Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

202 THIOPHENOL
exactly titrated with alkali and phenolphthalein. The yellow lead salts
and the colourless mercury salts are characteristic.
Experiment.-—A few drops of thiophenol are added to alcoholic
solutions of lead acetate and mercuric chloride.
The ease with which the hydrogen atom is detached from the sulphur
is noteworthy (analogy with H2S); even atmospheric oxygen gradually
converts the mercaptans into aryl or alkyl disulphides, and mild oxidising
agents do so at once :
2 K.SH —> E.8—S.E .
Experiment.-—A few drops of thiophenol on a watch-glass are
slowly evaporated to dryness on the water bath with a few cubic
centimetres of very dilute ammonia solution (fume chamber). An
oil remains which sets to a crystalline solid on cooling. It is phenyldisulphide,
melting point 61°.
On reduction the disulphides take up hydrogen and are reconverted
into mercaptans.
A biological example of this relationship is provided by cysteine and
cystine:
2 HOOC.CH . CH2
HOOC.CH . CH2
NH2 SH _2H NH2 S
Cysteine > I Cystine .
+2B^ NH28
HOOC.CH. CH2
Another is the behaviour of " glutathione " (Hopkins), a tripeptide
in which the NH2-group of cystine (or of cysteine) is combined with a
carboxyl group 1 of glutamic acid to form an amide grouping, and the
carboxyl group of cystine is likewise combined with the NH2-group of
glycine.
Cystine is one of the units from which the proteins are built up.
It is prepared by acid hydrolysis of keratin (from hair or horns).
The aromatic mercaptans react with chlorine to form aryl sulphur
chlorides (Zincke); phenyl sulphur chloride is a deep red, very reactive
liquid (Lecher) :
C6HS.8H + C12 —> C6HS8C1 + HC1.
From the mercaptans the sulphonic acids are re-formed by energetic
oxidation.
1 The one which is further from the NH2-group. Cf. Hanngton and Mead,
Biochem. J., 1935, 29, 1602.