MCAT Content / Liquid Phase Intermolecular Forces / Dipole Dipole Interactions

Dipole–Dipole Interactions

Topic: Liquid Phase Intermolecular Forces

Dipole–dipole interactions are a type of intermolecular force that exists when molecules with permanent dipoles align forming an electrostatic interaction.

Molecules that contain dipoles are called polar molecules. For example, a molecule of hydrogen chloride, HCl has a large permanent dipole. This is because there is a difference in electronegativity between H and Cl: Chlorine is more electronegative than hydrogen and pulls the electrons in the covalent bond towards itself. This created an uneven distribution of electrons, giving chlorine a slightly negative charge (δ-) and hydrogen a slightly positive charge (δ+). When two of these molecules align (see below), the opposite partial charges attract to one another in a weak intermolecular attraction, known as an electrostatic interaction. The interaction between the two dipoles is an attraction rather than full bond because no electrons are shared between the two molecules.

Water is another example of a polar molecule (oxygen is more electronegative than hydrogen). The dipole-dipole attractions between water molecules give water many of its properties, including its high surface tension. It also allows water to act as a polar solvent. It will dissolve other polar molecules easily due to the dipole-dipole interactions.

Dipole-Dipole interactions are not as strong as hydrogen bonds due to the lower electronegativity of the atoms involved, but stronger than London dispersion forces.

Don’t be fooled by molecules however that are not polar. Molecules often contain polar bonds because of electronegativity differences but have no overall dipole moment if they are symmetrical, this means the overall charges are evenly spread out. For example, in the molecule tetrachloromethane (CCl4), the chlorine atoms are more electronegative than the carbon atoms, and the electrons are drawn toward the chlorine atoms, creating dipoles. However, these carbon-chlorine dipoles cancel each other out because the molecular is symmetrical, and CCl4 has no overall dipole, so this molecule would not form intermolecular dipole-dipole interactions.

 

Practice Questions

 

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Key Points

• Dipole-Dipole interactions are electrostatic interactions that form between opposite partial charges of molecules with permanent dipoles (polar molecules).

• Dipole-Dipole interactions are weaker than hydrogen bonds but stronger than London dispersion forces (all these are much weaker than covalent bonds).

• These interactions do not form between molecules that have no overall polarity.

• Water is a good example of a polar solvent as it has a permanent dipole so is easily able to solvate polar and charged molecules.


Key Terms

Dipole: Occurs within a molecule when two atoms in a covalent bond have different affinities for electrons (electronegativities), so the ‘push and pull’ of their shared electrons results in one atom maintaining most of the electron density and a partial negative charge, leaving the other atom with a partial positive charge.

Polar:  A polar molecule is one that has uneven charge distribution. Factors that contribute to this include intramolecular dipoles and molecular geometry.

Electrostatic interaction: A term used to describe positive and negative attractions between opposite charges.



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