[kriggy]

@enthalpy: depends on what do you mean by few atoms? Usually you dont need more than 2 deuteriums for labeled reference compounds and using the conditions described, I got 6 protons (out of 8 ) fully deuterated (98% D incorporation according to NMR) and the last two without any deuterium at all.
I suggest you look into the paper because you get basically full deuteration of aromatic (with Pt/C catalyst) or aliphatic (with Pd/C catalyst) protons so there is really no need to do it in a way you desribe (for small amount of compound in laboratory scale).

[Enthalpy]

Hi Rolnor, Kriggy and the others,

I should have told it more clearly. The paper's single step achieves a strong replacement of H by D in the target molecule because D₂O is in large excess indeed.

To my understanding, less D₂O would limit the achieved D/H proportion at the target as water loses some D exchanged for H. There is no selectivity here, only some levelling among the molecules.

For some uses, for instance to produce deuterated molecules for sale, the consumption of D₂O would be a waste. To bring the same amount of D to the target, exploiting D₂O from <100% purity to >98% means <2% of the bought D is used, letting consume 50* too much D₂O.

My multi-step (or continuous) proposal, not quite convenient at lab scale for sure, lets save on the D₂O consumption by depleting it further and still achieving high D proportion in the target.

[wildfyr]

You can always collect your D2O/H2O mixture and repurify, but also you can use the same solution over and over. A 100:3 ratio is going to substitute D for H about as well as a 97:3 ratio. It's only after several cycles that you may start to see worse substitution.

[Enthalpy]

Yes.

Purifying D₂O and using it is often done at different companies.

Depending on the sought substitution and the excess of D₂O, it can serve again, but the consumption per amount of product remains.

Hence I certainly don't claim that my multistep setup is always necessary. It can be interesting in some circumstances, like the amount production of heavily substituted compounds.