Question

Describe the structure of the formaldehyde molecule, \(\mathrm{CH}_{2} \mathrm{O}\), in terms of hybrid orbitals, bond angles, and \(\sigma\) - and \(\pi\)-bonds. The \(\mathrm{C}\) atom is the central atom to which the other three atoms are attached.

Short answer

Formaldehyde has a central carbon atom hybridized to sp2, forming a trigonal planar shape with bond angles of ~120 degrees. There are three sigma bonds (two C-H and one C-O) and one pi bond (C=O).

Step-by-step solution

Determine the Hybridization of Carbon

The carbon atom in formaldehyde has two single bonds with hydrogen and one double bond with oxygen. To accommodate these bonds, carbon will use three hybridized orbitals. The electron configuration of carbon is 1s2 2s2 2p2. In its excited state, one of the 2s electrons is promoted to an empty 2p orbital, resulting in the configuration 1s2 2s1 2p3. Carbon then hybridizes to form three sp2 hybrid orbitals, which will form sigma bonds with hydrogen and oxygen atoms.

Describe the Bond Angles

The sp2 hybrid orbitals arrange themselves in the same plane at angles of approximately 120 degrees to each other to minimize electron-pair repulsion according to VSEPR theory, forming a trigonal planar shape.

Identify Sigma and Pi Bonds

In formaldehyde, each hydrogen atom is connected to the central carbon atom by a sigma bond formed from the overlap of an sp2 orbital of carbon with the 1s orbital of hydrogen. The oxygen atom is connected to the carbon atom by one sigma bond formed from the overlap of an sp2 orbital of carbon with an sp2 orbital from oxygen. The second bond between carbon and oxygen is a pi bond, resulting from the sideways overlap of the remaining p orbitals on both the carbon and oxygen atoms.

Key concepts

sp2 Hybridization

Understanding the structure of formaldehyde, particularly its molecular geometry and bond formation, requires knowing about hybridization. In formaldehyde, the carbon atom undergoes sp2 hybridization. But what does sp2 hybridization mean?

Hybridization is a concept in chemistry where atomic orbitals mix to form new, identical hybrid orbitals. For carbon in formaldehyde, sp2 hybridization occurs when one 2s orbital and two 2p orbitals blend together to make three sp2 hybrid orbitals, leaving one 2p orbital unhybridized. This hybridization facilitates the formation of three sigma bonds with the surrounding atoms.

Each sp2 hybrid orbital has an equal degree of s and p character, roughly 33% s and 67% p character. The unhybridized 2p orbital will later play a crucial role in forming a pi bond with oxygen. The sp2 hybridization is key to achieving the trigonal planar shape of the molecule, as it allows the bonded atoms to spread out evenly at 120-degree angles around the central carbon atom.

Bond Angles

Bond angles are pivotal in determining the shape and spatial configuration of a molecule. For formaldehyde, the bond angles are directly influenced by its sp2 hybridization state.

In a trigonal planar arrangement, the bond angles between the hybrid orbitals are ideally 120 degrees. The Valence Shell Electron Pair Repulsion (VSEPR) theory explains that in order to minimize the repulsion between the electron pairs, the orbitals adjust so that they are as far apart as possible. This theory underpins the reasoning behind the trigonal planar arrangement and its associated bond angles in formaldehyde.

The optimization of electron pair orientation results in the molecule's flat and angular shape. The measured bond angles in formaldehyde are indeed close to the theoretical 120 degrees, with slight variations that can occur due to the molecules' bond lengths and the influence of the non-bonded electrons.

Sigma and Pi Bonds

When discussing the types of bonds present in formaldehyde, we differentiate between sigma (σ) and pi (π) bonds. These bonds are types of covalent bonds that differ in the way the atomic orbitals overlap.

A sigma bond is the strongest type of covalent bond. It is formed by the head-on overlapping of atomic orbitals and is found in all single bonds. In formaldehyde, each hydrogen atom forms a sigma bond with the central carbon atom. This bond results from the overlap between the carbon's sp2 hybrid orbital and the 1s orbital of hydrogen. Furthermore, the carbon and oxygen atoms are also connected by a sigma bond, emerging from the overlap between an sp2 hybrid orbital of carbon with an sp2-like orbital of oxygen.

In addition to the sigma bonds, a pi bond is formed between carbon and oxygen, characterizing their double bond. The pi bond forms from the side-to-side overlap of the unhybridized p orbitals on carbon and oxygen. This bond allows for the delocalization of electrons, which imparts stability to the molecule. The combination of one sigma and one pi bond constitutes the double bond between carbon and oxygen in the formaldehyde molecule, which is pivotal for the molecule's reactivity.