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The types of intermolecular bonding and the evidence for their existence

Also, O—H---O hydrogen bonds are clearly stronger than N—H---N hydrogen bonds, as we see by comparing propanol with the amines. As expected, the presence of two hydrogen bonding functions in a compound raises the boiling point even further. Acetic acid the ninth entry is an interesting case. A dimeric species, shown above, held together by two hydrogen bonds is a major component of the liquid state.

Thus, the dimeric hydrogen bonded structure appears to be a good representation of acetic acid in the condensed state. A related principle is worth noting at this point. Although the hydrogen bond is relatively weak ca. The hydrogen bonds between cellulose fibers confer great strength to wood and related materials. Some decompose before melting, a few sublime, but a majority undergo repeated melting and crystallization without any change in molecular structure.

When a pure crystalline compound is heated, or a liquid cooled, the change in sample temperature with time is roughly uniform.

However, if the solid melts, or the liquid freezes, a discontinuity occurs and the temperature of the sample remains constant until the phase change is complete. For a given compound, this temperature represents its melting point or freezing pointand is a reproducible constant as long as the external pressure does not change.

The length of the horizontal portion depends on the size of the sample, since a quantity of heat proportional to the heat of fusion must be added or removed before the phase change is complete Now it is well known that the freezing point of a solvent is lowered by a dissolved solute, e.

This provides a useful means for establishing the identity or non-identity of two or more compounds, since the melting points of numerous solid organic compounds are documented and commonly used as a test of purity. The phase diagram below shows the melting point behavior of mixtures ranging from pure A on the left to pure B on the right. A small amount of compound B in a sample of compound A lowers and broadens its melting point; and the same is true for a sample of B containing a litle A.

The lowest mixture melting point, e, is called the eutectic point. The types of intermolecular bonding and the evidence for their existence example, if A is cinnamic acid, m. Below the temperature of the isothermal line ced, the mixture is entirely solid, consisting of a conglomerate of solid A and solid B.

Above this temperature the mixture is either a liquid or a liquid solid mixture, the composition of which varies. In some rare cases of nonpolar compounds of similar size and crystal structure, a true solid solution of one in the other, rather than a conglomerate, is formed.

Melting or freezing takes place over a broad temperature range and there is no true eutectic point. An interesting but less common mixed system involves molecular components that form a tight complex or molecular compound, capable of existing as a discrete species in equilibrium with a liquid of the same composition. Such a species usually has a sharp congruent melting point and produces a phase diagram having the appearance of two adjacent eutectic diagrams.

An example of such a system is shown on the right, the molecular compound being represented as A: Molecular complexes of this kind commonly have a 50: In addition to the potential complications noted above, the simple process of taking a melting point may also be influenced by changes in crystal structure, either before or after an initial melt. The existence of more than one crystal form for a given compound is called polymorphism.

Polymorphism Polymorphs of a compound are different crystal forms in which the lattice arrangement of molecules are dissimilar. These distinct solids usually have different melting points, solubilities, densities and optical properties. Many polymorphic compounds have flexible molecules that may assume different conformations, and X-ray examination of these solids shows that their crystal lattices impose certain conformational constraints.

Intermolecular Forces

When melted or in solution, different polymorphic crystals of this kind produce the same rapidly equilibrating mixture of molecular species. Polymorphism is similar to, but distinct from, hydrated or solvated crystalline forms.

The ribofuranose tetraacetate, shown at the upper left below, was the source of an early puzzle involving polymorphism. Several years later the same material, having the same melting point, was prepared independently in Germany and the United States.

Dipole–dipole interaction

Eventually, it became apparent that any laboratory into which the higher melting form had been introduced was no longer able to make the lower melting form. Microscopic seeds of the stable polymorph in the environment inevitably directed crystallization to that end.

  • In the case of H2O, it is the attraction between the partial positive charges on the H and the partial negative charges on the O;
  • Hydration is an example of ion-dipole interactions.

X-ray diffraction data showed the lower melting polymorph to be monoclinic, space group P2. The higher melting form was orthorhombic, space group P212121. Polymorphism has proven to be a critical factor in pharmaceuticals, solid state pigments and polymer manufacture. Some examples are described below.

  • As a result of these fluctuations, regions of equal and opposite partial charge arise in one of the molecules and give rise to a transient dipole;
  • Hence, the two molecules cohere.

Acetaminophen Acetaminophen is a common analgesic e. It is usually obtained as monoclinic prisms right on crystallization from water.

A less stable orthorhombic polymorph, having better physical properties for pressing into tablets, is shown on left. Two polymorphs of Acetaminophen. Quinacridone Quinacridone is an important pigment used in paints and inks. It has a rigid flat molecular structure, and in dilute solution has a light yellow color. Three polymorphs have been identified. Intermolecular hydrogen bonds are an important feature in all off these. The crystal colors range from bright red to violet.

Ranitidine The anti-ulcer drug ranitidine Zantac was first patented by Glaxo-Wellcome in 1978. Seven years later a second polymorph of ranitidine was patented by the same company. This extended the licensing coverage until 2002, and efforts to market a generic form were thwarted, because it was not possible to prepare the first polymorph uncontaminated by the second.

It displayed six polymorphic crystal forms.