Intermolecular Interactions (Molecular Associations)
Interactions that are inTERmolecular are among more than one molecule of a substance. These are the forces that attract the molecules of a liquid or solid to each other. Intermolecular interactions occur among nonpolar molecules as well as among polar ones, but polar interactions are much stronger than nonpolar ones are.
Polar Molecules:
Polar molecules contain one or more dipoles. They fall into two categories: polar protic, and polar aprotic. Polar protic molecules have protons (hydrogens) attached to electronegative atoms, namely F, O, or N. Polar aprotic molecules still contain dipoles, but there are no protons attached to F, O, or N. It is essential that you understand the distinction between protic and aprotic polar molecules in order to properly understand the organic chemistry substitution mechanisms.
Polar Aprotic Molecules:
Polar aprotic molecules have intermolecular interactions known as dipole-dipole interactions. The positive end of the dipole on one molecule is attracted to the negative end of the dipole on another molecule. This is an electrostatic interaction, and it is governed by Coulomb’s Law. Coulomb’s Law tells us that either a larger charge separation (leading to a larger molecular dipole) or a smaller distance between the molecules will strengthen the dipole-dipole interactions between them. There are many examples of polar aprotic molecules, including ketones, aldehydes, esters, tertiary amines and amides, HCl, HBr, HI, cyanides, nitro groups, and DMSO (dimethyl sulfoxide). Polar aprotic solvents destabilize nucleophiles, making them suitable solvents for Sn2 reactions in organic chemistry.
Polar Protic Molecules:
Polar protic molecules have intermolecular interactions known as hydrogen bonds. In spite of their name, note that hydrogen bonds are not true bonds. Rather, hydrogen bonds are an especially strong type of dipole-dipole interaction. The positive end of a dipole on one molecule (the H) is sometimes so strongly attracted to the negative end of another molecule’s dipole (the F, O, or N) that the H will dissociate from the first molecule and bind to the second one. You are already familiar with this process for water, which autoionizes to form H3O+ and OH-:
2 (H2O) <-> (OH)- + (H3O)+
Water is the most common example of a polar protic molecule, but some other examples include alcohols, carboxylic acids, primary and secondary amines and amides, and hydrogen fluoride. Polar protic solvents stabilize carbocations, making them suitable solvents for Sn1 reactions in organic chemistry.
Nonpolar Molecules:
Molecules that do not contain dipoles are called nonpolar molecules. They are held together by intermolecular interactions that are called London dispersion forces or van der Waals interactions. These interactions are due to temporary, induced dipoles. Where do these dipoles come from? To answer this, you must realize that the electrons in a bond or in an atom are not stationary. Rather, these electrons are constantly in motion. Sometimes, by sheer chance, they are not equally distributed on the atom or in the bond, and this inequality of charge forms a temporary dipole. It is temporary because soon thereafter, as the electrons continue to move, they distribute more equally again, and so the dipole goes away. But in the meantime, that short-lived dipole has affected its neighbors, and it has caused similar dipoles to form in them as well. This is why these dipoles are induced. So you can imagine a bunch of molecules, held together by small dipoles that are constantly forming and unforming. But there are always some dipoles present at any given time, and that is what holds the liquid or solid together.
Note that all substances contain dispersion forces, but these forces are only greatly significant for nonpolar elements and molecules that lack dipole-dipole interactions. In polar compounds, dispersion forces can be neglected.
