Problem 25 - Chemical Principles

Question

Rank the following molecules in order of increasing viscosity at \(50^{\circ} \mathrm{C}: \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{SH}, \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}, \mathrm{C}_{6} \mathrm{H}_{6}\).

Step-by-Step Solution

Answer

C6H6 < C6H5SH < C6H5OH

Step 1: Understand Molecular Structure and Intermolecular Forces

Viscosity is influenced by the strength of intermolecular forces within a liquid. Stronger intermolecular forces result in higher viscosity. Benzene (C6H6) has relatively weak dispersion forces. Both phenol (C6H5OH) and thiophenol (C6H5SH) have stronger hydrogen bonding due to the presence of OH or SH groups, respectively.

Step 2: Compare Hydrogen Bonding Potential

Phenol has hydrogen bonding due to the OH group, which is stronger than the hydrogen bonding in thiophenol with the SH group. The OH group can form stronger hydrogen bonds than the SH group because oxygen is more electronegative than sulfur and can form stronger dipole-dipole interactions.

Step 3: Rank the Molecules by Viscosity

We can now rank the molecules by viscosity. Benzene has the lowest viscosity due to weak dispersion forces. Thiophenol has a higher viscosity than benzene due to the presence of weak hydrogen bonds. Phenol will have the highest viscosity among the three since it can form the strongest hydrogen bonds.

Key Concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion that act between neighboring particles (molecules, atoms, or ions). These forces are responsible for holding substances together and determine many properties of substances, such as boiling points, melting points, and viscosity. There are several types of intermolecular forces, including London dispersion forces, dipole-dipole interactions, and hydrogen bonding.

The London dispersion force is the weakest intermolecular force and is present in all molecules, whether they are polar or nonpolar. It arises due to the instantaneous and induced dipoles that occur when electron clouds of adjacent molecules distort momentarily. Dipole-dipole interactions are stronger and occur between molecules that have permanent dipoles — this means that one end of the molecule has a slight negative charge, while the other end has a slight positive charge. Finally, hydrogen bonding is a special type of dipole-dipole interaction that occurs when a hydrogen atom is bonded to a highly electronegative atom, like oxygen, nitrogen, or fluorine, and is in close proximity to another electronegative atom.

Hydrogen Bonding

Hydrogen bonding is an especially strong type of intermolecular force that plays a crucial role in determining the physical properties of many compounds, including water and organic molecules. Hydrogen bonds occur when a hydrogen atom, which is covalently bonded to a highly electronegative atom such as oxygen, nitrogen, or fluorine, is electrostatically attracted to a lone pair of electrons on another electronegative atom within the same or a different molecule.

The strength of hydrogen bonding depends on the electronegativity of the atoms involved and the distance between the hydrogen and the electronegative atom it is attracted to. In the context of the exercise, the OH group present in phenol is highly effective at hydrogen bonding because oxygen is more electronegative than sulfur, which is present in thiophenol. This higher electronegativity allows oxygen to pull electron density towards itself more strongly, creating a greater dipole that is conducive to forming stronger hydrogen bonds.

Molecular Structure

The molecular structure significantly influences a molecule's physical properties and behaviors, such as viscosity. Viscosity is a measure of a fluid's resistance to flow and is directly affected by the shape and size of molecules as well as the type and strength of intermolecular forces present within the substance.

The molecular structure of a compound dictates how molecules pack together and how they interact with one another. For example, linear or symmetrical molecules like benzene (C6H6) tend to have lower viscosities because they can slide past one another more easily. In contrast, molecules with more complex structures or those with functional groups capable of strong intermolecular interactions, such as hydrogen bonding, will have higher viscosities because they are more likely to 'stick' together and hinder flow.