Enantiotropic forms

Two.isomers are known Cis, crysts (from toluene) exists in three enantiotropic forms, the transition point lies between 75.2 -78.1°, mp 99°, bp 118° at 14 mm Hg and Trans, crysts (from benz, toluene or ether), mp 104°, bp 120° at 14 mm Hg. 

OB to C02 —22.20% the salt exists in two enantiotropic forms with the properties listed in Table 1... 

An interesting test of the third law is possible when a solid is capable of existing in two or more modifications, i.e., enantiotropic forms, with a definite transition point. The entropy of the high temperature form (a) at some temperature above the transition point may, in some cases, be obtained in two independent ways. First, heat capacity measurements can be made on the form (/3) stable below the transition point, and the entropy at this temperature may then be determined in the usual manner. To this is then added the entropy of transition, thus giving the entropy of the o-f orm at the transition point (cf. first three lines of Table XVI). The entropy contribution of the a-form from the transition temperature to the chosen temperature is then obtained from heat capacity measurements on the o-form. The second procedure is to cool the ot-form rapidly below the transition point so that it remains in a metastable state. Its heat capacity can then be determined from very low temperatures up to temperatures above the normal transition point, and the entropy of the a-form is then obtained directly from these data. Measurements of this kind have been made with a number of substances, e.g., sulfur, tin, cyclohexanol and phosphine, and the entropies obtained by the two method have been found to be in close agreement. ... 

The DSC cui-ve for Form I of finasteride is characteristic of an enantiotropic form. Form I is more stable at lower temperatures, with conversion to the higher-temperature stable Form n at the transition point. Cooling samples back through the transition point (unmelted) to room temperature and below results in Form II only. Cooling of melted samples, whether starting with Form I or 11, results in the crystallization of Form II, with no conversion to Form I upon cooling to room temperature and... 

Figure 2. Gibbs free energy curves for a hypothetical system of polymorphs A, B, and C. The systems are classified as monotropic (forms A and C, forms B and C) or enantiotropic (forms A and B) with a transition temperature, T,. Melting points, T , for the polymorphs are shown by the intersection of the curves for the crystalline and liquid states. Adapted from Rodriguez-Spong et al., (2004) according to the relationships developed by Shalaev and Zografi (2002).

Figure 3.1a shows the free energy curves for the reversible enantiotropic Forms... 

Equilibrium Relations in the Case of Liquid Crystals.— Whether we are dealing here with substances in two strictly crystalline and enantiotropic forms (which we may call the solid and liquid crystalline forms), possessing a definite transition point, or whether we... 

We have already learned that certain substances are capable of existing in various crystalline forms, and these forms are so related to one another that at a given temperature the relative stability of each pair of polymorphic forms undergoes change. Since each crystalline variety of a substance must have its own solubility, there must be a break in the solubility curve at the temperature of transition of the two enantiotropic forms. At this point the two solubility curves must cut, for since the two forms are in equilibrium with respect to their vapour (p. 33), they must also be in equilibrium with respect to their solutions. 

In Fig. lb. Form II is stable at temperatures below the transition temperature and Form I is stable above T. At the transition temperature the two forms have the same solubility, and reversible transformation between enantiotropic Forms I and II can be achieved by temperature manipulation. The relative solubility of two polymorphs is a... 

I and III, were deduced to be enantiotropic pairs in the sense that their G vs. T curves crossed. Form III was found to be monotropic with respect to Form II, since the G vs. T curves did not cross below their melting points, and since there was no temperature at which Form El was the most stable polymorph. The solubility data illustrated in Fig. 1 were used to estimate a transition temperature of 74°C for the enantiotropic Forms I and II, while the reported enthalpy difference was 4.5 kcal/mol at 74°C and 2.54 kcal/mol at 25°C. The most stable polymorph below 74°C was Form I, whereas Form... 

In Figure 6.43b, form II is stable at temperatures below the transition temperature T and form I is stable above T. At the transition temperature both forms have the same solubility and reversible transformation between these two enantiotropic forms I and II can be effected by temperature manipulation. Figure 6.43c, however, depicts the intervention of metastable phases (the broken line extensions to the two solubility curves) which bear evidence of the importance of kinetic factors which for a time may override thermodynamic considerations. For example, if a solution of composition and temperature represented by point A (supersaturated with respect to both I and II) is allowed to crystallize it would not be unusual if the metastable form I crystallized out first even though the temperature would suggest that form II is the stable form. This would simply be an example of Ostwald s rule (section 5.7) being followed. This behaviour would occur, for example, if form II had the faster nucleation and/or crystal growth rates. However, if the crystals of form I were kept in contact with the mother liquor, transformation could occur as the more soluble form I crystals dissolve and the less soluble form II crystals nucleate and grow. 

Two types of polymorphism occur in lipids. The forms are enantiotropic. When each form is thermodynamically stable in a definite range of temperature and pressure, the forms are enantiotropic. Thus one enantiotropic form changes to another at a certain transition temperature. If, on the other hand, one of two forms is thermodynamically unstable, the two forms are said to be monotropic. The phase transitions in lipids are usually of the monotropic type, for example in glycerides. 

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