Question

Discuss the IMF of caffeine used in a TLC experiment with 1:1 Hexanes / Ethyl acetate as the solvent system and explain why the polarity of caffeine is high or low in this solvent?

Answer 1

IMF in Caffeine:-

Intermolecular forces also are known as van der Waals forces consist of Hydrogen bonding, London dispersion forces and Dipole-dipole forces. The intermolecular forces that are present in caffeine are London dispersion forces and Dipole-dipole forces.

Dipole-Dipole forces exist between the positive end of one polar molecule and the negative end of another polar molecule. Caffeine molecules have bond dipoles that measure the polarity of a chemical bond within a molecule. They occur whenever there is a separation of positive and negative charges. These bond dipoles within the molecule create an overall molecular dipole; therefore the molecule has one negative end and one positive end. These charged ends are attracted to other opposite charged ends creating a very strong intermolecular force. London Dispersion forces exist because the electrons are always in motion, temporarily becoming more positive or negative. The London force is the immediate attraction of electrons from one atom to the positive nuclei of other surrounding atoms. These strong intermolecular forces make it difficult to break the molecule apart, therefore more energy is needed causing a high melting and boiling point. The Melting point is 235°C and the boiling point is 178°C.

The polarity of Caffeine:-

Caffeine is a polar molecule due to the electronegativity difference between the carbon-oxygen single polar covalent bonds and carbon-nitrogen single polar covalent bonds. The atom with a higher electronegativity will have a slightly more negative charge due to the attraction of more electrons, causing an unequal sharing of electrons. This results in carbon having a partial positive charge and oxygen or nitrogen having a partial negative charge. The geometry of the caffeine molecular allows for it to be polar because the negative and positive ends don’t cancel each other out.

Polarity in the given solvent will be high because the solvent is also polar and caffeine is also polar so the spot will move faster and rf value will increase.