Question

Acetone, a common solvent used in some nail polish removers, has the molecular formula \(\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{O}\) and contains a carbon- oxygen double bond. (a) Propose two Lewis structures for acetone. (b) What is the geometry around the carbon atoms in each of the structures? (c) Which of the bonds in each structure are polar?

Short answer

(a) Two Lewis structures focus on \(\mathrm{CH}_3\mathrm{COCH}_3\); (b) Geometry: central carbon trigonal planar, terminal carbons tetrahedral; (c) C=O bond is polar.

Step-by-step solution

Identify Atoms and Bonding Capacity

Acetone has the molecular formula \(\mathrm{C}_3\mathrm{H}_6\mathrm{O}\). Knowing each atom's valency, carbon forms 4 bonds, hydrogen forms 1 bond, and oxygen forms 2 bonds. The carbon-oxygen bonds have a double bond as part of the ketone group characteristic in acetone.

Propose Lewis Structure 1

The first Lewis structure for acetone is drawn as \(\mathrm{CH}_3\mathrm{COCH}_3\) with central carbon atom double-bonded to an oxygen atom (C=O), and single-bonded to two \(\mathrm{CH}_3\) groups. This structure satisfies the valency of all involved atoms.

Propose Lewis Structure 2

A second valid Lewis structure for acetone could involve a resonance form, but in this exercise, it's more about featuring different arrangements. Realistically, for acetone with a double-bonded oxygen, it primarily exists in this one clear form, but exploring a plausible alternative involves moving the C-O double bond to other carbon positions to test resonance possibilities.

Determine Geometry Around Carbon Atoms

For both structures proposed, determine the geometry around each carbon. Acetone has trigonal planar geometry around the central carbon involved in the C=O due to sp2 hybridization (120-degree bond angles), while the end carbon atoms have tetrahedral geometry because they are bonded to three hydrogens and one carbon each.

Identify Polar Bonds

Identify bonds as polar or non-polar based on electronegativity differences. In both structures, the C=O bond is polar due to the significant difference in electronegativity between carbon and oxygen. The C-H bonds are weakly polar or considered nonpolar.

Key concepts

Acetone

Acetone is a simple organic molecule with the chemical formula \(\mathrm{C}_3\mathrm{H}_6\mathrm{O}\). It is primarily known as a solvent, commonly used in products like nail polish remover. Acetone is part of the ketone family, which includes compounds that feature the functional group \(\text{C=O}\) (carbon-oxygen double bond). When discussing its molecular structure, acetone is often depicted as \(\mathrm{CH}_3\mathrm{COCH}_3\), highlighting the bond between the carbon and oxygen.

• Acetone is colorless and miscible in water, which means it can mix with water in all proportions.
• This solvent is widely utilized in cleaning, thinning paint, and even in some medical and laboratory applications due to its ability to dissolve both polar and non-polar substances.
• Understanding its Lewis structure helps in exploring its behavior in chemical reactions and its polar characteristics.

Carbon-Oxygen Double Bond

The carbon-oxygen double bond is a defining feature of acetone's chemical structure. This type of bond, written as \(\text{C=O}\), indicates that two pairs of electrons are shared between the carbon and oxygen atoms. The presence of a double bond gives acetone some unique properties:

• Double bonds are stronger than single bonds, resulting in higher stability within the molecule.
• They also restrict the rotation of the bonded atoms, affecting the overall shape of the molecule.
• In acetone, the carbon-oxygen double bond contributes to the molecule’s reactivity, making it an efficient solvent.

Due to the high electronegativity of oxygen compared to carbon, the carbon-oxygen double bond is also a key aspect of acetone's polarity, which will be explored later.

Molecular Geometry

Understanding the molecular geometry of acetone is crucial for predicting how it interacts with other molecules. Around the central carbon in the carbon-oxygen double bond, the geometry is trigonal planar due to \(\text{sp}^2\) hybridization. This affects the molecule as follows:

• Trigonal planar geometry has bond angles of approximately 120 degrees.
• Flat molecular shape is important for understanding how acetone stacks or interacts in various chemical environments.
• On the other hand, the carbons at the ends of the acetone molecule are surrounded by hydrogen atoms and are \(\text{sp}^3\) hybridized, resulting in a tetrahedral geometry with bond angles close to 109.5 degrees.

This variation in geometry within a single molecule is important for anticipating intermolecular forces and reactivity with other compounds.

Polar Bonds

In acetone, bonds are classified as polar or non-polar based on electronegativity differences between the atoms involved. A polar bond occurs when there is a significant difference in electronegativity, leading to an uneven distribution of electron density. In acetone:

• The carbon-oxygen double bond is polar, as oxygen is more electronegative than carbon, causing a partial negative charge on the oxygen and a partial positive charge on carbon.
• As a result, the \(\text{C=O}\) bond acts as a site of reactivity, attracting polar solutes and playing a pivotal role in the solvent properties of acetone.
• On the contrary, the carbon–hydrogen bonds within acetone are considered nonpolar or weakly polar, because carbon and hydrogen have similar electronegativities.

Understanding where polar bonds are in acetone guides its interaction with other molecules, particularly in solvation and reaction mechanisms.