Question

In what kind of orbitals do the lone-pair electrons on the oxygen of acetone reside, and 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 sp^2 hybrid orbitals, and they are in the same plane as the methyl \(-\mathrm{CH}_{3}\) groups.

Step-by-step solution

Determine the molecular structure of acetone

Acetone's molecular formula is \(\mathrm{C}_{3}\mathrm{H}_{6}\mathrm{O}\), and its structure consists of a central carbon atom (C1) bonded to an oxygen atom (C1=O) and two methyl \(-\mathrm{CH}_{3}\) groups bonded to the central carbon.

Identify the hybridization of the oxygen atom

We need to determine the hybridization of the oxygen atom to identify the type of orbitals where the lone pairs reside. The oxygen atom forms a double bond with the central carbon and has two lone pairs. Since oxygen is involved in the formation of one \(\sigma\) bond and one \(\pi\) bond, the hybridization of the oxygen atom is sp^2.

Determine the type of orbitals holding the lone pairs

As the oxygen atom is sp^2 hybridized, it has three sp^2 hybrid orbitals. Two of these hybrid orbitals are used to make a \(\sigma\) bond with the central carbon atom and hold the oxygen's lone pairs, while the third one forms a \(\pi\) bond with the carbon atom using remaining p orbital.

Determine the orientation of the orbitals holding the lone pairs

Sp^2 hybridization suggests a trigonal planar geometry around the oxygen atom, with bond angles of 120° between orbitals. Since the \(-\mathrm{CH}_{3}\) groups and the oxygen atom are bonded to the same central carbon atom, they are in the same plane. Therefore, the sp^2 hybrid orbitals holding the lone pairs on the oxygen atom are in the same plane as the methyl \(-\mathrm{CH}_{3}\) groups. In conclusion, the lone-pair electrons on the oxygen atom of acetone reside in sp^2 hybrid orbitals, and they are in the same plane as the methyl \(-\mathrm{CH}_{3}\) groups.

Key concepts

Hybridization

Understanding hybridization is key to predicting chemical behavior and structure. In molecules, hybridization refers to the mixing of atomic orbitals to form new, equivalent hybrid orbitals. These hybrid orbitals are crucial as they define the geometry of molecules and the type of chemical bonds that form. For acetone, look at the oxygen atom in the molecule. Oxygen in acetone is sp\(^2\) hybridized. This means its orbitals are a mix of one s and two p orbitals, creating three sp\(^2\) hybrid orbitals.

• One sp\(^2\) orbital is used to form a \(\sigma\) bond with the carbon atom.
• The other two sp\(^2\) orbitals hold lone pair electrons.

Understanding this hybridization is essential because it tells us about the distribution of electrons and how molecules interact in space.

Lone Pair Electrons

Lone pair electrons are those electrons in an atom that are not involved in bonding and remain localized on one atom. In acetone, the oxygen atom has two sets of lone pair electrons. These are crucial for determining the properties and reactions of the molecule.In the case of acetone:

• The lone pairs reside in the sp\(^2\) hybrid orbitals of the oxygen atom.
• These orbitals are within the same plane as the carbon atoms to which the oxygen is bonded.

Lone pairs influence the molecule's polarity and reactivity. They can participate in hydrogen bonding or act as a site for chemical reactions, affecting how acetone interacts with other substances.

Trigonal Planar Geometry

Molecular geometry plays a key role in the understanding of chemical structures. For acetone, the relevant geometry around the oxygen atom is trigonal planar. This geometry is characterized by a flat, triangular arrangement of atoms around a central atom, within one plane.In a trigonal planar geometry:

• The bond angles are approximately 120°.
• All atoms and orbitals lie in the same plane.

The sp\(^2\) hybridization of the oxygen atom results in this particular geometry, ensuring that the lone pairs and the bonds to the carbon atom are evenly spaced. This is why in acetone, the structure is notably flat, with the lone pairs existing on the same plane as the methyl \(-\mathrm{CH}_{3}\) groups.