Question

In what kind of orbitals do the lone-pair electrons on the oxygen of acetone reside? Are they in the same plane as the methyl \(-\mathrm{CH}_{3}\) groups, or are they perpendicular to the methyl \(-\mathrm{CH}_{3}\) groups?

Short answer

Answer: The lone-pair electrons on the oxygen atom of acetone reside in orbitals that are perpendicular to the plane containing the methyl \(\mathrm{-CH}_3\) groups.

Step-by-step solution

Identify the molecular structure of acetone

Acetone is an organic compound with the formula \(\mathrm{CH}_3\mathrm{COCH}_3\). It contains an oxygen atom double-bonded to a central carbon atom, with two methyl groups (\(\mathrm{-CH}_3\)) bonded to the central carbon as well.

Determine the hybridization of the oxygen atom in acetone

The oxygen atom in acetone is double-bonded to the central carbon atom, fulfilling two of its valence positions. Additionally, it has two lone pairs, occupying the other two valence positions. Oxygen has two bonding positions and two lone pairs, making a total of four positions. Therefore, the oxygen atom in acetone is sp3 hybridized.

Determine the geometry of the acetone molecule

The central carbon atom in acetone is double-bonded to the oxygen atom and single-bonded to the two methyl \(\mathrm{-CH}_3\) groups. The central carbon has three bonding positions, so it is sp2 hybridized. The geometry of the acetone molecule around the central carbon is trigonal planar.

Determine the orientation of lone pairs of electrons on the oxygen atom

As the oxygen atom in acetone is sp3 hybridized, its geometry should be tetrahedral. In a tetrahedral geometry, the bond angles are approximately 109.5 degrees. Two corners of the tetrahedron represent the bonding positions for the double bond to the central carbon, and the other two corners represent the lone-pair orbitals. Since the central carbon atom has a trigonal planar geometry, all the atoms around it (two \(\mathrm{-CH}_3\) groups and oxygen) lie in the same plane. Therefore, the lone-pair orbitals on the oxygen atom are perpendicular to the plane containing the \(\mathrm{-CH}_3\) groups. In conclusion, the lone-pair electrons on the oxygen of acetone reside in orbitals that are perpendicular to the plane containing the methyl \(\mathrm{-CH}_3\) groups.