Problem 91 - Chemistry: The Central Science in SI Units, Global Edition

Question

Rubbing alcohol is the common name for 2 -propanol (isopropanol), \(\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}\). (a) Draw the three-dimensional structure of the 2 -propanol molecule and predict the geometry around each carbon atom. (b) Is the 2 -propanol molecule polar or nonpolar? (c) What kinds of intermolecular attractive forces exist between the 2 -propanol molecules? (d) The isomer 1-propanol has the same molecular weight as 2 -propanol, yet 2 -propanol boils at \(82.6^{\circ} \mathrm{C}\) and 1 -propanol boils at \(97.2^{\circ} \mathrm{C}\). Explain the difference.

Step-by-Step Solution

Answer

The 3D structure of 2-propanol has three carbon atoms with tetrahedral geometry. The molecule is polar due to the polar bond between carbon and oxygen atoms. Attractive forces between 2-propanol molecules include hydrogen bonds and dispersion forces. The difference in boiling points between 2-propanol and 1-propanol is explained by the position of the hydroxyl group, which affects the strength of hydrogen bond formation and intermolecular forces.

Step 1: Draw the 3D structure of 2-propanol

First, let’s start by drawing the 3D structure of the 2-propanol molecule. The molecular formula for 2-propanol is \(\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}\). The formula shows that it consists of three carbon atoms, seven hydrogen atoms, and one oxygen atom. The 2-propanol is an isopropanol, meaning that the OH group is attached to the central carbon. The 3D structure can be represented as: ``` H H H | | | H-C-C-C-O-H | | H H ``` Next, we will predict the geometry around each carbon atom.

Step 2: Predict the geometry around each carbon atom

In the 2-propanol molecule, each carbon atom is bonded to either three or four other atoms. According to the VSEPR (Valence Shell Electron Pair Repulsion) theory, the bond geometry around each carbon atom can be predicted as follows: - Around the first carbon (C1), there are 3 hydrogen atoms. Therefore, the bond angles will be approximately 109.5°, giving a tetrahedral geometry for this carbon atom. - Around the second carbon (C2), there are two hydrogen atoms, one carbon atom, and one oxygen atom. Hence, the bond angles would also be approximately 109.5°, resulting in a tetrahedral geometry for this carbon atom. - Around the third carbon (C3), there are 3 hydrogen atoms. Similar to C1, the bond angles will be approximately 109.5°, leading to a tetrahedral geometry for this carbon atom.

Step 3: Determine polarity of the 2-propanol molecule

To determine the polarity of the 2-propanol molecule, we need to evaluate the overall distribution of the electron charge in the molecule. Since oxygen is more electronegative than carbon and hydrogen, there is a polar bond between the latter carbon and oxygen atoms in the molecule, which results in a molecular dipole. Therefore, the 2-propanol molecule is polar.

Step 4: Identify intermolecular attractive forces

The intermolecular attractive forces between 2-propanol molecules are primarily due to the presence of the polar OH group. This interaction can lead to the formation of hydrogen bonds between the molecules. Moreover, dispersion forces (London forces) also exist between the molecules, which are attributed to the instantaneous polarization and the temporary dipoles formed in the molecules.

Step 5: Explain the difference in boiling points between 2-propanol and its isomer, 1-propanol

In both isomers, 1-propanol and 2-propanol, the molecules have the same molecular weight and similar structure. However, they differ in the position of the hydroxyl group (OH group), which affects the intermolecular forces present in the liquid. In the 1-propanol molecule, the OH group is bonded to the terminal carbon atom, allowing for more extensive hydrogen bond formation between the 1-propanol molecules. This situation leads to stronger intermolecular forces, which cause higher boiling points. On the other hand, the 2-propanol molecule has the OH group attached to the middle carbon atom. This positioning limits the opportunity for stronger hydrogen bond formation between the molecules, resulting in weaker intermolecular forces. Consequently, 2-propanol has a lower boiling point than 1-propanol.

Key Concepts

Molecular Geometry

In 2-propanol, understanding the molecular geometry involves looking at each carbon atom individually. Each carbon atom in 2-propanol is part of a unique environment, because of how atoms are arranged around them. According to VSEPR theory, which helps us predict shapes of molecules, atoms and electron pairs position themselves to minimize repulsion.

For 2-propanol:

• The first carbon (C1) has bonds with three hydrogen atoms, forming a tetrahedral geometry with a bond angle close to 109.5°.
• The second carbon (C2) is bonded to two hydrogen atoms, one carbon atom, and one oxygen atom, also forming a tetrahedral shape.
• The third carbon (C3), similar to C1, binds with three hydrogen atoms, maintaining the tetrahedral geometry.

Understanding these geometries is fundamental because it influences other properties like polarity and intermolecular forces.

Polarity

The polarity of a molecule significantly affects its physical properties, such as solubility and boiling point. For 2-propanol, polarity arises due to differences in electronegativity between the atoms in the molecule.

In the 2-propanol molecule:

• The oxygen atom is more electronegative than both carbon and hydrogen, resulting in polar covalent bonds between the oxygen and the carbon it's attached to, as well as with hydrogen.
• This leads to a dipole moment, where one end of the molecule is slightly negative and the other end slightly positive.

Thus, 2-propanol is considered a polar molecule, which directly impacts how it interacts with other molecules, contributing to its chemical behavior.

Intermolecular Forces

Intermolecular forces are crucial in understanding how molecules interact with each other. In 2-propanol, the presence of the hydroxyl (OH) group means there are specific forces at play.

For 2-propanol:

• The OH group allows for hydrogen bonding, a strong intermolecular force, occurring between the slightly positive hydrogen atom of one molecule and the slightly negative oxygen atom of another.
• Additionally, dispersion forces (or London dispersion forces) come into play due to temporary dipoles that develop in the molecules.

These forces are key in explaining why 2-propanol has certain boiling points and solubility characteristics, as opposed to molecules with weaker intermolecular forces.

Boiling Point

The boiling point of a molecule is influenced by its structure and the intermolecular forces between molecules. In 2-propanol and its isomer 1-propanol, the difference in boiling points despite similar molecular weights is notable.

Here's why:

• 1-propanol's OH group is bonded to an end carbon, maximizing the potential for hydrogen bonding due to less steric hindrance. This results in stronger intermolecular forces and a higher boiling point of 97.2°C.
• In 2-propanol, the OH group is attached to the central carbon, which limits hydrogen bonding interactions due to spatial arrangement. This results in weaker intermolecular forces and thus a lower boiling point of 82.6°C.

This demonstrates how a seemingly small structural change can influence physical properties like boiling point based on the strength and extent of intermolecular interactions.