NITRO DERIVATIVES OF PHENOL^493Violent boiling of picric acid with a concentrated solution of sodium hydroxide
may bring about decomposition of the former:
NaOH
C 6 H 2 (NO 2 ) 3 OH NH 3 + HCN + HNO 2 (7)Hypochlorites decompose picric acid, which undergoes chlorination to form
chloropicrin. The reaction is carried out with calcium hypochlorite. The calcium
hydroxide formed in the reaction neutralizes the hydrochloric acid formed. This
is a commercial method of preparation of chloropicrin.
85°C
C 6 H 2 (NO 2 ) 3 OH + 11HOC1 -> 3 CCl 3 .NO 2 + 3CO 2 + 6H 2 O + 2HCl (8)
When reacted with chlorine, aqua regia or potassium chlorate in the presence
of hydrochloric acid, picric acid yields chloranil along with chloropicrin:Cl 2
C 6 H 2 (NO 2 ) 3 OH -> CCl 3 .NO 2 + (9)If the reaction is stopped just when the formation of chloropicrin starts. the presence
of 6-chloro-2,4- and 4-chloro-2,6-dinitrophenols can be also detected along with it.
When heated to a temperature above its melting point, picric acid can react
with sulphur. Addition of sulphur lowers the ignition temperature of picric acid
(see below).
Action of heat. According to Kast [40], picric acid, when heated to 160°C
begins to decompose slightly and an insignificant evolution of gases occurs.
At 200-210°C the evolution of gases is stronger and heating at 260°C for half an
hour results in explosion. The explosion may not occur below temperatures of
300-310°C when a small quantity of the acid is heated more rapidly.
Micewicz and Majkowski [54] reported that the initiation temperature of picric
acid is 243-288°C on slow heating, while it rises to 337-346°C on rapid heating.
T. Urbanski and Pillich [58] found that picric acid with a 5% addition of
sulphur exploded at 251°C and with 10% at 247°C whereas pure picric acid exploded
at 330°C.
Picric acid melts prior to burning giving a sooty flame. The heat of fusion lowers
the temperature of the layer adjacent to the burning one, so the substance may either
stop burning or continue to burn only gently. In large quantities, burning of picric
acid may proceed slowly, especially when it is spread over a larger area. If, however,
the product is concentrated on a small area or when it is in a confined space, ex-
plosion may occur. Burning the product containing metal picrates may result in
explosion. Similarly a potential danger exists when burning picric acid is in contact