Factors Affecting Boiling Point
* Intermolecular Forces: The stronger the intermolecular forces between molecules, the more energy is required to overcome these forces and cause a change of state from liquid to gas. The key forces we need to consider are:
* Hydrogen Bonding: The strongest intermolecular force. It occurs when a hydrogen atom is bonded to a highly electronegative atom (like oxygen, nitrogen, or fluorine).
* Dipole-Dipole Interactions: Occur between polar molecules.
* London Dispersion Forces: Weakest intermolecular force, present in all molecules, and become stronger with increasing molecular size.
Comparing the Molecules
1. Aniline (C6H5NH2):
* Hydrogen Bonding: Aniline can participate in hydrogen bonding because the nitrogen atom in the amino group (NH2) is highly electronegative.
* Dipole-Dipole Interactions: Aniline is a polar molecule due to the presence of the amino group.
2. Phenol (C6H5OH):
* Hydrogen Bonding: Phenol can also participate in hydrogen bonding due to the oxygen atom in the hydroxyl group (OH).
* Dipole-Dipole Interactions: Phenol is also a polar molecule.
3. Toluene (C6H5CH3):
* No Hydrogen Bonding: The methyl group (CH3) in toluene cannot participate in hydrogen bonding.
* Dipole-Dipole Interactions: Toluene is a non-polar molecule.
4. Benzene (C6H6):
* No Hydrogen Bonding: Benzene is a non-polar molecule.
Conclusion
* Aniline has the highest boiling point: Its ability to form hydrogen bonds and its polar nature contribute to strong intermolecular forces.
* Phenol has a higher boiling point than toluene and benzene: While phenol can participate in hydrogen bonding, it has weaker hydrogen bonding than aniline because oxygen is less electronegative than nitrogen.
* Toluene has a higher boiling point than benzene: Toluene has a larger molecular size and stronger London Dispersion forces than benzene.
Therefore, the order of increasing boiling points is:
Benzene < Toluene < Phenol < Aniline
