Question

Rationalize the differences in physical properties in terms of intermolecular forces for the following organic compounds. Compare the first three substances with each other, compare the last three with each other, and then compare all six. Can you account for any anomalies? $$\text{Misplaced hline}$$

Short answer

Comparing the physical properties of the given organic compounds, we can attribute the differences in boiling points, melting points, and enthalpy of vaporization to the types of intermolecular forces present in each molecule. Nonpolar compounds (Benzene, Naphthalene, and Carbon Tetrachloride) mainly have dispersion forces, while polar compounds with functional groups (Acetone, Acetic Acid, and Benzoic Acid) exhibit stronger dipole-dipole interactions and hydrogen bonding. The general trend of increasing boiling and melting points can be observed from nonpolar to polar compounds and from dispersion forces to dipole-dipole interactions and hydrogen bonding. Anomalies, such as the similar boiling points of benzene and carbon tetrachloride, may arise due to differences in molecular shapes affecting molecular interactions and packing.

Step-by-step solution

Benzene (C6H6), Naphthalene (C10H8), Carbon Tetrachloride (CCl4)

These compounds mainly have dispersion forces, which are the weakest among the intermolecular forces. The strength of these forces depends on the molecular size and shape, with larger and more extended molecules exhibiting stronger dispersion forces. - Benzene: bp = 80°C, mp = 6°C, ΔHvap = 33.9 kJ/mol - Naphthalene: bp = 218°C, mp = 80°C, ΔHvap = 51.5 kJ/mol - Carbon Tetrachloride: bp = 76°C, mp = -23°C, ΔHvap = 31.8 kJ/mol Naphthalene has the highest boiling point and melting point among the three due to its larger size and extended structure, leading to stronger dispersion forces. Carbon tetrachloride and benzene have similar boiling points, but carbon tetrachloride has a lower melting point. This could be due to the difference in molecular shapes, which affect how the molecules pack together. #Step 2: Compare the last three compounds#

Acetone (CH3COCH3), Acetic Acid (CH3CO2H), Benzoic Acid (C6H5CO2H)

These compounds have polar functional groups, leading to stronger intermolecular forces such as dipole-dipole interactions and hydrogen bonding. - Acetone: bp = 56°C, mp = -95°C, ΔHvap = 31.8 kJ/mol (dipole-dipole) - Acetic Acid: bp = 118°C, mp = 17°C, ΔHvap = 39.7 kJ/mol (dipole-dipole and hydrogen bonding) - Benzoic Acid: bp = 249°C, mp = 122°C, ΔHvap = 68.2 kJ/mol (dipole-dipole and hydrogen bonding) Acetic acid and benzoic acid have higher boiling points and melting points compared to acetone due to the presence of hydrogen bonding. Additionally, benzoic acid has higher boiling and melting points than acetic acid because of its larger molecular size, which leads to stronger dispersion forces. #Step 3: Compare all six compounds# Notice the trend of increasing boiling and melting points from nonpolar molecules to polar ones with functional groups that can form hydrogen bonding. - Nonpolar compounds (dispersion forces only): Carbon tetrachloride < Benzene < Naphthalene - Polar compounds (dipole-dipole and hydrogen bonding): Acetone < Acetic Acid < Benzoic Acid. Anomalies may be found in the comparison, such as the similar boiling points of benzene and carbon tetrachloride. This could be due to differences in molecular shapes affecting the way molecules interact and pack together. Overall, the types and strengths of intermolecular forces present have a significant impact on the physical properties of these organic compounds.

Key concepts

Dispersion Forces

Dispersion forces are a type of weak intermolecular force that occur between atoms and nonpolar molecules. These forces, also known as London dispersion forces, are the weakest type of intermolecular interaction. They arise from temporary fluctuations in the electron distribution within atoms and molecules, creating instantaneous dipoles. This temporary dipole induces a dipole in nearby molecules, resulting in attraction.

Despite their weak nature, dispersion forces can have significant effects when larger molecules with more electrons are involved.

• Molecular size and shape influence the strength of dispersion forces.
• Larger and more elongated molecules generally exhibit stronger dispersion forces.

In the case of benzene, naphthalene, and carbon tetrachloride, their boiling and melting points vary due to differences in their molecular sizes and shapes, which affect the strength of their dispersion forces. For example, naphthalene, being the largest among the three, showcases stronger dispersion forces, resulting in higher boiling and melting points.

Dipole-Dipole Interactions

Dipole-dipole interactions are attractive forces between polar molecules, arising from electrical interactions between the dipoles of different molecules. These forces are stronger than dispersion forces because they involve permanent dipoles.

The strength of dipole-dipole interactions depends on:

• The magnitude of the dipoles: Larger dipoles result in stronger interactions.
• The polarity of molecules: More polar molecules experience stronger dipole-dipole forces.

When comparing acetone, acetic acid, and benzoic acid, dipole-dipole interactions contribute to the overall increase in their boiling and melting points. For instance, acetone experiences dipole-dipole interactions, but it's weaker compared to acetic acid and benzoic acid due to the absence of hydrogen bonding, which enhances the intermolecular forces further.

Hydrogen Bonding

Hydrogen bonding is a special type of strong dipole-dipole interaction that occurs when a hydrogen atom is bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine. This results in a significant dipole, allowing a hydrogen bond to form between the hydrogen atom of one molecule and the electronegative atom of another.

The impact of hydrogen bonding on physical properties can be profound:

• Compounds with hydrogen bonding often have significantly higher boiling and melting points compared to those without.
• It also affects solubility and viscosity properties.

In the context of the compounds discussed, acetic acid and benzoic acid both exhibit hydrogen bonding. This contributes to their higher boiling and melting points compared to acetone, which lacks this type of interaction. The presence of hydrogen bonding, alongside dipole-dipole interactions, makes these compounds more stable in their liquid and solid states, requiring more energy to change phases.

Boiling Point

The boiling point of a substance is the temperature at which its vapor pressure equals the atmospheric pressure, allowing it to change from a liquid to a gas. It is a good indicator of the strength of intermolecular forces present within a substance. Higher boiling points suggest stronger intermolecular attractions.

Factors affecting the boiling point include:

• The type and strength of intermolecular forces: Stronger forces lead to higher boiling points.
• Molecular size and structure: Larger molecules often have higher boiling points due to increased surface area for dispersion forces.

In the exercise, naphthalene has a higher boiling point than benzene and carbon tetrachloride, attributed to its larger molecular size and the resulting stronger dispersion forces. On the other hand, benzoic acid has the highest boiling point among the second set of compounds because it experiences both hydrogen bonding and strong dipole-dipole interactions.

Melting Point

Melting point is the temperature at which a solid becomes a liquid, indicating the energy required to overcome intermolecular forces holding the solid structure together. Like boiling points, melting points can help infer the strength of these forces.

Important influences on melting point include:

• The types of intermolecular forces: Stronger forces yield higher melting points.
• Molecular packing and shape: How well molecules pack can impact the melting point.

In the compounds analyzed, naphthalene's high melting point highlights the effectiveness of dispersion forces given its molecular structure. Conversely, benzoic acid has a significantly higher melting point than both acetone and acetic acid due to the enhanced stability from hydrogen bonding and its ability to form a robust solid lattice. This reflects the complex interplay between intermolecular forces and molecular architecture.