224 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES
This means that steric barriers exist which must be overcome to form
an addition compound owing to nonplanarity of the nitro compound. The
steric factor could also explain the fact that the addition compound reacts
only slowly with bromine, whereas indene itself rapidly adds bromine to form
a dibromide.
A detailed list of addition compounds formed by sym- trinitrobenzene, 2,4,6-tri-
nitrotoluene, picric acid and other polynitro compounds is given in the appropriate
paragraphs devoted to these nitro compounds.
It is important to add that some molecular addition compounds of sym- tri-
nitrobenzene with hydrocarbons can serve for identification and purification of
hydrocarbons (Veibel [141]). Trinitrofluorenone seems to be of particular value
(Orchin and Woolfolk [142], Lepley [154]).
As shown by T. Urbanski [143] between 1933 and 1937, there is a group of
molecular addition compounds whose existence cannot be explained by all the
points mentioned above. They are addition compounds of certain nitro compounds
with esters of nitric acid. Thus many aromatic mononitro compounds form addition
compounds with mannitol hexanitrate, and some aromatic trinitro compounds
do so with erythritol tetranitrate (Vol. II). On the basis of these facts the author
suggests that two main reasons are responsible for the formation of these addition
compounds :
(1) Assembly of a great number of nitro groups in both components: nitric
ester and nitro compound. Incidentally the sum of these groups is eight in the
above mentioned examples.
As shown by Hackel [144] glycerine trinitrate (nitroglycerine) does not form
addition compounds with any of the aromatic nitro derivatives (mono-, di- or
tri-nitro compounds). This is probably due to the inadequate number of nitro
groups in nitroglycerine.
(2) The polarity of nitric ester. All esters forming addition compounds pos-
sess high polarity. On the contrary, the ester of low polarity-penthaerythritol tetra-
nitrate does not give addition compounds with any of the above mentioned aro-
matic nitro derivatives.
Complex compounds between nitro compounds and SbCl 5 and HCl possess
a quite different and clearly ionic character (Klages, Mühlbauer and Uhl [145]).
They are stable below 0°C and at 0°C they readily lose HCl. The heat of decompo-
sition (5-6 kcal/mole) suggests that a hydrogen bond is present in the complex
to stabilize the salts. The authors suggest the structure