Polar molecules, condensation

We have considered the weak van der Waals forces that cause the condensation of covalent molecules. The formation of an ionic lattice results from the stronger interactions among molecules with highly ionic bonds. But most molecules fall between these two extremes. Most molecules are held together by bonds that are largely covalent, but with enough charge separation to affect the properties of the molecules. These are the molecules we have, called polar molecules. 

Polar molecules like II2O show apparent polymerization to an extent quite impossible in the gas phase at low pressures. The dipole field interaction, which is of the order of 1 ev., results in an artificial multilayer physical adsorption at pressures and temperatures where ordinarily only a minute fraction of the first layer would exist. Since multilayer adsorption is quite liquid-like, the high degree of polymerization can be explained. It is interesting to note that at low fields individual peaks show some substructure, which could be due to alignment differences at the time of ionization or could correspond to ionization from different layers within the adsorbate. It is hoped to study physical adsorption near the condensation point at low pressure with nonpolar rare gas atoms to see if layer structure can be elucidated in this way. 

All molecules, including polar molecules, interact by the London mechanism. However, if the molecules are polar, they may also interact by the dipole-dipole mechanism. The latter is particularly important when the molecules are in a condensed phase and do not rotate freely (the interaction then has a longer range because it varies as 1/r3 rather... 

Dipo/e Moment The evidence comes from an examination of the dielectric constant of the hydrogen halides. In an electric field, say between the plates of a condenser, molecules that have a charge separation within them will tend to orient themselves with the electric field. Such molecules behave like electric dipoles (Fig. 4.3) and are called polar molecules. The extent of orientation is reflected by a change in the dielectric... 

Answer In solution, the negative charges on chondroitin sulfate repel each other and force the molecule into an extended conformation. The polar molecule also attracts many water molecules (water of hydration), further increasing the molecular volume. In the dehydrated solid, each negative charge is counterbalanced by a counterion, such as Na+, and the molecule collapses into its condensed form. 

We may conclude that many-body forces are not important for the structure of solid hydrogen chloride (for further details see Sections 4.3 and 5). The energy of interaction in the dimer and in the solid fit very well into our relations. This is more a test of our assumptions of binary potentials in equations 8 and 18 than a limit on the role of many-body forces because the only available value was derived from cluster calculations based on the assumption of pairwise additivity. From the concepts and data discussed in this section it is obvious that an accurate description of clusters and condensed phases formed from polar molecules like HF and H20 which are both characteristic hydrogen bond donors and acceptors, requires a proper consideration of many-body forces. 

Hydrogen cyanide condenses at 25.6°C to a liquid with a very high dielectric constant (107 at 25°C). Here, as in similar cases, such as water, the high dielectric constant is due to association of intrinsically very polar molecules by hydrogen bonding. Liquid HCN is unstable and can polymerize violently in the absence of stabilizers in aqueous solutions polymerization is induced by ultraviolet light. 

Conditions are very different in condensed phases, however. As has been shown above all but the ion-ion and ion-induced dipole interactions alter their values upon randomization. The contribution that is most dramatically altered — increased — by fixing the molecules is the dipole-dipole part. We quote from Davies . . predominance of the dispersion energy is a characteristic of non-polar molecules or of the gas phase only. In liquids or solids where the molecules are at much closer distances, the random orientation which reduces the dipole-dipole term to dependence upon r in the gaseous state, is far less likely to be maintained. Dipolar molecules can then assume fixed orientations with respect to one another with greatly increased energies of interaction . Thus dipole-dipole, ion-dipole and to a lesser extent, dipole-induced dipole interactions become more important in condensed phases. (For a comparison between gaseous and solid HCl, see ° p. 166)... 

In this initial step, an aldol condensation takes place. The solvent of this reaction is an ionic liquid. It is a very polar solvent that is able to stabilize polar molecules and can act as an acceptor for hydrogen bonds. Thus, it is able to shift the equilibrium towards the enol tautomers. After the aldol addition, the intermediate can eliminate water via a second enol intermediate. Although the substrates are both aldehydes, only one product is formed because 19 lacks an acidic position, so it cannot become a nucleophile. Owing to its tendency to form homodimers, a two-fold excess of propionaldehyde 20 is needed. 

Non-polar molecules have no permanent dipole and cannot form normal bonds. The non-polar noble gases, however, condense to liquid and ultimately form solids if cooled sufficiently. This suggests that some form of intermolecular force holds the molecules together in the liquid and solid state. The amount of energy (Box 4.8) required to melt solid xenon is 14.9kJ mol-1, demonstrating that cohesive forces operate between the molecules. 

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