Monday, April 6, 2015

Melting Points

Melting occurs as a result of applying sufficient heat to a solid so that it moves from its solid phase to its liquid phase. The temperature at which this transformation takes place is the compound’s melting point. Each compound has a specific melting point; thus, when working with an unknown solid compound or a known solid compound whose purity is in doubt, one important measurement that chemists often take is the compound's melting point. Fortunately, taking a melting point is relatively easy to do. They then use this temperature to help identify the unknown solid compound or to verify that the compound is impure.

Four factors that influence a compound’s melting point are symmetry, polarity, hydrogen bonding, and molecular weight. Before looking at those factors, here is some information that you need to keep in mind about solids. As you learned in Section 4.1, a crystalline solid is composed of molecules arranged in a regular pattern. To arrange themselves in this pattern, the molecules normally have the same conformation and the same specific orientation relative to each other. Figure 4.6 shows a hypothetical crystalline solid. Above the solid are molecules A and B. They are both the same as the molecules in the solid, but neither molecule A nor molecule B can fit into the crystalline lattice. Molecule A has an incorrect conformation; and although molecule B has the correct conformation, it has an incorrect orientation.


The more symmetrical a molecule is, the better it fits into a crystalline structure, and the higher is its melting point. Because molecules must have a similar conformation to form a crystalline lattice, any molecules with the “wrong” symmetry do not easily fit into the growing crystal. For example, consider the following isomeric

compounds—pentane with a melting point of –130oC and 2,2-dimethylpropane with a melting point of –17oC. Pentane exists in a large variety of conformations that are similar in energy to each other. Below are three of its many possible staggered conformations.



Because the rotational energy barrier for the various conformations of pentane is very low, pentane exists as a mixture of conformations except at a very low temperature. Thus, pentane does not readily form a solid. It favors the liquid phase over the solid phase in the solid-liquid equilibrium.
In contrast to the conformational mobility of pentane, 2,2-dimethylpropane has only one conformation of the carbon skeleton:


All the hydrogen atoms of 2,2-dimethylpropane are equivalent, and the molecule has a very high level of symmetry. This symmetry means that each molecule has the “right” conformation to form a crystal. Also, this symmetry increases the probability that each molecule has the correct orientation to fit into a growing crystal. Thus, 2,2-dimethylpropane readily forms a crystalline lattice. It favors its solid form in the solid-liquid equilibrium and requires a higher temperature than pentane to convert from its solid form to its liquid form.
Another important factor contributing to the melting point of a compound is its polarity. The more polar a compound is, the stronger are its intermolecular attractions and the higher is its melting point. For example, benzyl alcohol melts at –15oC whereas benzoic acid melts at 122oC.



Both compounds are polar, but the intermolecular attractions of benzoic acid are much stronger than those of benzyl alcohol, as is shown by the fact that benzoic acid forms a relatively stable dimer. This dimer is held together by attractions, called hydrogen bonds, between the polar oxygens and the acidic hydrogens of the carboxylic acid functional groups. Hydrogen bonds influence the melting point of a solid, but they influence the boiling point of a liquid even more



The molecular weight of a compound influences its melting point. As the molecular weight increases in a homologous series of compounds, so does the melting point. The reason the melting point increases with the weight is that it takes more energy to separate larger molecules from a crystalline structure than it takes to separate smaller ones. Although the increase in melting point holds true over a long series, other factors at times override the effects of molecular weight for molecules of similar size. Two of these factors are symmetry and hydrogen bonding. For example, because methane molecules are symmetrical and pack into a crystalline lattice better than propane molecules do, methane’s melting point (–182.5 oC) is seven degrees higher than propane's (–189.7 oC) despite its lower molecular weight.






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