Q69E - Organic Chemistry (Mcmurry)

Question

21-69 We said in Section 21-6 that mechanistic studies on ester hydrolysishave been carried out using ethyl propanoate labeled with in theether-like oxygen. Assume that${}^{\mathbf{\text{18}}}\mathbf{\text{O}}\mathbf{-}$ labeled acetic acid is your onlysource of isotopic oxygen, and then propose a synthesis of the labeledethyl propanoate.

Step-by-Step Solution

Answer

Ester hydrolysis results in the formation of numerous $\mathbf{\text{> C=O}}$ bond cleavage methods.

$R-\stackrel{O}{\stackrel{\parallel }{C}}-O{R}^{\text{'}}+OH\stackrel{Slow}{\leftrightarrow }R-\stackrel{O}{\stackrel{|}{\underset{OH}{\underset{|}{C}}}}-O{R}^{\text{'}}\stackrel{}{\leftrightarrow }R-\stackrel{O}{\stackrel{\parallel }{C}}-OH+O{R}^{\text{'}}\to R\stackrel{O}{\stackrel{\parallel }{C}}-O^{\text{'}}+R-OH$

$R-\stackrel{O}{\stackrel{\parallel }{C}}-O{R}^{\text{'}}+OH\stackrel{Slow}{\leftrightarrow }R-\stackrel{O}{\stackrel{|}{\underset{OH}{\underset{|}{C}}}}-O{R}^{\text{'}}\stackrel{}{\leftrightarrow }R-\stackrel{O}{\stackrel{\parallel }{C}}-OH+O{R}^{\text{'}}\to R\stackrel{O}{\stackrel{\parallel }{C}}-O^{\text{'}}+R-OH$

Step 1: Introduction to the Concept

Alkyl oxygen cleavage or acyl oxygen cleavage can both cause ester hydrolysis.

Step 2: Solution Explanation

The pathway of ester hydrolysis that leads to the creation of labeled ethyl propanoate using  labeled acetic acid is shown below.

Because kinetic studies indicate that a reaction is a bimolecular event, the reaction of the second-order reaction involves an addition elimination sequence in which the nucleophilic displacement  happens in a  tetrahedral intermediate, resulting in acyl oxygen bond cleavage.

$R-\stackrel{O}{\stackrel{\parallel }{C}}-O{R}^{\text{'}}+OH\stackrel{Slow}{\leftrightarrow }R-\stackrel{O}{\stackrel{|}{\underset{OH}{\underset{|}{C}}}}-O{R}^{\text{'}}\stackrel{}{\leftrightarrow }R-\stackrel{O}{\stackrel{\parallel }{C}}-OH+O{R}^{\text{'}}\to R\stackrel{O}{\stackrel{\parallel }{C}}-O^{\text{'}}+R-OH$ 

${}^{\mathbf{\text{18}}}\mathbf{\text{O}}$ labeling experiments and stereo chemistry analyses have yielded evidence for acyl oxygen bond cleavage. When ethyl propanoate was hydrolyzed by NCOH in water supplemented with, ${}^{\mathbf{\text{18}}}\mathbf{\text{O}}$the outcome was ETOH with no ${}^{\mathbf{\text{18}}}\mathbf{\text{O}}$  label.
 $R-\stackrel{O}{\stackrel{\parallel }{C}}-O{R}^{\text{'}}+OH\stackrel{Slow}{\leftrightarrow }R-\stackrel{O}{\stackrel{|}{\underset{OH}{\underset{|}{C}}}}-O{R}^{\text{'}}\stackrel{}{\leftrightarrow }R-\stackrel{O}{\stackrel{\parallel }{C}}-OH+O{R}^{\text{'}}\to R\stackrel{O}{\stackrel{\parallel }{C}}-O^{\text{'}}+R-OH$

data-custom-editor="chemistry" $\mathbf{\text{sec }}\mathit{â}\mathit{€}\mathbf{"}\mathit{b}\mathit{u}\mathit{t}\mathit{y}\mathit{l}$ alcohol of reaction $+{13.8}^{\circ }$is obtained once again when sec butyl alcohol of reaction $+{13.8}^{\circ }$ is transformed into the benzoate ester and that ester is hydrolyzed by di alkali. This perfect configuration preservation shows that the bond between oxygen and the acyl group fissions.