Physical properties and intermolecular forces

The physical properties of a compound are determined by the attractive forces between the individual molecules, called intermolecular forces. It is often diflBcult to use the molecular structure alone to predict a precise melting point or boUing point for a compound. However, a few simple trends will allow us to compare compounds to each other in a relative way, for example, to predict which compound will boil at a higher temperature. 

All intermolecular forces are electrostatic—that is, these forces occur as a result of the attraction between opposite charges. The electrostatic interactions for neutral molecules (with no formal charges) are often classified as (1) dipole-dipole interactions, (2) hydrogen bonding, and (3) fleeting dipole-dipole interactions. 

In the hquid phase, the molecules are firee to tumble in space, but they do tend to move in such a way so as to attract each other more often then they repel each other. The resulting net attraction between the molecules results in an elevated melting point and boiling point. To iUusttate this, compare the physical properties of isobutylene and acetone  

Isobutylene lacks a significant dipole moment, but acetone does have a net dipole moment. Therefore, acetone molecules will experience greater attractive interactions than isobutylene molecules. As a result, acetone has a higher melting point and higher boifing point than isobutylene. 

Hydrogen bond interaction between molecules of water 

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Polyesters are another important class of polyols. There are many polyester types used, so a generic structure is shown in Scheme 4.4. They are often based on adipic acid and either ethylene glycol (ethylene adipates) or 1,4-butanediol (butylene adipates). Polyesters, because of the polar carbonyl groups, contribute more to intermolecular forces, and physical properties such as tear and impact resistance are often improved by using them. They are also utilized for their solvent and acid resistance and light stability. Relatively poor hydrolytic stability is... 

EXAMPLE 12-6 Relating Intermolecular Forces and Physical Properties... 

The polarity of molecules also creates attractive forces between molecules that cause the molecules to stick together. These attractive forces are called Intermolecular Forces. The physical properties of melting point, boiling point, vapor pressure, evaporation, viscosity, surface tension, and solubility are related to the strength of attractive forces between molecules. 

Substitution of fluorine for hydrogen in an organic compound has a profound influence on the compound s chemical and physical properties. Several factors that are characteristic of fluorine and that underHe the observed effects are the large electronegativity of fluorine, its small size, the low degree of polarizabiHty of the carbon—fluorine bond and the weak intermolecular forces. These effects are illustrated by the comparisons of properties of fluorocarbons to chlorocarbons and hydrocarbons in Tables 1 and 2. 

We have to refine our atomic and molecular model of matter to see how bulk properties can be interpreted in terms of the properties of individual molecules, such as their size, shape, and polarity. We begin by exploring intermolecular forces, the forces between molecules, as distinct from the forces responsible for the formation of chemical bonds between atoms. Then we consider how intermolecular forces determine the physical properties of liquids and the structures and physical properties of solids. 

Electrical effects are the major factor in chemical reactivities and physical properties. Intermolecular forces are usually the major factor in bioactivities. Either electrical effects or intermolecular forces may be the predominant factor in chemical properties. Steric effects only occur when the substituent and the active site are in close proximity to each other and even then rarely account for more than twenty-five percent of the overall substituent effect. 

Inter- and intramolecular forces (imf) are of vital importance in the quantitative description of structural effects on bioactivities and chemical properties. They can make a significant contribution to chemical reactivities and some physical properties as well. Types of intermolecular forces and their present parameterization are listed in Table 750. 

Intermolecular forces are responsible for the condensed states of matter. The particles making up solids and liquids are held together by intermolecular forces, and these forces affect a number of the physical properties of matter in these two states. Intermolecular forces are quite a bit weaker than the covalent and ionic bonds discussed in Chapter 7. The latter requires several hundred to several thousand kilojoules per mole to break. The strength of intermolecular forces are a few to tens of kilojoules per... 

Chapter 7, Physical Properties, provides examples of intermolecular forces and how they affect the state of matter and the physical properties of familiar compounds. 

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