  1998 John Wiley & Sons, Ltd. KEYWORDS: benzothiazole-2-thiol; formation mechanism Benzothiazole-2-thiol (mercaptobenzthiazole; MBT) 1 is an important bulk chemical used as an accelerator in the vulcanization of rubbers. 1 It is prepared by a process originating in 1927 in which aniline, carbon disulphide and sulphur in equimolar amounts are heated together in a closed autoclave at 220–250°C over a period of about 4 h (the Kelly Process). 2 The stoichiometry of the reaction is as follows: The yield of the desired product can reach 90% but there are many minor byproducts; more than 50 can be separated by liquid chromatography and most have been identified (see Figure 1). Several mechanistic schemes have been put forward to explain the formation of the principal product, some of which are indicated below and all of which are now shown to be incorrect. 1. The formation and sulphurization of phenyl isothiocyanate (Scheme 1): 3 Phenyl isothiocyanate is formed in only very minor amounts whereas, according to this hypothesis, it should be obtained as a major initial product. Furthermore, although the authors claimed that its reaction with sulphur leads to MBT, the present work does not substantiate this. No reaction appears to take place between phenyl isothiocyanate and sulphur at 230°C. Thiocarbanilide forms readily and quantitatively from the reaction between aniline and carbon disulphide and the present work has shown that MBT results from heating this substance with sulphur (Scheme 2). The earlier workers assumed the role of sulphur was to supply the ring sulphur atom, now shown to be incorrect, and could not account for specific ortho attack if this were the mechanism. 2. The thiolation of phenylcarbamic acid (3 2–5 (Scheme 3): the initial product, 3, formed from aniline and carbon disulphide was proposed to react with sulphur to form the 2-thiol, which then cyclizes. An alternative version proposes sulphurization of diphenyldithiobiuret (4), followed by cyclization. 3. The sulphurization of aniline and cyclization: 6 aniline and sulphur do indeed react together at tempera- tures well below those of the process to form a mixture of thiols and bis(anilino)polysulphides. The products are discussed below (see Figure 3). The present work can eliminate this as a major pathway although it is shown to account for 3–4% of the product. The argument against this reaction as a major route to MBT is that it would require specific attack in the ortho position, an unlikely situation and one which is demonstrably untrue. Thiola- tion of aniline by sulphur is shown by the present study to be random in its orientation. Two key observations which must find an explanation in any mechanistic scheme are first, the formation of a carbon–sulphur bond exclusively in the ortho position of the aniline unit, for which an intramolecular attack of sulphur on carbon seems the only possibility, and second, the sudden incursion of reaction at 180 °C. Other uncertainties before the present work were the origin of JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 11, 1–9 (1998) *Correspondence to: N. S. Isaacs, Department of Chemistry, University of Reading, Whiteknights P.O. Box 224, Reading RG6 2AD, UK. E-mail: n.s.isaacs@reading.ac.uk Contract/grant sponsor: Monsato.  1998 John Wiley & Sons, Ltd. CCC 0894–3230/98/010001–09 $17.50