4-NITRO-5-METHYL-2-SULPHOBEN2:OIC ACID 
207 
Remsen and Saunders^^ made the phenol derivatives of o-sul- 
phobenzoie acid by heating the high melting acid chloride with 
phenol. They obtained only the symmetrical diphenyl ester. 
Remsen and McKee^^ found that the unsymmetrical acid chlor- 
ide gave the same results. More thorough work by Humphreys, 
in which careful attention was paid to the temperature of the 
reaction, shows that at 40° the symmetrical acid chloride gives 
both the diphenyl ester and some of the phenyl ester sulphone 
shloride, while the unsymmetrical gives only the phenyl ester 
sulphone chloride ; at 60° both acid chlorides give equal amounts 
of the above mentioned products; at 130° both acid chlorides 
give only the diphenyl ester. It would seem that the behavior 
is largely a function of the temperature, and that it is not safe 
to draw any general conclusions from the experimental data. 
Chambers^^ found that even at low temperatures the p-nitro-o- 
sulphobenzoic acid, when a very weak alkaline solution was 
used, as in the Schotten-Baumann reaction, gave the diphenyl 
ester. 
In every instance noted, a water soluble red substance is also> 
formed, probably a phthalein. No investigation has been made 
of it, and none was made here in spite of its evident presence. 
It is removed in the washing of the diphenyl ester with water. 
It shows the properties of an indicator. Alkalies, including 
a.mmonia, give a deep purple color, mineral acids a salmon pink 
solution. With acetic and tartaric acids, even in the presence of 
hydrochloric acids, the color is lemon yellow. However, excess, 
of sulphuric acid prevents the development of the yellow color. 
The material for these tests was the unpurified substance ob- 
tained by heating phenol and the acid chloride together at a 
high temperature for some time. The difference in the be- 
havior of the acids may be due to the presence of other sub- 
stances than the phthalein proper. 
Three methods were used in preparing the diphenyl ester. 
First, the acid chloride was warmed with phenol. The produc- 
tion of phthalein was so marked, that this method was used 
only for a trial preparation. The diester appears to be the 
only product outside of the phthalein. 
Second, the Schotten-Baumann method was tried. The acid 
chloride was dissolved by gentle warming in a slight excess of 
3iAm. Chem. J., 17, 353, 1895. 
32Am. Chem. J., 18, 799, 1896. 
33Am. Chem. J., 30, 292, 1903. 
3^Am. Chem. J., 30, 374, 1903. 
