Q21-75E - Organic Chemistry (Mcmurry)

Question

21-75 One frequently used method for preparing methyl esters is by reactionof carboxylic acids with diazomethane, ${\mathbf{\text{CH}}}_{\mathbf{\text{2}}}{\mathbf{\text{N}}}_{\mathbf{\text{2}}}$

The reaction occurs in two steps: (1) protonation of diazomethane bythe carboxylic acid to yield methyldiazonium ion,, ${\mathbf{\text{CH}}}_{\mathbf{\text{3}}}{{\mathbf{\text{N}}}_{\mathbf{\text{2}}}}^{\mathbf{+}}$plus a carboxylate ion; and (2) reaction of the carboxylate ion with ${\mathbf{\text{CH}}}_{\mathbf{\text{3}}}{{\mathbf{\text{N}}}_{\mathbf{\text{2}}}}^{\mathbf{+}}$ .
 (a) Draw two resonance structures of diazomethane, and account forstep 1.
(b) What kind of reaction occurs in step 2?

Step-by-Step Solution

Answer

a) The resonance structures of diazomethane,

$\begin{array}{l}\underset{H}{\overset{H}{\underset{\left|}{\overset{\right|}{C}}}}=N=\stackrel{+}{N}\underset{}{\leftrightarrow }\underset{\updownarrow }{{\text{H}}_{2}C-\stackrel{+}{N}=N}\\ \underset{}{\leftrightarrow }{\text{ H}}_{2}C=\underset{+}{N}=N^{-}\end{array}$

b) 

The reaction occurs in step 2

Step 1: Introduction to the Concept

The arrangement of chemical bonds between atoms in a molecule (in anion or radical with many atoms), particularly which atoms are chemically bound to which other atoms with what kind of chemical bond, as well as any geometric shape information needed to uniquely identify the type of molecule.

Step 2: Determination of resonance structure of CH 3 N 2 +

a)

The resonance structures of diazomethane,

 $\begin{array}{l}\underset{H}{\overset{H}{\underset{\left|}{\overset{\right|}{C}}}}=N=\stackrel{+}{N}\underset{}{\leftrightarrow }\underset{\updownarrow }{{\text{H}}_{2}C-\stackrel{+}{N}=N}\\ \underset{}{\leftrightarrow }{\text{ H}}_{2}C=\underset{+}{N}=N^{-}\end{array}$

Step 3: Determination of kind of reaction occurs

b)

As illustrated above, the reaction of step 2 includes a nucleophilic acyl substitution wherein the oxygen nucleophilic assaults the carbon of diazomethane.

Nucleophilic acyl substitution happens on the carbonyl carbon by an addition elimination process.