Question

One frequently used method for preparing methy esters is by reaction of carboxylic acids with diazomethane, \(\mathrm{CH}_{2} \mathrm{~N}_{2}\). The reaction occurs in two steps: (1) protonation of diazomethane by the carboxylic acid to yield methyldiazonium ion, \(\mathrm{CH}_{3} \mathrm{~N}_{2}^{+},\) plus a carboxylate ion; and (2) reaction of the carboxylate ion with \(\mathrm{CH}_{3} \mathrm{~N}_{2}^{+}\). (a) Draw two resonance structures of diazomethane, and account for step 1. (b) What kind of reaction occurs in step \(2 ?\)

Short answer

(a) Draw resonance forms and show protonation of \(\mathrm{CH_2 N_2}\) . (b) Step 2 is a nucleophilic substitution reaction.

Step-by-step solution

Drawing Resonance Structures of Diazomethane

Diazomethane (\(\mathrm{CH}_{2} \mathrm{~N}_{2}\)) can be drawn in two major resonance forms. The first structure has a neutral carbon bonded to two hydrogens and a nitrogen atom with a triple bond to another nitrogen atom, which carries a lone pair. In this structure, there is no formal charge on carbon, carbon is sp-hybridized, and the nitrogen carrying the lone pair has a formal negative charge. The second resonance structure can be drawn by shifting the lone pair of electrons on the terminal nitrogen to form a double bond with the adjacent nitrogen and moving electrons from the N=N bond to form a lone pair on the central nitrogen, making it positively charged and creating a zwitterionic form. This movement gives a positive formal charge on the central nitrogen and a negative formal charge on the terminal nitrogen.

Explaining Step 1 - Protonation to Form Methyldiazonium Ion

In the first step of the reaction, the carboxylic acid donates a proton to diazomethane, specifically to the carbon atom, forming the methyldiazonium ion (\(\mathrm{CH}_{3} \mathrm{~N}_{2}^{+}\)) and a carboxylate ion. This occurs because the resonance structure of diazomethane has a lone pair on its nitrogen that can stabilize the positive charge allowing protonation of the carbon. The resulting methyldiazonium ion has a positive charge on nitrogen that stabilizes the ion.

Identifying Reaction Type in Step 2

In the second step of the process, the carboxylate ion (\(\mathrm{RCOO}^{-}\)) reacts with methyldiazonium ion (\(\mathrm{CH}_{3} \mathrm{~N}_{2}^{+}\)). This is a nucleophilic substitution reaction where the carboxylate ion acts as a nucleophile, attacking the positively charged carbon in the methyldiazonium ion, which results in the formation of a methyl ester and nitrogen gas. The nitrogen gas leaves as a byproduct, driving the reaction forward.

Key concepts

Methyl Esters

Methyl esters are a type of ester where the alcohol component is methanol. These compounds are organic esters formed by the reaction of an acid and methanol. Methyl esters feature prominently in various industries, including pharmaceuticals and biofuels production.
A notable reaction for preparing methyl esters is through the esterification of carboxylic acids using diazomethane. This reaction converts the acid to its methyl ester in a streamlined manner. The process is highly efficient because it forms water as the only byproduct, ensuring a clean reaction.
The widespread applicability of methyl esters in synthesis makes them valuable. They act as intermediates in chemical reactions and are used to produce a wide array of chemical goods.

Resonance Structures

Resonance structures are different ways to represent the distribution of electrons within a molecule. These structures highlight potential electron arrangements, indicating that electrons are delocalized over a molecule rather than confined between a set of atoms.
Diazomethane (\(CH_2 N_2\)) is an excellent example of a molecule with resonance structures. This molecule can be depicted in two primary ways:

• The first resonance structure shows a neutral carbon atom bonded to two hydrogens and a nitrogen atom with a triple bond to another nitrogen. This form has no formal charge on the carbon, but a negative charge on the nitrogen holding a lone pair.
• The second structure involves electron shifts that generate a zwitterionic form, transferring lone pair electrons and creating distinct charges. The terminal nitrogen becomes negatively charged, while the central nitrogen carries a positive charge.

Understanding these structures is vital, as they explain the stability and reaction mechanisms of diazomethane.

Nucleophilic Substitution

Nucleophilic substitution is a type of chemical reaction in which a nucleophile replaces a leaving group attached to a carbon atom in a molecule. This reaction is essential in organic chemistry, as it forms the basis for synthesizing various complex molecules and compounds.
In the case of preparing methyl esters using diazomethane and carboxylic acids, the second step is a nucleophilic substitution reaction. Here, the carboxylate ion (\(RCOO^-\)) acts as the nucleophile. It targets the methyldiazonium ion (\(CH_3 N_2^+\)), attacking the carbon atom that holds the positive charge.
This reaction leads to the creation of a methyl ester and the release of nitrogen gas as a byproduct. The drive for this reaction is enhanced by the stability of nitrogen gas as a molecule, which helps propel the reaction forward smoothly.

Carboxylic Acids

Carboxylic acids are organic compounds characterized by the presence of a carboxyl group (\(-COOH\)). They are widely recognized for their acidity, resulting from the ability to donate a proton from the hydroxyl group, forming a carboxylate ion.
In the reaction with diazomethane, carboxylic acids serve as the proton donor. This protonation is essential in generating the methyldiazonium ion, a key intermediate in forming methyl esters.
Carboxylic acids vary in properties and reactivity depending on their structural components. Their uses span from the synthesis of complex esters to participation in various biochemical processes. This versatility makes understanding carboxylic acids crucial for students and professionals in chemistry-related fields.