CBr4 has a zero dipole moment and a boiling point of 189.5°C; CH3Br has a dipole moment of 0.05 D and a boiling point of 3.56°C.Briefly explain why this polar molecule has a lower boiling point than the non-polar molecule.
The Correct Answer and Explanation is :
The difference in boiling points of CBr₄ and CH₃Br despite their differing dipole moments can be attributed to the nature and strength of the intermolecular forces acting on these molecules, as well as their molecular sizes.
CBr₄: Non-Polar Molecule
CBr₄ (Carbon Tetrafluoride) is a non-polar molecule, despite the fact that the individual C-Br bonds are polar. The molecular geometry of CBr₄ is tetrahedral, and the dipoles of the individual bonds cancel out due to the symmetry of the molecule, resulting in a net dipole moment of zero. The primary intermolecular force in CBr₄ is London dispersion forces, which arise from the temporary fluctuations in electron distribution that induce dipoles in neighboring molecules. These dispersion forces are generally weaker than the dipole-dipole interactions present in polar molecules. However, the size and mass of the CBr₄ molecule (due to the presence of large bromine atoms) contribute to relatively stronger dispersion forces compared to smaller, less polar molecules. This allows CBr₄ to have a relatively high boiling point of 189.5°C.
CH₃Br: Polar Molecule
CH₃Br (Methyl Bromide) is a polar molecule because of the significant difference in electronegativity between carbon and bromine, which results in a permanent dipole moment of 0.05 D. The molecule is asymmetrical, meaning the dipoles do not cancel out. As a result, dipole-dipole interactions between CH₃Br molecules exist, which are generally stronger than London dispersion forces. However, the boiling point of CH₃Br is much lower (3.56°C) compared to CBr₄, despite the presence of dipole-dipole forces. This can be explained by the fact that CH₃Br is much smaller than CBr₄ and its molecular mass is lower. The overall weaker London dispersion forces in CH₃Br (due to its smaller size) are unable to compensate for the effects of weaker dipole-dipole interactions, resulting in a lower boiling point.
Conclusion
While CH₃Br has a permanent dipole and experiences dipole-dipole interactions, the significantly larger size of CBr₄ leads to stronger London dispersion forces, ultimately giving CBr₄ the higher boiling point. Thus, the boiling point of a molecule is not determined by dipole moment alone but by a combination of all intermolecular forces and the size and mass of the molecules involved.