One frequently used method for preparing methyl esters is by...

One frequently used method for preparing methyl 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 $2 ?$

One frequently used method for preparing methyl 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 $2 ?$

Key Concepts

Resonance Structures

Resonance structures refer to the different ways electrons can be arranged in a molecule without changing the overall connectivity of the atoms. For molecules like diazomethane, drawing multiple resonance structures helps illustrate the delocalization of charge and electron density between the carbon and the adjacent nitrogen atoms, which is key to understanding its reactivity, particularly how it can be protonated by an acid.

Protonation Mechanism

The protonation mechanism involves the transfer of a proton from an acid to a basic site on a molecule. In the context of the reaction between a carboxylic acid and diazomethane, the carboxylic acid donates a proton to diazomethane, leading to the formation of a methyldiazonium ion. This step demonstrates how acids can activate weak bases by protonation, making the molecule more electrophilic and setting the stage for further reaction.

Nucleophilic Substitution Reaction

Nucleophilic substitution reactions occur when a nucleophile (a species with an electron pair) attacks an electrophilic center, displacing a leaving group. In the reaction discussed, after protonation forms the methyldiazonium ion, the carboxylate ion acts as a nucleophile and attacks this electrophilic center. This step exemplifies a nucleophilic substitution process, where the nucleophile's attack leads to the formation of a new bond and ultimately the methyl ester product.

Transcript

00:01 Okay, this problem is having us describe a reaction between diazomethane and carboxylic acid.

00:06 So first i'm going to write down my carboxylic acid and then my diazomethane.

00:12 So diazomethine is ch2n2, so i'm going to write down my ch2, and i'm going to double bond that to a nitrogen.

00:20 If that is double bonded to nitrogen, i'm going to double on this nitrogen to another one.

00:25 So the center one has a positive charge, and this rightmost one has a negative charge.

00:32 So if that has a negative charge and this has a positive charge, then this is a neutral molecule.

00:38 Okay, so this is one resonance form.

00:41 This is one.

00:42 Another resonance form would be if i were to manipulate this compound in a way that i can move the positive and negative charges around.

00:49 So i'm going to move the electrons from this rightmost nitrogen onto this double bond to create a triple bond.

00:56 And if i do that, then this nitrogen will have exceeded its octet.

00:59 So i need to manipulate this molecule even further so that, that does not occur.

01:04 So i don't have an exceeded octet on that nitrogen.

01:06 So i'm going to move the electrons from this carbon nitrogen bond onto this carbon.

01:11 I'm moving everything to the left.

01:13 Okay, so i should have the molecule in which i have a lone pairs on the carbon.

01:22 So that is now a single bond.

01:23 I have the positive charge of my nitrogen still, but i have a triple bond between the nitrogen's instead.

01:29 Okay, so i just moved my positive and negative charges around...

Please give Ace some feedback