Difference Between Dipole-Dipole and London Dispersion Forces

Key Difference – Dipole-Dipole vs London Dispersion Forces 

Dipole-dipole and London dispersion forces are two attraction forces found between molecules or atoms; they directly affect the boiling point of the atom /molecule. The key difference between Dipole-Dipole and London Dispersion forces is their strength and where they can be found. The strength of London dispersion forces is relatively weaker than dipole-dipole interactions; however both of these attractions are weaker than ionic or covalent bonds. London dispersion forces can be found in any molecule or sometimes in atoms, but dipole-dipole interactions are only found in polar molecules.

What is Dipole-Dipole Force?

Dipole-dipole interactions occur when two oppositely polarized molecules interact through space. These forces exist in all molecules that are polar. Polar molecules are formed when two atoms have an electronegativity difference when they form a covalent bond. In this case, atoms can’t share electrons evenly between two atoms due to the electronegativity difference. The more electronegative atom attracts the electron cloud more than the less electronegative atom; so that the resulting molecule possesses slightly positive end and slightly negative end. The positive and negative dipoles in other molecules can attract each other, and this attraction is called dipole-dipole forces.

What is London Dispersion Force?

London dispersion forces are considered as the weakest intermolecular force between adjacent molecules or atoms. London dispersion forces result in when there are fluctuations in electron distribution in the molecule or atom. For example; these types of attraction forces arise in neighboring atoms due to an instantaneous dipole on any atom. It induces dipole on neighboring atoms and then attracts each other through weak attraction forces. The magnitude of the London dispersion force depends on how easily electrons on the atom or in the molecule can be polarized in response to an instantaneous force. They are temporary forces that may be available in any molecule since they have electrons.

What is the difference between Dipole-Dipole and London Dispersion Forces?

Definition:

Dipole-Dipole Force: Dipole-dipole force is the attraction force between the positive dipole of a polar molecule and the negative dipole of another oppositely polarized molecule.

London Dispersion Force: London dispersion force is the temporary attractive force between adjacent molecules or atoms when there is fluctuation in the electron distribution.

Nature:

Dipole-Dipole Force: Dipole-dipole interactions are found in polar molecules such as HCl, BrCl, and HBr.  This arises when two molecules share electrons unevenly to form a covalent bond. The electron density shifts towards the more electronegative atom, resulting slightly negative dipole at one end and slightly positive dipole in the other end.

London Dispersion Force: London dispersion forces can be found in any atom or molecule; the requirement is an electron cloud. London dispersion forces are found in non-polar molecules and atoms too.

Strength:

Dipole-Dipole Force: Dipole-dipole forces are stronger than the dispersion forces but weaker than ionic and covalent bonds. The average strength of dispersion forces varies between1-10 kcal/mol.

London Dispersion Force: They are weak because London dispersion forces are temporary forces (0-1 kcal/mol).

Affecting Factors:

Dipole-Dipole Force: The affecting factors for the strength of dipole-dipole forces are electronegativity difference between atoms in the molecule, molecular size and the shape of the molecule. In other words, when the bond length increases the dipole interaction decreases.

London Dispersion Force: The magnitude of London dispersion forces depends on several factors. It increases with the number of electrons in the atom. Polarizability is one of the important factors that affect the strength in London dispersion forces; it is the ability to distort the electron cloud by another atom/molecule. Molecules having lesser electronegativity and larger radii have higher polarizability. In contrast; it is difficult to distort the electron cloud in smaller atoms since electrons are very close to the nucleus.

Example:

Rn- The larger the atom, easy to polarize (Higher polarizability) and possesses the strongest attractive forces. Helium is very small and difficult to distort and result in weaker London dispersion forces.

Image Courtesy:

1. Dipole-dipole-interaction-in-HCl-2D By Benjah-bmm27 (Own work) [Public domain], via Wikimedia Commons

2. Forze di London By Riccardo Rovinetti (Own work) [CC BY-SA 3.0], via Wikimedia Commons