Question

Draw a Lewis structure for the \(N, N\) -dimethylformamide molecule. The skeletal structure is Various types of evidence lead to the conclusion that there is some double bond character to the \(\mathrm{C}-\mathrm{N}\) bond. Draw one of more resonance structures that support this observation.

Short answer

The Lewis structure for N, N-dimethylformamide (DMF) has Carbon as the central atom, double bonded to Oxygen and single bonded to Nitrogen, with two Hydrogen atoms single bonded to Nitrogen. The total valence electrons are 18. Two resonance structures can be drawn for DMF, as shown below: Resonance structure 1: O // C=N / \ H H Resonance structure 2: O " || H-C-N / H These resonance structures show that there is some double bond character in the C-N bond, as supported by the evidence.

Step-by-step solution

Determine the total number of valence electrons

First, count the valence electrons for each atom in the molecule. Here, we have 1 Carbon (C), 1 Nitrogen (N), 1 Oxygen (O), and 4 Hydrogen (H) atoms. The number of valence electrons for each atom is: - Carbon (C): 4 - Nitrogen (N): 5 - Oxygen (O): 6 - Hydrogen (H): 1 Total valence electrons: 4(1 Carbon) + 5(1 Nitrogen) + 6(1 Oxygen) + 1(4 Hydrogens) = 18 valence electrons.

Determine the central atom and skeletal structure

Choose the least electronegative atom (excluding Hydrogen) as the central atom. In this case, Carbon is the least electronegative, so it will be the central atom. The skeletal structure given in the problem is O " || H-C-N / H

Arrange the other atoms around the central atom

Place the Oxygen atom double bonded to Carbon, Nitrogen atom single bonded to Carbon, and two Hydrogen atoms single bonded to Nitrogen as shown in the skeletal structure above. We have now used 10 of the 18 valence electrons.

Distribute electrons to satisfy the octet rule

Now, distribute the remaining 8 valence electrons to satisfy the octet rule around each atom (except Hydrogen, which follows the duet rule). However, since we have used all of the electrons, every atom in the molecule is already satisfying the octet (or duet) rule, and no further distribution is needed.

Modify the structure if any double or triple bonds are required

The current Lewis structure shows one double bond between Carbon and Oxygen atom and a single bond between Carbon and Nitrogen atoms. Since the evidence suggests that there is some double bond character to the C-N bond, we must draw resonance structures.

Draw resonance structures that support the double bond character to the C-N bond

Resonance structures are formed by moving electrons, not atoms. We can move one of the double bonds between Carbon and Oxygen to form a double bond with Nitrogen atom as shown below: Resonance structure 1: O // C=N / \ H H Resonance structure 2: O " || H-C-N / H The actual molecule is a hybrid of these resonance structures, suggesting a partial double bond character in the C-N bond.

Key concepts

Resonance Structures

Resonance structures are essential for understanding molecules where electron distribution is not fixed but rather can be represented in multiple ways. These structures are alternative ways to arrange electrons that stabilize molecules when a single Lewis structure is insufficient. For example, in N,N-dimethylformamide, although we initially draw a single bond between the carbon and nitrogen atoms, evidence suggests a partial double bond character. This arises because the electron distribution can resonate between different structures.

• Resonance involves delocalization of electrons, which helps in stabilizing the molecule.
• The true structure of the molecule is a hybrid of these resonance structures.

In practice, this means moving electrons—not atoms—to depict different resonance structures. This resonance ensures some bonds have characteristics of double bonds, even in a seemingly single-bonded structure.

Valence Electrons

Understanding valence electrons is crucial for drawing Lewis structures as they are the electrons involved in chemical bonding. These are the outermost electrons that determine how atoms bond and interact with each other. For N,N-dimethylformamide, we calculated total valence electrons by summing the valence electrons for each atom:

• Carbon (4 electrons)
• Nitrogen (5 electrons)
• Oxygen (6 electrons)
• Hydrogen (1 electron each, for a total of 4)

Totaling 18 valence electrons, these electrons are then distributed among atoms in the molecule to satisfy the chemical rules of bonding, like the octet rule. Proper distribution ensures correct electron pairing and molecule stability.

Octet Rule

The octet rule is a fundamental principle in chemistry stating that atoms tend to adjust their electrons to achieve a noble gas configuration, often having eight electrons in their outer shell. In the case of N,N-dimethylformamide, each atom (excluding hydrogen, which follows the duet rule) aims to complete its valence shell:

• Carbon achieves 8 electrons by sharing with adjacent atoms.
• Nitrogen and Oxygen also aim to fill or share up to 8 electrons.
• Hydrogen atoms, given their simplicity, follow the duet rule and only need 2 electrons.

Following the octet rule allows us to verify the distribution of electrons around each atom after the initial drawing of a molecule's structure and adjust as necessary to achieve stability.

Dimethylformamide

Dimethylformamide (DMF) is a commonly used organic solvent in the laboratory noted for its ability to dissolve both polar and non-polar substances. Its structure consists of a formamide group linked with two methyl groups on the nitrogen atom, leading to its characteristic name. The electronic structure of DMF provides insights into its chemical reactivity and uses:

• Its double bond character in the C-N bond contributes to a planar configuration.
• The resonance structures impact its physical properties like boiling point and solubility.
• It's frequently employed in synthetic chemistry due to its relatively high dielectric constant.

Understanding its molecular structure, such as through resonance and electron distribution, enhances its efficient use in various chemical and industrial applications.