Question

Acetone, \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CO}\), is widely used as an industrial solvent. (a) Draw the Lewis structure for the acetone molecule, and predict the geometry around each carbon atom. (b) Is the acetone molecule polar or nonpolar? (c) What kinds of intermolecular attractive forces exist between acetone molecules? (d) 1-Propanol, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\), has a molecular weight that is very similar to that of acetone, yet acetone boils at \(56.5^{\circ} \mathrm{C}\) and 1-propanol boils at \(97.2^{\circ} \mathrm{C}\). Explain the difference.

Short answer

In summary, acetone has a Lewis structure with a central carbon atom double bonded to an oxygen and single bonded to two other carbon atoms. Its geometry is trigonal planar for the central carbon and tetrahedral for the other carbon atoms. The molecule is polar due to the presence of a dipole moment in the C=O bond. Intermolecular attractive forces include dipole-dipole interactions and London dispersion forces. The higher boiling point of 1-propanol compared to acetone can be attributed to the presence of the hydroxyl group, which allows for stronger hydrogen bonding interactions.

Step-by-step solution

Draw the Lewis structure for acetone

To draw the Lewis structure for acetone, consider that the molecule has the following formula: \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CO}\). Acetone will have a central carbon atom that forms double bonds with an oxygen atom and single bonds with two other carbon atoms. The Lewis structure can be represented as: ``` O ═ C / \ C C | | | | H H H H ```

Predict the geometry around each carbon atom

Using the Valence Shell Electron Pair Repulsion (VSEPR) model, we can predict the geometry around each carbon atom based on its hybridization. The central carbon atom has sp2 hybridization as it's forming 3 bonding pairs (2 single and 1 double) and no lone pairs. Hence, the geometry around the central carbon atom would be trigonal planar. The other two carbon atoms have sp3 hybridization as they are each forming 4 bonding pairs with hydrogen atoms and no lone pair. Therefore, the geometry around these two carbon atoms would be tetrahedral.

Determine if the acetone molecule is polar or nonpolar

A molecule is polar if the sum of the individual bond dipole moments is not equal to zero and it has an uneven distribution of electron charge. In the case of acetone, the polar C=O bond has a dipole moment that is not equal to zero. The C-C and C-H bonds are less polar in comparison, but they have some polarity as well. Due to the geometry of the molecule, the dipole moments of C-H and C-C bonds do not cancel out the dipole moment of the C=O bond. As a result, the acetone molecule is polar.

Identify the intermolecular attractive forces between acetone molecules

Acetone molecules experience different types of attractive forces between them: 1. Dipole-dipole interaction: Since acetone is a polar molecule, it has dipole-dipole interactions between the positive charges of one molecule and the negative charges of another molecule. 2. London dispersion forces: All molecules, including nonpolar and polar molecules, experience London dispersion forces due to a temporary uneven distribution of electrons in their electron clouds, which results in temporary dipoles. Thus, the intermolecular attractive forces between acetone molecules are dipole-dipole interactions and London dispersion forces.

Explain the difference in boiling points between acetone and 1-propanol

Acetone and 1-propanol have similar molecular weights, but different boiling points. 1-propanol has a higher boiling point because of the presence of the hydroxyl group \(-\mathrm{OH}\). This group allows for hydrogen bonding between 1-propanol molecules. Hydrogen bonding is a stronger intermolecular force than dipole-dipole interactions and London dispersion forces, which are present between the acetone molecules. The stronger intermolecular forces in 1-propanol require more energy to break, which is why it has a higher boiling point than acetone, despite having a similar molecular weight.

Key concepts

Lewis structure

When discussing the Lewis structure of acetone, (\((\text{CH}_3)_2 \text{CO}\)), the fundamental aspect is understanding how atoms are connected via bonds and how electrons are arranged. In the Lewis structure:

• The central carbon atom forms a double bond with an oxygen atom, which is crucial for the molecule's characteristics.
• This carbon also forms single bonds with two methyl groups (CH3).
• The hydrogens are attached to the carbon atoms, completing the molecule to look like this:``` O ═ C / \ C C | | H H | | H H```
• Each line between atoms represents a bonding pair of electrons.

The Lewis structure is a simple, yet effective way to visualize how atoms coalesce to form more complex molecules.

VSEPR model

The Valence Shell Electron Pair Repulsion (VSEPR) model helps us predict molecular geometries by considering electron pair repulsion. It suggests that electron pairs around a central atom will arrange themselves as far apart as possible for minimal repulsion. For acetone's central carbon:

• The central carbon has sp2 hybridization, with three bonding pairs (two single bonds, one double bond) and no lone pairs.
• This arrangement forms a trigonal planar geometry, giving it a flat, three-sided shape around the central atom.
• Each of the outer carbon atoms in the methyl groups exhibits sp3 hybridization, leading to a tetrahedral shape, as each is bound to three hydrogens and one central carbon.

Understanding VSEPR allows us to forecast the three-dimensional structure and the overall geometric nature of the molecule.

Intermolecular forces

Intermolecular forces are the forces that hold molecules together and affect physical properties like boiling and melting points. They differ from the strong covalent bonds within the molecule:

• **London dispersion forces** are present in all molecules due to the instantaneous dipoles created by electron movement and are evident between acetone molecules.
• These forces are generally weak but increase with larger molecules.
• In acetone, **dipole-dipole interactions** also play a role due to its polar nature, providing stronger attractions than London dispersion forces alone.

Recognizing these forces helps us understand how acetone behaves, particularly in relation to solvents and evaporation.

Dipole-dipole interactions

In polar molecules like acetone, dipole-dipole interactions occur due to the presence of positive and negative ends. This happens in acetone because:

• The C=O bond in acetone is highly polar, with oxygen being more electronegative than carbon, creating a significant dipole.
• These permanent dipoles attract oppositely charged ends of neighboring molecules.
• This interaction is stronger than London dispersion forces but weaker than hydrogen bonds.

Dipole-dipole interactions significantly influence the physical properties of a substance, such as its boiling point and solubility in polar solvents.

Hydrogen bonding

Hydrogen bonding is a special type of dipole-dipole interaction that occurs when hydrogen is covalently linked to highly electronegative atoms (e.g., nitrogen, oxygen, or fluorine). Although acetone lacks hydrogen bonds, understanding this concept is key when comparing it to substances that do. For example:

• 1-Propanol can form hydrogen bonds due to its hydroxyl group (-OH), where hydrogen is bonded to oxygen.
• Hydrogen bonds significantly increase boiling points due to their strength.
• This explains why 1-propanol boils at a higher temperature than acetone, despite their similar molecular weights.

Hydrogen bonding plays a crucial role in defining the physical properties and interactions of molecules.