Question

Describe the structure of the formamide molecule, \(\mathrm{H}_{2} \mathrm{CONH}_{2}\), in terms of hybrid orbitals, bond angles, and \(\sigma\) - and \(\pi\)-bonds. The \(\mathrm{C}\) atom is bonded to two \(\mathrm{H}\) atoms, a terminal \(\mathrm{O}\) atom, and the \(\mathrm{N}\) atom. The \(\mathrm{N}\) atom is also bonded to two \(\mathrm{H}\) atoms.

Short answer

In formamide (\text{H}_2\text{CONH}_2), the carbon atom is sp^3 hybridized with approximately 109.5-degree bond angles, the oxygen atom is sp^2 hybridized, and nitrogen is sp^3 hybridized. The molecule includes sigma bonds (C-H, C-N, C-O, and N-H) and one pi bond (C-O).

Step-by-step solution

Determine the hybridization of the carbon atom

To determine the hybridization of the carbon atom in formamide, count the number of sigma bonds and lone pairs associated with it. Carbon forms two sigma bonds with hydrogen, one sigma bond with oxygen, and one sigma bond with nitrogen, totaling four sigma bonds. This indicates sp^3 hybridization for the carbon atom.

Determine the hybridization of the oxygen atom

The oxygen atom is bonded to the carbon with a double bond which consists of one sigma bond and one pi bond, and it has two lone pairs. The lone pairs and the sigma bond mean the oxygen has three regions of electron density, suggesting sp^2 hybridization for the oxygen atom.

Determine the hybridization of the nitrogen atom

The nitrogen atom is bonded to the carbon with a single bond and to two hydrogen atoms, with one lone pair remaining. Considering these three regions of electron density around nitrogen, it has sp^3 hybridization.

Describe the bond angles

According to the sp^3 hybridization of the carbon and nitrogen, the ideal bond angles are approximately 109.5 degrees. However, the pi bond between the carbon and oxygen atoms will cause some distortion, reducing the bond angle between the carbon-oxygen double bond and the adjacent sigma bonds.

Identify the sigma and pi bonds

In formamide, the carbon is involved in four sigma bonds (C-H, C-H, C-N, and C-O). The oxygen is involved in one sigma bond (C-O) and one pi bond (C-O) with carbon. Nitrogen forms three sigma bonds (N-C and two N-H).

Key concepts

sp3 Hybridization

The concept of sp3 hybridization is foundational to understanding the molecular structure of compounds like formamide. Hybridization occurs when atomic orbitals mix to create new, identical hybrid orbitals. In the context of formamide, the carbon atom undergoes sp3 hybridization. This process involves the mixing of one s orbital and three p orbitals from the second energy level of carbon, producing four equivalent sp3 hybrid orbitals. Each of these orbitals can form a sigma bond with other atoms.

Similarly, the nitrogen in formamide is also sp3 hybridized, indicating it has four regions of electron density which include three sigma bonds and one lone pair. The four areas of electron density around the carbon and nitrogen atoms lead to a tetrahedral geometry. In a perfect tetrahedral structure, the bond angles are approximately 109.5 degrees. However, in reality, the presence of different substituents can slightly alter these angles due to variations in electronegativity and the repulsion between electrons.

Sigma and Pi Bonds

Understanding sigma (σ) and pi (π) bonds is essential when examining the bonding in molecules such as formamide. Sigma bonds are the first bonds formed between two atoms. They are characterized by the head-to-head overlap of orbitals, allowing for free rotation of the bonded atoms around the bond axis.

In formamide, each bond connecting the carbon atom to hydrogen (C-H), the bond between carbon and nitrogen (C-N), and one part of the carbon-oxygen bond (C-O) are sigma bonds. These bonds provide the skeleton of the molecule's structure. On the other hand, pi bonds are formed due to the side-to-side overlap of p orbitals and are found in double and triple bonds. They do not allow for such rotation. In formamide's case, the second bond in the carbon-oxygen double bond is a pi bond, which adds rigidity to the structure and influences the bond angles.

Bond Angles

The term bond angles refers to the angle between two adjacent bonds at an atom. In an sp3 hybridized molecule such as formamide, the ideal bond angles are around 109.5 degrees due to the tetrahedral arrangement of hybrid orbitals. However, these angles can be affected by factors such as the presence of double bonds or lone pairs that exert different amounts of repulsion.

In formamide, the presence of the rigid pi bond in the carbon-oxygen double bond will slightly reduce the bond angles next to it, compared to the ideal tetrahedral angle. This is due to the increased electron density above and below the plane of the sigma bonds, which tends to compress the space they occupy. As a result, the bond angle associated with the sp2 hybridized oxygen will be closer to 120 degrees, while those around the sp3 hybridized carbon and nitrogen may be slightly less than 109.5 degrees due to the differences in the substituents' size and electronegativity.