London dispersion forces The relatively

B. The best solvents for a solute have intermolecular bonds of similar strength to the solute ( like dissolves like ). Bromine is a non-polar molecule with intermolecular attractions due to weak London dispersion forces. The relatively strong hydrogen bonding in H20 and NH3 and the very strong electrostatic attractions in molten NaCI would make each of them a poor solvent for Br2 because these molecules would prefer to remain attracted to one another. CS2 is a fairly small non-polar molecule. 

Solutions of nonpolar solutes in nonpolar solvents represent the simplest type. The forces involved in solute-solvent and solvent-solvent interactions are all London dispersion forces and relatively weak. The presence of these forces resulting in a condensed phase is the only difference from the mixing of ideal gases. As in the latter case, the only driving force is the entropy (randomness) of mixing. In an ideal solution (AW, = 0) at constant temperature the free energy change will be composed solely of the entropy term ... 

London dispersion forces are relatively weak forces that arise among noble gas atoms and in nonpolar molecules. London forces arise horn instantaneous dipoles that develop when one atom (or molecule) momentarily distorts the electron cloud of another atom (or molecule). London forces are typicaUy weaker than either permanent dipole-dipole forces or covalent bonds. 

There are probably several factors which contribute to determining the endo exo ratio in any specific case. These include steric effects, dipole-dipole interactions, and London dispersion forces. MO interpretations emphasize secondary orbital interactions between the It orbitals on the dienophile substituent(s) and the developing 7t bond between C-2 and C-3 of the diene. There are quite a few exceptions to the Alder rule, and in most cases the preference for the endo isomer is relatively modest. For example, whereas cyclopentadiene reacts with methyl acrylate in decalin solution to give mainly the endo adduct (75%), the ratio is solvent-sensitive and ranges up to 90% endo in methanol. When a methyl substituent is added to the dienophile (methyl methacrylate), the exo product predominates. ... 

Van der Waals forces are intermolecular and are classified as (i) dipole-dipole interactions, (ii) dipole-induced-dipole interactions and (iii) London dispersion forces which operate between atoms as the result of the nucleus not always being at the centre of mass of the surrounding electrons. The hydrogen bond is regarded as a special form of dipole-dipole interaction, because the positive end of dipolar species containing hydrogen atoms is the relatively unshielded proton. 

Relatively weak forces of attraction that exist between nonpolar molecules are called van der Waals forces or London dispersion forces. Dispersion forces between molecules are much weaker than the covalent bonds within molecules. Electrons move continuously within bonds and molecules, so at any time one side of the molecule can have more electron density than the other side, which gives rise to a temporary dipole. Because the dipoles in the molecules are induced, the interactions between the molecules are also called induced dipole-induced dipole interactions. 

The carbon sheets in graphite are separated by a distance of 335 pm and are held together by only London dispersion forces. Atmospheric gases can be absorbed between the sheets, thus enabling the sheets to easily slide over one another. As a result, graphite has a slippery feel and can be used as a lubricant. Because the sheets are so far apart, it s relatively difficult for an electron to hop from one sheet to the next and the electrical conductivity in the direction perpendicular to the sheets is therefore about 104 times smaller than the conductivity parallel to the sheets. 

A nonpolar liquid like heptane (C7H16) has intermolecular bonds with relatively weak London dispersion forces. Heptane is immiscible in water because the attraction that water molecules have for each other via hydrogen bonding is too strong. Unlike Na+ and CP ions, heptane molecules cannot break these bonds. Because bonds of similar strength must be broken and formed for solvation to occur, nonpolar substances tend to be soluble in nonpolar solvents, and ionic and polar substances are soluble in polar solvents like water. Polar molecules are often called hydrophilic and non-polar molecules are called hydrophobic. This observation is often stated as like dissolves like. Network solids (e.g., diamond) are soluble in neither polar nor nonpolar solvents because the covalent bonds within the solid are too strong for these solvents to break. 

Both dipole-dipole and London Dispersion forces are subclasses of van der Waal interactions. When two polar molecules approach one another, a natural attraction known as dipole-dipole forces is created between oppositely charged ends. The relative intensity of dipole-dipole forces may be represented by Eq. 2 ... 

In contrast to dipole-dipole forces, London Dispersion interactions are much weaker in nature since they involve nonpolar molecules that do not possess permanent dipole moments. The only modes for molecular attraction are through polarization of electrons, which leads to the creation of small dipole-dipole interactions and mutual attractive forces. Since electron polarization occurs much more readily for electrons farther from the nucleus, this effect is more pronounced for molecules that are larger with a greater number of electrons, especially positioned on atoms with a high atomic number, consisting of more diffuse orbitals. These induced dipole forces are responsible for the liquefaction of gases such as He and Ar at low temperatures and pressures. The relative strength of London Dispersion forces is described by Eq. 3 ... 

While nonpolar molecules can experience only London dispersion forces, polar molecules experience both dipole-dipole forces and London dispersion forces. Determining how much each force adds to the overall force of attraction between polar molecules is not easy. London dispersion forces also exist between ions in ionic compounds, but they are quite small relative to ionic forces and can almost always be overlooked. 

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