Higher melting and boiling points signify stronger noncovalent intermolecular forces.Ĭonsider the boiling points of increasingly larger hydrocarbons. The overarching principle involved is simple: the stronger the noncovalent interactions between molecules, the more energy that is required, in the form of heat, to break them apart. The observable melting and boiling points of different organic molecules provides an additional illustration of the effects of noncovalent interactions. In the last example, we see the three IMFs compared directly to illustrate the relative strength IMFs to boiling points. The H-bonding of ethanol results in a liquid for cocktails at room temperature, while the weaker dipole-dipole of the dimethylether results in a gas a room temperature. The structural isomers with the chemical formula C 2H 6O have different dominant IMFs. Branching creates more spherical shapes noting that the sphere allows the maximum volume with the least surface area. The visual image of MO theory can be helpful in seeing each compound as a cloud of electrons in an all encompassing MO system. However, as the carbon chain is shortened to create the carbon branches found in isopentane and neopentane the overall surface area of the molecules decreases. When comparing the structural isomers of pentane (pentane, isopentane, and neopentane), they all have the same molecular formula C 5H 12. In the table below, we see examples of these relationships. ![]() ![]() Since all compounds exhibit some level of London dispersion forces and compounds capable of H-bonding also exhibit dipole-dipole, we will use the phrase "dominant IMF" to communicate the IMF most responsible for the physical properties of the compound. When comparing compounds with the same IMFs, we use size and shape as tie breakers since the London dispersion forces increase as the surface area increases. H-bonding > dipole-dipole > London dispersion (van der Waals) Therefore, we can compare the relative strengths of the IMFs of the compounds to predict their relative boiling points. Large molecules have more electrons and nuclei that create van der Waals attractive forces, so their compounds usually have higher boiling points than. The stronger the IMFs, the lower the vapor pressure of the substance and the higher the boiling point. Arrange these compounds by their expected boiling point (highest to lowest). Intermolecular forces (IMFs) can be used to predict relative boiling points. ![]() It is very important to apply this rule only to like compounds.
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