Q68E - Organic Chemistry (Mcmurry)

Question

21-68 When a carboxylic acid is dissolved in isotopically labeled water, thelabel rapidly becomes incorporated into both oxygen atoms of the carboxylic acid. Explain.
 $\text{R}-\stackrel{\text{O}}{\stackrel{\parallel }{\text{C}}}-\text{OH }\stackrel{{\text{H}}_{\text{2}}\text{O}}{\to }\text{ R}-\stackrel{\text{O}}{\stackrel{\parallel }{\text{C}}}-\text{OH}$

Step-by-Step Solution

Answer

Between the two oxygen of the carboxylate anion's highly resonance stabilized oxygen, there occurs a scramble for the radioactive label.

 $R-\stackrel{O}{\stackrel{\parallel }{-}}-OH+H_{2}O\stackrel{}{\rightleftarrows }R-\underset{OH}{\underset{|}{\stackrel{OH}{\stackrel{|}{-}}}}-OH$

$R-\stackrel{O}{\stackrel{\parallel }{-}}-OH+H_{2}O\stackrel{}{\rightleftarrows }R-\underset{OH}{\underset{|}{\stackrel{OH}{\stackrel{|}{-}}}}-OH\stackrel{}{\rightleftarrows }R-\stackrel{O}{\stackrel{\parallel }{-}}-OH+H_{2}O$

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: Solution Explanation

The exchange of radioactive label at the oxygen reflects scrambled aria resonance interaction at the acyl oxygen, and the reaction is a nucleophilic acyl substitution.

$R-\stackrel{O}{\stackrel{\parallel }{-}}-OH+H_{2}O\stackrel{}{\rightleftarrows }R-\underset{OH}{\underset{|}{\stackrel{OH}{\stackrel{|}{-}}}}-OH$

When traveling to the right , the OH group is added from a water molecule in the solution, but when going to the left, any of the three OH groups can be explained. Thus,

$R-\stackrel{O}{\stackrel{\parallel }{-}}-OH+H_{2}O\stackrel{}{\rightleftarrows }R-\underset{OH}{\underset{|}{\stackrel{OH}{\stackrel{|}{-}}}}-OH\stackrel{}{\rightleftarrows }R-\stackrel{O}{\stackrel{\parallel }{-}}-OH+H_{2}O$

Unlabeled, mono-labeled, and di-labeled carboxylic acid molecules achieve equilibrium.