Reaction of acetone with D3 O^+ yields hexadeuterioacetone. That is, all the hydrogens in aceton

Reaction of acetone with D3 O^+ yields hexadeuterioacetone....

Key Concepts

Keto-Enol Tautomerism

This concept involves the interconversion between a carbonyl compound (the keto form) and its corresponding enol form. In many carbonyl compounds such as acetone, the hydrogen atoms adjacent to the carbonyl group (alpha hydrogens) are acidic and can be removed under acidic or basic conditions. The resulting enol, which contains a C=C double bond and an -OH group, can readily revert back to the keto form. This equilibrium is central to many reactions of carbonyl compounds including isotopic exchange processes.

Acid-Catalyzed Proton (Deuteron) Exchange

Acid-catalyzed hydrogen exchange occurs when the alpha hydrogens of a carbonyl compound are replaced by deuterons (D+) from a deuterated acid like D3O+. Under acidic conditions, protonation of the carbonyl oxygen increases the acidity of the adjacent hydrogens, facilitating their removal and the formation of an enol intermediate. This enol can then capture a deuteron from the solvent, and through repeated cycles of enolization and re-protonation (or deuteronation in this case), all exchangeable hydrogens can be replaced by deuterium.

Electrophilic Activation in Mercury-Catalyzed Reactions

Mercuric ion-catalyzed addition reactions, such as the hydration of alkynes, rely on electrophilic activation of unsaturated carbon–carbon bonds. In these mechanisms, the mercuric ion coordinates to the multiple bond, rendering it more susceptible to attack by a nucleophile such as water. Although the reaction with acetone and deuterated water doesn’t use mercury, the analogy lies in forming an activated intermediate (like the enol here) which then undergoes nucleophilic attack (in this case by D3O+). This parallel highlights the common theme of electrophilic activation to facilitate subsequent chemical transformations.

Isotopic Labeling with Deuterium

Isotopic labeling involves replacing a specific atom in a molecule with its isotope—in this example, replacing hydrogen with deuterium—to trace reaction pathways or alter physical properties. In the acetone deuteration process, repeated cycles of acid-catalyzed proton-deuteron exchange, driven by the keto-enol equilibrium, lead to a molecule in which nearly all the exchangeable hydrogens are substituted with deuterium. This method is widely used in mechanistic studies, spectroscopy, and in the synthesis of labeled compounds for research purposes.

Transcript

00:02 In problem 9 .48, we are asked to provide a mechanism for changing all the hydrogens on acetone into deuteriums.

00:13 Now, we are asked to look at the mercury -catalyzed hydrogenation of an alkynes to get an idea of the correct mechanism for this problem.

00:26 At the end of the mercury -catalyzed mechanism, there is the keto -enyl ptotomerism that is present.

00:34 In many ketones.

00:37 I will draw the keto and enol forms below.

00:42 The keto form is more preferred than the email form, and this will be the major product.

01:13 So the keto form is on the left, and the enol form is on the right.

01:18 It is by this mechanism that the deuteriums will be added.

01:24 I will show the electron, the mechanisms with the electron flow below.

01:31 And i will react to them.

01:32 With deuterium.

01:37 So we start out with our keto form and each end of the carbon will have three hydrogens, but i'll only focus on the right side in this problem.

02:00 This molecule will then be reacted with d3o as opposed to h3o and the keto will pick up one of the deuteriums.

02:14 So we have od3, two lone pairs on oxygen and the lone pairs on the keto oxygen will pick up one of the deuteriums attached to the od3.

02:58 So this oxygen will pick up a deuterium leaving its electrons, and this will go back and forth with the next intermediate.

03:13 In the next intermediate, we have the oxygen with a double bond to the carbon.

03:32 That will now have a deuterium.

03:38 And this will have a positive charge.

03:42 We still have the same three hydrogens on the end.

04:03 And this will have another resonant structure, which will help predict the enol form and how the deuteriums are incorporated.

04:37 So the electrons will fold up from our keto form...

Please give Ace some feedback