Why Do CH3OH, CH3Cl, and CH3Br Have Different Boiling Points?

[Paulo2014]

The boiling points of three compounds are: CH3OH, 65C; CH3Cl, –24C; CH3Br, 4C

Discuss the relative strengths of the intermolecular forces in liquid samples of these three compounds.

what should I include in my discussion?

 

[Borek]

Explanation how boiling points depend on the intermolecular forces?

 

[Blueshift5]

In order to discuss the relative strengths of the intermolecular forces in these three compounds, it is important to first understand what intermolecular forces are. Intermolecular forces are the attractive forces between molecules that hold them together in a liquid or solid state. These forces are weaker than the chemical bonds within a molecule, but they still play a significant role in determining the physical properties of a substance, such as boiling point.

In the case of CH3OH, CH3Cl, and CH3Br, all three compounds contain a polar covalent bond between the carbon and hydrogen atoms. This results in a partial positive charge on the hydrogen atom and a partial negative charge on the carbon atom. This polarity leads to the formation of dipole-dipole interactions, which are the strongest type of intermolecular force present in these compounds.

In addition to dipole-dipole interactions, CH3OH also contains hydrogen bonding, which is a special type of dipole-dipole interaction that occurs when hydrogen is bonded to a highly electronegative atom, such as oxygen or nitrogen. Hydrogen bonding is significantly stronger than regular dipole-dipole interactions, and therefore, CH3OH will have the strongest intermolecular forces among the three compounds.

On the other hand, CH3Cl and CH3Br do not have hydrogen bonding, but they do have dipole-dipole interactions. However, CH3Br has a larger molar mass and a larger atomic radius than CH3Cl, which results in stronger London dispersion forces between its molecules. London dispersion forces are the weakest type of intermolecular force, but they become more significant as the size of the molecule increases. Therefore, CH3Br will have stronger intermolecular forces than CH3Cl.

In conclusion, the relative strengths of the intermolecular forces in these three compounds can be ranked as follows: CH3OH > CH3Br > CH3Cl. This is due to the presence of hydrogen bonding in CH3OH and the larger molar mass of CH3Br, which leads to stronger intermolecular forces. These differences in intermolecular forces also explain the difference in boiling points, with CH3OH having the highest boiling point due to its stronger intermolecular forces.