Question

The boiling point of methylcyclohexane \(\left(\mathrm{C}_{7} \mathrm{H}_{14}, \mathrm{MW} 98.2\right)\) is \(101^{\circ} \mathrm{C}\). The boiling point of perfluoromethylcyclohexane \(\left(\mathrm{C}_{7} \mathrm{~F}_{14}, \mathrm{MW} 350\right)\) is \(76^{\circ} \mathrm{C}\). Account for the fact that although the molecular weight of perfluoromethylcyclohexane is over three times that of methylcyclohexane, its boiling point is lower than that of methylcyclohexane.

Short answer

Question: Explain the reason behind the boiling point difference between methylcyclohexane and perfluoromethylcyclohexane despite the difference in their molecular weights. Answer: Despite having a higher molecular weight, perfluoromethylcyclohexane has a lower boiling point than methylcyclohexane due to the competing intermolecular forces: strong dipole-dipole interactions from the carbon-fluorine bonds and strong repulsions between fluorine atoms. These competing forces weaken the overall intermolecular attractions, leading to a lower boiling point compared to methylcyclohexane, which experiences only van der Waals forces.

Step-by-step solution

Methylcyclohexane has the molecular formula \(\mathrm{C}_{7} \mathrm{H}_{14}\), with a molecular weight of 98.2, and it boils at \(101^{\circ}\mathrm{C}\). Perfluoromethylcyclohexane has the molecular formula \(\mathrm{C}_{7} \mathrm{F}_{14}\), with a molecular weight of 350, and it boils at \(76^{\circ} \mathrm{C}\). Now, we need to compare the intermolecular forces present in these two compounds that govern their boiling points. #Step 2: Compare the intermolecular forces in the compounds#

Methylcyclohexane, a hydrocarbon, has mostly van der Waals interactions (also known as London dispersion forces) between its molecules. Van der Waals forces are weak, temporary dipoles that arise from the random distribution of electrons around the molecules. The strength of these forces increases with the molecular weight and surface area of the molecule. Perfluoromethylcyclohexane, on the other hand, is a perfluorocarbon, which means all the hydrogen atoms are replaced by fluorine atoms. Fluorine is highly electronegative, so the carbon-fluorine bonds create more significant dipole-dipole interactions. However, these forces are counteracted by the strong repulsions between the negatively charged fluorine atoms on adjacent molecules. This repulsion between fluorine atoms weakens the intermolecular forces and can contribute to a lower boiling point. #Step 3: Relate molecular weight to intermolecular forces and boiling points#

Generally, a higher molecular weight corresponds to stronger van der Waals interactions, which would typically lead to higher boiling points. In this case, although perfluoromethylcyclohexane has a much higher molecular weight than methylcyclohexane, its boiling point is lower. This can be attributed to the competing forces at play in the perfluoromethylcyclohexane: stronger dipole-dipole interactions due to the carbon-fluorine bonds, and strong repulsions between the fluorine atoms. #Step 4: Conclude the explanation#

Although perfluoromethylcyclohexane has a higher molecular weight than methylcyclohexane, its boiling point is lower due to the competing intermolecular forces: strong dipole-dipole interactions from the carbon-fluorine bonds and strong repulsions between fluorine atoms. These competing forces weaken the overall intermolecular attractions, leading to a lower boiling point compared to methylcyclohexane, which experiences only van der Waals forces.

Key concepts

Intermolecular Forces

When it comes to understanding the boiling point of substances, intermolecular forces play a crucial role. These forces are the attractions or repulsions which act between neighboring particles (atoms, molecules, or ions). They are different from intramolecular forces, which exist within particles and hold individual molecules together. Intermolecular forces determine the physical properties of substances, including their boiling points, melting points, and solubilities.

There are several types of intermolecular forces, but the main ones include dispersion forces (also known as London dispersion forces), dipole-dipole interactions, and hydrogen bonding. Dispersion forces are the weakest and arise due to temporary fluctuations in electron density within molecules. Dipole-dipole interactions occur when molecules with permanent dipoles attract each other. The stronger these intermolecular forces are, the more energy is required to overcome them, which leads to higher boiling points.

In the case of the exercise given, methylcyclohexane has a lower molecular weight and experiences predominantly van der Waals interactions, explaining its moderate boiling point, while the competing forces in perfluoromethylcyclohexane result in a surprisingly lower boiling point despite its higher molecular weight.

Molecular Weight

Molecular weight (or molecular mass) is fundamentally the total mass of all atoms present in a molecule. It is often measured in atomic mass units (amu) or Daltons. The concept of molecular weight is vital when comparing the properties of molecules, as it has a direct correlation with the strength of van der Waals interactions. As a molecule's size and mass increase, so does the number of electrons. This leads to more significant dispersion forces, which can have a direct impact on the boiling point.

Typically, substances with higher molecular weights have higher boiling points because larger molecules have greater electron clouds, which makes them more polarizable. This leads to stronger London dispersion forces, requiring more thermal energy to overcome. However, as in the textbook's exercise, other factors such as the nature of the atoms involved and their respective electronegativities may alter this expected trend.

Van der Waals Interactions

Van der Waals interactions, also known as London dispersion forces, are a type of intermolecular force present in all molecules, whether they are polar or nonpolar. They originate from the temporary polarization of the electron cloud around molecules, leading to the formation of instantaneous dipoles. When this happens, the temporary negative charge on one molecule attracts the temporary positive charge on another, creating a weak attraction between the two.

The strength of van der Waals interactions is influenced by the size and shape of molecules. Larger molecules with more electron clouds have stronger van der Waals interactions, and this trend is exemplified by comparing molecular weights and boiling points in a homologous series. In the textbook exercise, the relatively simple structure of methylcyclohexane means its molecules are primarily influenced by van der Waals forces, which correlates with its molecular weight and contributes to its boiling point.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between molecules that possess permanent dipoles, meaning one end of the molecule has a partial negative charge while the other end has a partial positive charge. These permanent dipoles are attracted to the opposite charges on neighboring molecules. Dipole-dipole interactions are generally stronger than van der Waals interactions but weaker than hydrogen bonds.

One common instance of dipole-dipole interactions is found in molecules with highly electronegative atoms, such as fluorine, which can create strong dipolar molecules. In the exercise scenario, the carbon-fluorine bonds in perfluoromethylcyclohexane result in strong dipole-dipole interactions. However, the surrounding electronegative fluorine atoms also exert strong repulsions on each other, counteracting the typical effects seen with increased molecular weight. This balance of forces results in a lower boiling point for perfluoromethylcyclohexane compared to what might be expected based solely on its molecular weight.