Video Discussing Dipole Intermolecular Forces. Their structures are as follows:. Asked for: order of increasing boiling points. Compare the molar masses and the polarities of the compounds. Compounds with higher molar masses and that are polar will have the highest boiling points.
The first compound, 2-methylpropane, contains only C—H bonds, which are not very polar because C and H have similar electronegativities. It should therefore have a very small but nonzero dipole moment and a very low boiling point.
As a result, the C—O bond dipoles partially reinforce one another and generate a significant dipole moment that should give a moderately high boiling point. The C—O bond dipole therefore corresponds to the molecular dipole, which should result in both a rather large dipole moment and a high boiling point. Thus far, we have considered only interactions between polar molecules.
Other factors must be considered to explain why many nonpolar molecules, such as bromine, benzene, and hexane, are liquids at room temperature; why others, such as iodine and naphthalene, are solids.
What kind of attractive forces can exist between nonpolar molecules or atoms? This question was answered by Fritz London — , a German physicist who later worked in the United States.
In , London proposed that temporary fluctuations in the electron distributions within atoms and nonpolar molecules could result in the formation of short-lived instantaneous dipole moments , which produce attractive forces called London dispersion forces between otherwise nonpolar substances.
Consider a pair of adjacent He atoms, for example. On average, the two electrons in each He atom are uniformly distributed around the nucleus. Because the electrons are in constant motion, however, their distribution in one atom is likely to be asymmetrical at any given instant, resulting in an instantaneous dipole moment. The net effect is that the first atom causes the temporary formation of a dipole, called an induced dipole , in the second. Interactions between these temporary dipoles cause atoms to be attracted to one another.
These attractive interactions are weak and fall off rapidly with increasing distance. Doubling the distance therefore decreases the attractive energy by 2 6 , or fold. Instantaneous dipole—induced dipole interactions between nonpolar molecules can produce intermolecular attractions just as they produce interatomic attractions in monatomic substances like Xe. The reason for this trend is that the strength of London dispersion forces is related to the ease with which the electron distribution in a given atom can be perturbed.
In small atoms such as He, the two 1 s electrons are held close to the nucleus in a very small volume, and electron—electron repulsions are strong enough to prevent significant asymmetry in their distribution. In larger atoms such as Xe, however, the outer electrons are much less strongly attracted to the nucleus because of filled intervening shells. As a result, it is relatively easy to temporarily deform the electron distribution to generate an instantaneous or induced dipole.
The ease of deformation of the electron distribution in an atom or molecule is called its polarizability. Because the electron distribution is more easily perturbed in large, heavy species than in small, light species, we say that heavier substances tend to be much more polarizable than lighter ones. For similar substances, London dispersion forces get stronger with increasing molecular size.
The polarizability of a substance also determines how it interacts with ions and species that possess permanent dipoles. The strengths of London dispersion forces also depend significantly on molecular shape because shape determines how much of one molecule can interact with its neighboring molecules at any given time. Neopentane is almost spherical, with a small surface area for intermolecular interactions, whereas n -pentane has an extended conformation that enables it to come into close contact with other n -pentane molecules.
As a result, the boiling point of neopentane 9. All molecules, whether polar or nonpolar, are attracted to one another by London dispersion forces in addition to any other attractive forces that may be present. In general, however, dipole—dipole interactions in small polar molecules are significantly stronger than London dispersion forces, so the former predominate. Arrange n -butane, propane, 2-methylpropane [isobutene, CH 3 2 CHCH 3 ], and n -pentane in order of increasing boiling points.
Determine the intermolecular forces in the compounds, and then arrange the compounds according to the strength of those forces. The substance with the weakest forces will have the lowest boiling point.
The four compounds are alkanes and nonpolar, so London dispersion forces are the only important intermolecular forces.
These forces are generally stronger with increasing molecular mass, so propane should have the lowest boiling point and n -pentane should have the highest, with the two butane isomers falling in between. Of the two butane isomers, 2-methylpropane is more compact, and n -butane has the more extended shape. Consequently, we expect intermolecular interactions for n -butane to be stronger due to its larger surface area, resulting in a higher boiling point.
Molecules with hydrogen atoms bonded to electronegative atoms such as O, N, and F and to a much lesser extent, Cl and S tend to exhibit unusually strong intermolecular interactions. Larger atoms and molecules have more electrons.
This leads to larger dipoles being established. London dispersion forces increase the larger the atomic size. Molecules with a permanent dipole are polar. Polar molecules display attractions between the oppositely charged ends of the molecules.
This type of intermolecular bond is stronger than London dispersion forces with the same number of electrons. Hydrogen bonding is the strongest type of intermolecular bond.
It is a specific type of permanent dipole to permanent dipole attraction that occurs when a hydrogen atom is covalently bonded to a highly electronegative element such as nitrogen, oxygen or fluorine.
They are often liquids or gases at room temperature. Energy is transferred to a substance to melt or boil it. This energy is needed to overcome the intermolecular forces of attraction between the molecules. The more energy needed, the higher the melting point or boiling point. For example, the longer the alkane molecule, the higher the boiling point.
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