Intermolecular Forces: London, Dipole, Induced-Dipole, and Hydrogen Bonding, Study notes of Biochemistry

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TYPES OF

INTERMOLECULAR FORCES

by

Dr. Deepak Choudhary

Assistant Professor

Department of Pharmaceutical Sciences,

Mohanlal Sukhadia University, Udaipur

Intermolecular forces may be attractive or repulsive.

• Johannes D van der Waals, Dutch, was the first to postulate intermolecular forces in developing a theory to account for properties of real gases. Van der waals forces include
• London forces
• Dipole - dipole forces
• Dipole - induced dipole forces Other intermolecular forces are
• Ion - dipole interactions
• Ion - induced dipole interactions
• Hydrogen Bonding

The London dispersion force is the weakest intermolecular force. The London dispersion force is a temporary attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles. This force is sometimes called an induced dipole induced dipole attraction. London forces are the attractive forces that cause non polar substances to condense to liquids and to freeze into solids when the temperature is lowered sufficiently. DIPOLE - DIPOLE FORCES Dipole-dipole forces are attractive forces between the positive end of one polar molecule and the negative end of another polar molecule. Dipole-dipole forces have strengths that range from 5 kJ to 20 kJ per mole. They are much weaker than ionic or covalent bonds and have a significant effect only when the molecules involved are close together (touching or almost touching).

INDUCED - DIPOLE FORCES Ion – induced dipole forces An ion-induced dipole attraction is a weak attraction that results when the approach of an ion induces a dipole in an atom or in a non polar molecule by disturbing the arrangement of electrons in the non polar species. Dipole – Induced Dipole Forces A dipole-induced dipole attraction is a weak attraction that results when a polar molecule induces a dipole in an atom or in a non polar molecule by disturbing the arrangement of electrons in the non polar species.