α - protons in D2O

[caffeine jitters]

Hello all,
I am wondering about base-promoted vs. acid-catalyzed deuteration. I know that when we talk about base-promoted halogenation, the base will give us a multi-substituted alpha carbon. Conversely, with the acid, we will get a product where only one of the alpha-hydrogens is substituted with halogen. So, what happens with deuteration? Should we expect to see all the hydrogens substituted for deuterium when a base is used, and only one when acid is used?

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[Syxx]

For deuterium exchange reactions, it doesn't matter which one you choose to use (acid or base), all alpha hydrogens will be replaced if you have an excess of deuterium.

The difference in the acid catalyzed reactions stems from the effect that adding a very electronegative group has on the keto to enol tauterism. Remember in acidic conditions you need to be able to form the enol for the reaction to proceed. In doing this the reaction goes through a carbocation intermediate at the carbonyl carbon. When you try to do this a second time (after the first substitution) the inductive effect of bromine now makes this a very unfavorable intermediate and inhibits the enol from forming.

 

[caffeine jitters]

For deuterium exchange reactions, it doesn't matter which one you choose to use (acid or base), all alpha hydrogens will be replaced if you have an excess of deuterium.

The difference in the acid catalyzed reactions stems from the effect that adding a very electronegative group has on the keto to enol tauterism. Remember in acidic conditions you need to be able to form the enol for the reaction to proceed. In doing this the reaction goes through a carbocation intermediate at the carbonyl carbon. When you try to do this a second time (after the first substitution) the inductive effect of bromine now makes this a very unfavorable intermediate and inhibits the enol from forming.

awesome explanation, thank you for your help