Question

Write the important resonance structures which contribute to the resonance hybrid of diazomethane and show how these can be used to rationalize the formation of methyl acetate from diazomethane and acetic acid.

Short answer

The resonance structures of diazomethane are CH2=N+=N· and CH2-+≡N-N·. These structures facilitate the nucleophilic attack of the acetate ion (CH3-C(=O)O^-) on the positively charged carbon atom in diazomethane. The reaction between diazomethane (CH2N2) and acetic acid (CH3-C(=O)-OH) can be explained by a three-step mechanism involving proton transfers and nucleophilic attacks, eventually leading to the formation of methyl acetate (CH3-C(=O)-OCH3) and nitrogen gas (N2).

Step-by-step solution

Draw diazomethane molecule

First of all, we need to draw the diazomethane molecule. Diazomethane has the molecular formula CH2N2, which we can represent as CH2=N+=N·.

Draw the resonance structures of diazomethane

Now, we will draw the two important resonance structures of diazomethane: 1. CH2=N+=N· 2. CH2-+≡N-N· These structures show the delocalization of electrons in the diazomethane molecule.

Draw acetic acid molecule

Next, we need to draw the acetic acid molecule. Acetic acid has the molecular formula CH3COOH, which can be represented as CH3-C(=O)-OH.

Show the formation of methyl acetate from diazomethane and acetic acid

To explain the formation of methyl acetate from diazomethane and acetic acid, let's follow these steps: 1. Proton transfer: In the first step, the unshared electron pair on the nitrogen atom in the diazomethane molecule attacks the hydrogen of the acetic acid molecule. This results in the formation of a \(CH_3C(=O)O^-\) ion (acetate ion) and a protonated diazomethane molecule. CH2=N+=N· + CH3-C(=O)-OH → CH2=N+=NH^+ + CH3-C(=O)O^- 2. Nucleophilic attack: The negatively charged oxygen atom of the acetate ion attacks the positively charged carbon atom of the protonated diazomethane molecule, forming a new carbon-oxygen bond. CH2=N+=NH^+ + CH3-C(=O)O^- → CH2-N+=NH - CH3-C(=O)-O 3. Proton transfer: Finally, the protonated nitrogen atom donates a proton to the negatively charged oxygen atom. CH2-N+=NH - CH3-C(=O)-O → CH2=N+=N· + CH3-C(=O)-OH The overall result is that diazomethane and acetic acid have reacted to form a new molecule, methyl acetate (CH3-C(=O)-OCH3) and nitrogen gas (N2). CH2=N+=N· + CH3-C(=O)-OH → CH3-C(=O)-OCH3 + N2 The resonance structures of diazomethane facilitate the nucleophilic attack of the acetate ion on the positively charged carbon atom in diazomethane, which leads to the formation of methyl acetate.

Key concepts

Understanding Diazomethane

Diazomethane is a highly reactive yellow gas with the chemical formula \(CH_2N_2\). It consists of a methylene group \(CH_2\) covalently bonded to a diazo group \(N_2\). This gas is often used in organic chemistry for its role as a methylating agent and its ability to create carbenes, which are compounds with a bivalent carbon atom having two electrons that can form two covalent bonds.
The resonance structures of diazomethane play a crucial role in its reactivity:

• The first resonance structure \(CH_2=N^+=N^-\) suggests a strong partial positive charge on the nitrogen atom bonded to the carbon, making the molecule electron-deficient at this site.
• The alternative resonance form \(CH_2^-N\equiv N^+\) suggests a certain degree of electron delocalization, stabilizing the intermediate during chemical reactions.

These resonance structures highlight how the electrons can be distributed across the molecule, enabling nucleophilic reactions.

The Role of Acetic Acid

Acetic acid, with the formula \(CH_3COOH\), is a key player in many chemical reactions, including the one with diazomethane. A simple molecule, acetic acid is known for its weak acidity due to the carboxyl group \(COOH\).

The acidity of acetic acid allows it to donate a proton (H^+) to other molecules, making it reactive in different contexts. In the case of forming methyl acetate, acetic acid doesn’t just act as a reactant but plays an essential role in the mechanism by:

• Protonating parts of other molecules, changing their electronic structure and reactivity.
• Forming an acetate ion (CH_3COO^-) when it loses a hydrogen (H^+), which is crucial for the subsequent reactions.

The process involving acetic acid ensures the generation of intermediates that further react with diazomethane to form the desired product through a series of electron transfers.

Formation of Methyl Acetate

The transformation of diazomethane and acetic acid into methyl acetate is a fascinating journey of molecular interactions. Methyl acetate is an ester with the formula \(CH_3COOCH_3\), commonly used as a solvent. The reaction mechanism involves several steps driven by the reactive nature of both diazomethane and acetic acid:

• Proton Transfer: The lone pair on nitrogen in diazomethane attacks the hydrogen (H^+) in acetic acid. This forms a protonated diazene (CH_2=N^+=NH^+) and an acetate ion (CH_3COO^-).
• Nucleophilic Attack: The acetate ion then attacks the cationic center in the protonated diazomethane to form a new carbon-oxygen bond. This results in an intermediate with a linked structure of diazomethane and acetate.
• Final Proton Transfer: The nitrogen donates a proton back to stabilize the oxygen, resulting in the regeneration of diazomethane and formation of methyl acetate.

This reaction exemplifies how resonance structures and the electronic nature of organic molecules drive chemical transformations. The production of nitrogen gas (N_2) as a byproduct is also significant as it pushes the reaction forward by stabilizing the product.