Question

Freon, \(\mathrm{CCl}_{2} \mathrm{~F}_{2},\) and dichloromethane, \(\mathrm{CH}_{2} \mathrm{Cl}_{2},\) are common organic substances. Freon is a gas with a normal boiling point of \(-29.8^{\circ} \mathrm{C}\); dichloromethane's normal boiling point is \(39.6^{\circ} \mathrm{C}\). Which statement is the best explanation of these data? (a) Dichloromethane can form hydrogen bonds, but freon cannot. (b) Dichloromethane has a larger dipole moment than freon. (c) Freon is more polarizable than dichloromethane.

Short answer

The exercise seems to be flawed since none of the given options (a, b, or c) can sufficiently explain the difference in boiling points between Freon and dichloromethane. More information might be needed to determine the correct explanation for the difference.

Step-by-step solution

Analyze the possibility of hydrogen bonding

Hydrogen bonding occurs when hydrogen is directly bonded to highly electronegative atoms like fluorine, nitrogen, or oxygen. In the given molecules, dichloromethane (CH2Cl2) has hydrogen atoms bonded with carbon. Similarly, Freon (CCl2F2) does not have any hydrogen atom directly bonded to a highly electronegative atom. So, hydrogen bonding is not the reason for the difference in boiling points. Thus, option (a) is not the correct answer. Step 2: Compare the dipole moments of both molecules

Compare the dipole moments of both molecules

The dipole moment is a measure of the polarity of a bond. Molecules with larger dipole moments have stronger intermolecular forces, due to increased attraction between their opposite charges, which leads to higher boiling points. However, both dichloromethane and Freon do not have significant differences in electronegativity, and their overall molecular shapes are symmetrical, thus, their dipole moments are quite small, and their difference in dipole moments would not be a significant factor. Therefore, option (b) should not be the correct answer. Step 3: Assess the polarizability of the molecules

Assess the polarizability of the molecules

Polarizability is the ease with which the electron cloud around a molecule can be distorted. Larger molecules with more electrons are generally more polarizable. Polarizable molecules have increased London dispersion forces, which strengthen the intermolecular forces, causing the boiling point to be higher. Comparing the two molecules, Freon has more electrons than dichloromethane, making it more polarizable. However, Freon has a lower boiling point, indicating that it has weaker intermolecular forces than dichloromethane. This contradicts the notion that greater polarizability leads to a higher boiling point. Hence, option (c) is not the correct answer. Given that options (a), (b), and (c) are the only choices provided and our analysis indicates that neither hydrogen bonding, dipole moment difference, nor polarizability explains the difference in boiling points, the exercise seems to be flawed. More information might be needed to determine the correct explanation for the difference.

Key concepts

Boiling Point

The boiling point of a substance is crucial in determining its physical state under certain conditions. It is the temperature at which the vapor pressure of a liquid equals the external pressure. Generally, stronger intermolecular forces lead to higher boiling points, as more energy is required to overcome these forces for the liquid to transition to a gas.
In the exercise, Freon ( \(\mathrm{CCl}_{2} \mathrm{~F}_{2}\) ) has a much lower boiling point than dichloromethane ( \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) ). This suggests that dichloromethane has stronger intermolecular forces than Freon, despite being more polarizable.
Understanding the boiling point differences between molecules can provide insight into the types and strengths of intermolecular forces present.

Intermolecular Forces

Intermolecular forces (IMFs) are the forces that mediate interaction between molecules. The primary types are London dispersion forces, dipole-dipole interactions, and hydrogen bonding.
* **London Dispersion Forces:** These are weak forces arising from temporary dipoles induced when the electrons in molecules shift around. Larger molecules induce stronger forces because they have more electrons that can shift.
* **Dipole-Dipole Interactions:** These occur in polar molecules where permanent dipoles attract each other.
* **Hydrogen Bonding:** A special case of dipole-dipole interaction, occurring when hydrogen is bonded to highly electronegative atoms like F, N, or O.
In our case, neither Freon nor dichloromethane forms hydrogen bonds, so London dispersion forces and dipole-dipole interactions primarily determine their boiling points.

Polarizability

Polarizability relates to how easily an electron cloud in a molecule can be distorted, influencing the magnitude of the London dispersion forces. Factors increasing polarizability include the size of the electron cloud and the presence of electrons in outer orbitals that are shielded from the nucleus.
In larger molecules like Freon, increased polarizability might suggest stronger dispersion forces. However, in this example, despite Freon being more polarizable due to more electrons, this did not result in a higher boiling point, indicating that factors other than polarizability influence boiling point discrepancies more prominently in these cases.

Dipole Moment

The dipole moment is a vector quantity that represents the separation of charges in a molecule. It is calculated by multiplying the charge magnitude with the distance between the charges.
A molecule with a high dipole moment indicates a strong tendency to align in an electric field, demonstrating higher polarity. Higher dipole moments contribute to stronger dipole-dipole interactions, resulting in higher boiling points.
Both Freon and dichloromethane have certain polar characteristics, but their structural symmetry and minimal electronegativity differences mean their dipole moments do not drastically differ. Hence, the dipole moment does not play a key role in the boiling point variation for these substances.

Hydrogen Bonding

Hydrogen bonding is an especially strong type of dipole-dipole interaction. This special bond forms when hydrogen is covalently bonded to fluorine, nitrogen, or oxygen. The bond results in significant dipole interaction with nearby molecules, leading to much higher boiling points.
Importantly, neither Freon ( \(\mathrm{CCl}_{2} \mathrm{~F}_{2}\) ) nor dichloromethane ( \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) ) engages in hydrogen bonding. Hydrogen is not bonded to any of these highly electronegative elements in either compound. Thus, hydrogen bonding is not a player in determining the differences in boiling point for these molecules.