Nitroglycerine, labile form

There are two modifications of nitroglycerine, differing in freezing point and crystalline forms. With respect to crystal structure the form melting at the lower temperature represents a labile form (Fig. 6) which can be transformed spontaneously into the higher melting stable form of nitroglycerine (Fig. 7). 

The conditions necessary for the formation of each crystal form were examined by Hibbert [15]. He found that if recently produced nitroglycerine is mixed with wood meal or pulverized glass and cooled to the temperature of —40°C, while being stirred vigorously, the labile form is produced. By introducing a crystal obtained in this way into another portion of cooled nitroglycerine, crystallization of the labile form can be induced. 

Hackel has extended Hibbert s observations concerning the influence of certain substances on the formation of nitroglycerine crystals of different forms. Thus, adding urethane, phenylurethane, collodion, nitrocotton, cyclonite, tetryl, dinitrobenzene, or centralites I and II promotes the formation of the labile form. Admixtures of siliceous earth, trinitrobenzene, TNT, favour the creation of the stable form. 

Hibbert [15] assumed that the labile form of nitroglycerine belongs to the triclinic system, while according to Flink [19] the stable form has orthorhombic bipyramidal crystals. 

The freezing and melting temperatures of these transformations have been established by Kast [14] who also observed that very pure nitroglycerine is transformed on freezing mainly into the labile crystals, whereas a less pure sample often crystallizes in the stable form.,... 

On the basis of the dipole moment values and viscosity measurements of the two forms of nitroglycerine, de Kreuk [20] considered that the difference between the two forms is produced by rotational isomerism. According to this hypothesis the labile and stable forms would correspond to cis- and trans-isomers respectively. In a non-polar solvent the traits form predominates. In a polar solvent the content of the cis form increases and reaches a maximum in liquid nitroglycerine. 

Dinitrodiglycol exists in two crystalline forms—a stable form melting at a temperature of +2°C and a labile one whose melting point is —10.9°C (thus resembling nitroglycerine). At 20°C it is an oily liquid of density 1.385 g/cm3, boiling and simultaneously decomposing at 160°C. Its refractive index is 1.4517 (at 20°C). 

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