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.

Short answer

2-propanol is polar due to the OH group, exhibits hydrogen bonding, and boils lower than 1-propanol due to less extensive intermolecular forces.

Step-by-step solution

Draw the Three-Dimensional Structure

The molecule 2-propanol, or isopropanol, comprises three carbon atoms arranged in a chain. In a three-dimensional representation, C1 and C2 will have a tetrahedral shape since each is bonded to four atoms. The carbon atom at the center (C2) will also have a hydrogen and a hydroxyl group (OH) attached, representing its bond angles deviating slightly due to the larger electron cloud of the OH group.

Determine Molecular Geometry

Each carbon atom in 2-propanol is part of a tetrahedral geometry. The carbons are sp3 hybridized: the central carbon (C2) is connected to three hydrogen atoms and one hydroxyl group, while the terminal carbon atoms (C1 and C3) are each bonded to three hydrogen atoms and one other carbon atom.

Assess Molecular Polarity

2-propanol is a polar molecule primarily due to the presence of the hydroxyl group. The OH group has a significant dipole moment, making the overall molecular geometry polar despite the nonpolar C-C and C-H bonds. This dipole arises from the difference in electronegativity between oxygen and hydrogen.

Identify Intermolecular Forces

In 2-propanol, intermolecular forces include hydrogen bonding due to the OH group, as well as dipole-dipole interactions and London dispersion forces present in all molecules. Hydrogen bonds are the strongest interactions here, accounting for the molecule's relatively high boiling point.

Compare Boiling Points of Isomers

1-propanol also has hydrogen bonding capabilities like 2-propanol; however, 1-propanol can form a linear chain, which facilitates more extensive intermolecular hydrogen bonding. This linear arrangement allows for stronger intermolecular interactions in the liquid phase, explaining why 1-propanol has a higher boiling point than 2-propanol.

Key concepts

Molecular Geometry

2-propanol, also known as isopropanol, has a molecular formula of \( \text{C}_3\text{H}_7\text{OH} \). Its molecular geometry can be visualized by considering the arrangement of the atoms. The molecule consists of a chain of three carbon atoms. Each carbon exhibits a tetrahedral geometry, typical for sp3 hybridized carbon atoms. This means each carbon atom forms bonds with four other atoms.

In 2-propanol, the central carbon atom (C2) is bonded to one hydroxyl (OH) group, two hydrogen atoms, and one carbon atom, maintaining a tetrahedral shape. The terminal carbon atoms (C1 and C3) are each bonded to three hydrogen atoms and the central carbon atom. This gives the molecule its characteristic branched structure, which is important for its other properties.

Molecular Polarity

2-propanol is a polar molecule. The reason for its polarity primarily hinges on the presence of the hydroxyl group (OH). The oxygen in this group is more electronegative than the hydrogen, which creates a significant dipole moment across the molecule.

While the carbon-carbon and carbon-hydrogen bonds are nonpolar due to similar or equal electronegativity, the OH group disrupts this balance. The uneven distribution of electron density in the molecule leads to 2-propanol's overall polar nature. This polarity influences how 2-propanol interacts with other substances and its solubility in different solvents.

Intermolecular Forces

The intermolecular forces present in 2-propanol are a key factor in determining its properties, such as its boiling point. The primary force is hydrogen bonding, due to the OH group's strong attraction to other polar molecules, particularly involving hydrogen and oxygen.

In addition to hydrogen bonds, 2-propanol also exhibits dipole-dipole interactions, stemming from its polar nature. Furthermore, like all molecules, 2-propanol is subject to London dispersion forces. These are weak forces resulting from temporary dipoles that occur due to electron motion within molecules. Together, these forces play a crucial role in the molecule's physical characteristics and behavior.

Boiling Point Comparison

When comparing the boiling points of 2-propanol and its isomer 1-propanol, intermolecular interactions are key. Although both can form hydrogen bonds, the arrangement of atoms in the two molecules affects their boiling points.

1-propanol possesses a linear arrangement, which allows it to form more extensive hydrogen bonds compared to the branched structure of 2-propanol. These stronger intermolecular interactions in 1-propanol require more energy to break, resulting in a higher boiling point of 97.2°C, as opposed to 2-propanol's boiling point of 82.6°C. This difference underscores the impact molecular structure has on boiling point and physical properties.

Isomerism in Alcohols

Isomerism is a fascinating concept where compounds with the same molecular formula have different structures. In the case of alcohols, isomerism can lead to differences in physical and chemical properties. For 2-propanol and 1-propanol, they are structural isomers of each other.

Structural isomerism in alcohols arises from the different connectivity of the oxygen and carbon atoms within the molecule. This results in variations in molecular structure, such as branching, which in turn influences properties like boiling point, solubility, and reactivity. Understanding isomerism helps in recognizing how small changes in structure can massively alter a compound's behavior and uses.