Melting point of organic compounds

The melting points of organic compounds follow approximately the same trend as their boiling points. There are some anomalies, however, due to more complex forces of bonding in solids. 

Austin, J. 1930. A Relation Between the Molecular Weights and Melting Points of Organic Compounds. J. Am. Chem. Soc. 52, 1049. 

To predict the melting point of organic compounds, Tesconi and Yalkowsky (2000) recommend a method that uses the group contribution method based on the works of Simamora and Yalkowsky... 

Joback s method is an improved version of earlier methods, and attempts to estimate many properties of all the organic compounds. Table 5.13 shows the recommended Joback coefficients to build up the boiling and melting points of a compound, according to the formula... 

Crystalline organic compounds typically have a characteristic melting point. In some cases, a compound may have more than one arrangement in the crystal and such polymorphs will exhibit differences in melting behavior. For the most part, however, the melting point of a compound is characteristic and invariant for a pure sample. When contaminated by a second substance, however, the melting (or freezing) point is typically lowered. 

The melting point of a compound is the temperature at which the solid and liquid phases are in equilibrium at one atmosphere pressure is specified because the melting process involves a change in volume and is therefore pressure dependent. Since the melting point can be determined easily experimentally, it is the most commonly reported physical property for organic compounds. However, in the absence of a rigorous theory of fusion, it is one of the most difficult to predict. 

Walters, A., P. Myrdal, and S. Yalkowsky. 1995. A Method for Estimating the Boiling Points of Organic Compounds From Their Melting Points. Chemosphere. 31, 4, 3001-3008. 

Walters, A.E., P.B. Myrdal, and S.H. Yalkowsky. 1995. A method for estimating the boiling points of organic compounds from their melting points. Chemosphere 31 3001-08. 

Yalkowsky, S.H., J.F. Krzyzaniak, and P.B. Myrdal. 1994. Relationships between melting point and boiling point of organic compounds, bid. Eng. Chem. Res. 33 1872-77. 

No physical properties of 1,2 and 1,3-diazetes have been reported since they are very energtic molecules and can only be detected by spectroscopic means. On the other hand, 1,2- and 1,3-diazetidines have been found to be mostly solids, depending on their substituents. Decomposition has been observed at the melting points of many compounds, especially in the case of 1,3-diazetidines. These compounds are mostly soluble in the usual organic solvents such as ether, chloroform, acetone, etc. However, dimethyl sulfoxide (DMSO) has been used to dissolve diazetidinium salts. Diazetidine derivatives are mostly good solids that can be purified by recrystallization. Liquid compounds can be distilled under reduced pressure without decomposition. 

C. The increase in melting and boiling points of organic compounds is related to the polarity of functional groups. 

The D TA method also provides a simple and accurate way of determining the melting, boiling, and decomposition points of organic compounds. Generally, the data appear to be more consistent and reproducible than those obtained with a hot stage or a capillary tube. 

The application of mixed melting point determinations to the identification of organic compounds has been described in Section 1,17. In order to gain experience the student should carry out the following simple experiment. 

The methods of preparation of some of the more important derivatives of a number of classes of organic compounds are described in the various Sections dealing with their reactions and characterisation. These Sections conclude with tables incorporating the melting points and boiling points of the compounds themselves, and also the melting points of selected derivatives. For convenience, the references to the various tables are collected below. 

The metallic salts of trifluoromethanesulfonic acid can be prepared by reaction of the acid with the corresponding hydroxide or carbonate or by reaction of sulfonyl fluoride with the corresponding hydroxide. The salts are hydroscopic but can be dehydrated at 100°C under vacuum. The sodium salt has a melting point of 248°C and decomposes at 425°C. The lithium salt of trifluoromethanesulfonic acid [33454-82-9] CF SO Li, commonly called lithium triflate, is used as a battery electrolyte in primary lithium batteries because solutions of it exhibit high electrical conductivity, and because of the compound s low toxicity and excellent chemical stabiUty. It melts at 423°C and decomposes at 430°C. It is quite soluble in polar organic solvents and water. Table 2 shows the electrical conductivities of lithium triflate in comparison with other lithium electrolytes which are much more toxic (24). 

---