Weygand/Hilgetag Preparative Organic Chemistry

Cleavage of sulfur bonds 675
from chlorobenzene by successive sulfonation at the para-position, nitration,
amination, and removal of the sulfo group. 658
2. Cleavage of the S-H bond
Chlorination of thiophenols leads, with cleavage of the S-H bond, to arenesulfenyl
chlorides, 659 " 661 the first example of the series having been prepared
by Zincke as early as 1911: 662
RSH + Cl2
> RSC1 + HC1
According to investigations by Lecher and Holschneider, 663 the arenesulfenyl
chloride that is first formed from the thiophenol reacts with unchanged thiol
to give the disulfide, which in a third step undergoes chlorolysis to 2 moles of
arenesulfenyl chloride. In general, electron-attracting substituents increase the
stability of arenesulfenyl chlorides since they lower the electron density at
the Cl-S bond and thus make removal of the chlorine atom more difficult.
In such preparations it is important to exclude moisture, so as to avoid hydrolysis,
and to work at low temperatures with shielding from light, so as to avoid
C-halogenation.
General directions for preparation of sulfenyl chlorides: 664 The required thiophenol (0.1 mole)
is added to anhydrous carbon tetrachloride (50 ml) in a three-necked flask fitted with a stirrer,
dropping funnel, calcium chloride tube, and thermometer. Then a solution of chlorine (7.09 g,
0.01 mole) in anhydrous carbon tetrachloride (100 ml) is dropped in, with stirring, whilst
the temperature is not allowed to rise above —1° (—12° for compounds having two methyl
groups on an aromatic ring). Stirring is continued for a further 1 h, then the solvent is removed
in a vacuum and the residue is distilled at reduced pressure. Yields are 80-90%.
Benzene-, halobenzene-, toluene-, xylene-, and halotoluene-sulfenyl chlorides have been
obtained according to these instructions.
This synthesis of sulfenyl chlorides can also be used in the aliphatic series
although here the limitation applies that most alkylsulfur halides are too unstable
to be isolated from solution. In general, and also in the aromatic series,
sulfenyl chlorides are easier to prepare than the bromides, whilst the iodides
are known only in a few cases and the fluorides not at all. As an example,
Schneider, 665 chlorinating methanethiol in anhydrous carbon tetrachloride
at -15°, obtained dimethyl disulfide dichloride [chloro(methyl)(methylthio)sulfonium
chloride] which passed into methanesulfenyl chloride when allowed
to warm slowly to room temperature:
658 H. P. Schultz, Org. Syn., 31, 45 (1951).
659 N. Kharasch, S. J. Potempa, and H. L. Wehrmeister, Chem. Rev., 39, 269 (1946).
660 M. B. Sparke, J. L. Cameron, and N. Kharasch, J. Amer. Chem. Soc, 75, 4907
(1953).
661 W. E. Truce and M. F. Amos, /. Amer. Chem. Soc, 73, 3013 (1951).
662 T. Zincke, Ber. Deut. Chem. Ges., 44, 769 (1911).
663 H. Lecher and F. Holschneider, Ber. Deut. Chem. Ges., 57, 755 (1924).
664 L. Almasi and A. Hantz, Chem. Ber., 94, 728 (1961).
665 E. Schneider, Chem. Ber., 84, 911 (1951).