Electronic Structure and Bonding

Intermolecular Forces

All organic molecules have London (dispersion) forces. Some organic molecules have dipole-dipole interactions, and even fewer have hydrogen bonding. Ionic interactions are seen between cations and anions, and ion-dipole forces can be seen between an ion and a polar molecule. Intermolecular forces are responsible for the melting point, boiling point, and water solubility or insolubility of organic molecules.

Attractive forces between molecules, or intermolecular forces, account for the melting point, boiling point, and water solubility or insolubility of organic molecules. The term intermolecular force refers to the attractive or repulsive force between a molecule and a nearby molecule, atom, or ion. There are three types of attractive forces between neutral atoms or molecules, referred to collectively as van der Waals forces. Van der Waals forces are dipole-dipole interaction, London dispersion force, and hydrogen bonding, or the intermolecular force between permanent molecular dipoles.

1. Dipole-dipole interaction: the attractive or repulsive force between two polar molecules resulting from partial positive and partial negative charges of the molecules

2. Hydrogen bond: a weak intermolecular bond between a hydrogen atom of one molecule and a highly electronegative atom (usually fluorine, oxygen, or nitrogen) of a nearby molecule

3. London dispersion force (or induced dipole-induced dipole): an interaction between induced, instantaneous dipoles in all atoms and molecules

Dipole-dipole forces are present in molecules containing an unequal sharing of electrons. For example, HCl is polar, carrying a partial negative charge on chlorine and a partial positive charge on hydrogen. In a solution, these molecules arrange themselves with the partial positive side of one molecule attracting the partial negative side of another molecule and vice versa.
Hydrochloric acid (HCl) molecules in a solution exhibit dipole-dipole intermolecular attraction between the partial positive charge of the hydrogen atom and the partial negative charge of the chlorine atom.
Hydrogen bonds involve dipole-dipole interactions between molecules containing a hydrogen bound to a more electronegative atom, such as nitrogen (N), oxygen (O), or fluorine (F), and another nearby molecule containing an electron lone pair on an oxygen, nitrogen, or fluorine. The hydrogen attached to the electronegative element carries a partial positive charge, which is attracted to the lone pair of electrons on the other molecule. Hydrogen bonding differs from dipole-dipole interactions because when hydrogen is bound to O, N, or F, the hydrogen atom's nucleus (a single proton) is exposed. Hydrogen bonds are the strongest of the van der Waals intermolecular forces, but are about a tenth of the strength of an average covalent bond.
Hydrogen bonds involve dipole-dipole interactions and are the strongest of the van der Waals intermolecular forces.
Induced dipole interactions, also called London or dispersion forces, result in attractive forces between nonpolar molecules, such as chlorine gas, Cl2, and hydrocarbons such as ethane (CH3CH3{\rm{CH_3{-}CH_3}}) and propane (CH3CH2CH3{\rm{CH_3{-}CH_2{-}CH_3}}). Since the electrons are constantly moving, there are instances during which electrons may all go to the same side of the molecule at once, creating a temporary dipole and a momentary intermolecular attraction.