Please see this article:

Freisleben A, Schieberle P, Rychlik M.

J Agric Food Chem. 2002 Aug 14;50(17):4760-8.

The problem you have is that the amino group is ortho directing (the para position being occupied by the carboxylic acid), and your carboxylic acid directs meta to itself. This is why you only get D atoms substituting ortho to your amine. A possible way around this problem is to oxidise your amino to a nitro group (trifluroperaceitc acid should do this), then you will have two meta directing groups para to each other - they are unfortunately both deactivating, so you will need quite forcing conditions to deuterate all 4 ring hydrogens. When you have done this, then reduce the nitro back to amino. Long winded, old fashioned and quite messy, but this is the best suggestion I can offer. Good luck

Starting with p-toluidine, thus having two activating o-, p- directing group on benzene ring will do it. After H/D exchange experiment oxidize the methyl group to COOH by an oxidizing agent like permanganate.

Good luck

For the H/D four ring protons exchange of p-toluidine:

40.0 gms of p-toluidine-hydrochloride dissolved in 50 ml D20 are refluxed for 2 h and evaporated to dryness. The residue is dissolved in D20 (ca 40 mill the solution acidified to pH 3 with DC1 and heated for 24 h at 250°C for total ring deuteration in a V2A container provided with a glass-vessel. The mixture is evaporated to dryness and the residue recrystallised from D20 or D20-steam distilled after neutralisation.

--- C.G.B.Frischkorn, H.Kohler, B.Rose, D.Unterlugauer, H.M.Conrad, Journal of Labelled Compounds and Radiopharmaceuticals (1978), 14(4), 507-13.

* You still have to oxidize the methyl to carboxyl

But if you would like to start with p-aminobenzoic acid then need to see the reference provided by Dr. Fathi Aqra

4-Aminobenzoic acid (potassium salt ~ 3 g) was suspended with palladium on activated charcoal (~4 mg Pd) in deuterium oxide (15 mL) and heated at 200 °C in an autoclave for 2-3 h. The autoclave was cooled with cold water and the suspension was filtered and washed repeatedly with redistilled water. The solution was then lyophilized and the above process was repeated twice to give pure target molecule.

--- A. Freisleben , P. Schieberle , M. Rychlik, J Agric Food Chem. (2002) 14;50(17):4760-8

Good luck

Dear Dr.Adel Amer /Dr. Paul Carolan,

Thanks for the solution, I have tried Dr. Fathi Aqra process. but it is not working practically. I also tried deuteration of 4-nitrobenzoic acid as suggested by Dr. Paul Carolan bu no luck.The ring is highly deactivated.

But i think it may work with p-toluidine

thanks

In our lab, we have made varous deuterated aromatic compounds. Our condition is that heating a compound in D2O-10% Pd/c(10%w/w)-5% Pt/C(5%w/w)-NaBH4(trace) under microwave. 1-3 cycles. D%>98%.