Q19-8P - Organic Chemistry (Mcmurry)

Question

The oxygen in water is primarily (99.8%) 16O, but water enriched with the heavy isotope ${}^{18}\mathbf{O}$ is also available. When an aldehyde or ketone is dissolved in ${}^{18}\mathbf{O}$-enriched water, the isotopic label becomes incorporated into the carbonyl group. Explain.

${\mathbf{R}}_2\mathbf{C}\mathbf{=}\mathbf{O}\mathbf{+}{\mathbf{H}}_2\mathbf{O}\rightleftharpoons {\mathbf{R}}_2\mathbf{C}\mathbf{=}\mathbf{O}\mathbf{+}{\mathbf{H}}_2\mathbf{O}$ where O = ${}^{18}\mathbf{O}$

Step-by-Step Solution

Answer

Step-by-Step Solution Step 1: Isotope labeling

Isotope labeling is a technique used in the “investigation of rhe mechanism” of the chemical reaction.

E.g.,Hydrolysis of ester, when carried out with labeled oxygen $\left({}^{18}\mathbf{O}\right)$ , results in the formation of alcohol with labeled oxygen atoms which concludes that the “cleavage of the alkyl - Oxygen” bond has occurred.

Step 2:Incorporation of Isotope labeling into a carbonyl group

The nucleophile addition reaction of the water on the ketone/aldehyde is shown as follows:

Labeled oxygen ${}^{\text{18}}\text{O}$(represented by *)

Nucleophile addition reaction

The steps that involve nucleophile addition of water (neutral nucleophile) on the carbonyl carbon of aldehyde or ketone are all reversible.

As all the steps are reversible, the mechanism of nucleophile addition on the carbonyl carbon, the reverse mechanism, can be shown, responsible for incorporating labeled oxygen ${}^{\text{18}}\text{O}$(represented by *) on the carbonyl carbon.

 

The reverse reaction of nucleophile addition of water in aldehyde/ketone is shown as follows:

Reverse nucleophile addition reaction