[cupid.callin]

Which one is more acidic, (A) or (B).

Please also tell me the reason.

[NTUstudent]

I don't think that their acidity differs so much!

[cupid.callin]

I know that but anyhow i need to know which one has more acidity

Is it something that D has more +I effect than H. and if it is so please tell me why?

[orgopete]

Is that the real question? It looks as though it is a question to explain how hydrogen can become exchanged with deuterium atoms during treatment with base.

[cupid.callin]

Whatever it might be but still it is a valid question and indeed a very good one as no one still can tell me anything about it 

Please Someone Answer

[jake.n]

I agree that any difference between the two would be more interesting to a physical chemist than an organic chemist.  Anyway, here is the only thing I can find on the topic:

http://www.ncbi.nlm.nih.gov/pubmed/17358063

[orgopete]

Kudos to jake.n for the reference. Unfortunately, I do not have access to the actual article itself, but from the abstract, it appears as though they may be measuring the exchange rate at the ortho carbon as a C-H bond is mentioned. I can understand that a deuterium isotope effect could be found for the exchange. Otherwise, I am with NTUstudent in expecting little if any difference in the OH acidity due to the isotope effect.

Is there a difference in acidity of trideuteromethanol vs methanol or trideuteroacetic acid vs acetic acid?

[cupid.callin]

I checked my notes again and found this written somewhere...

"+I effect of D is more than H"
and also
"C-D bond is stronger than C-H bond"

without any reason...

can anyone explain me why?

[NTUstudent]

if +I effect of deuterium is more, then we can think as if there were methyl groups instead of deuteriums(ignoring steric effect, just to see clearly +I effect). then it comes that in resonance structure of dehydrogenated phenol, minus charge at carbon wtih methyl which is destabilize this structure, so phenol with to deuteriums will be less acidic! that's my opinion, but it might be wrong cause of other effects!

but you know, i can't relate strength of CD bond!

[g-bones]

This looks like a KIE problem to me.  Resonance of your negative charge onto those carbons "rehybridizes" them from an sp2 center to an sp3 center (not completely though because its just one resonance structure).  shifting from sp2 to sp3 yields an inverse secondary kinetic isotope effect meaning that deuterium actually favors rehybridizing a little more than hydrogen which should make that structure slightly more stable.  hope this helps.

[cupid.callin]

@ g-bones
sorry but actually the second one is less acidic.

@NTU
but how can i assume it as methyl? though it explains the answer ... it makes no sense to imagine D as CH3.

[NTUstudent]

ok! i am sorry! i think it was confusing example!
then i will try this:
after deprotonation, we have to anions! draw their resonances!
then you see that in the second phenol at minus charged carbon we have deuterium, and in the first phenol we have hydrogens! because deuterium has more +I effect it destabilize more this minus charged structure!

[cupid.callin]

Why is the +I effect of D more than H?

[cupid.callin]

Anyone?

[MissPhosgene]

It has to do with charge "concentration". For example, a small ion has a larger charge concentration than a larger ion of the same charge. Since deuterium is slightly bigger than H, the effective nuclear charge decreases, and therefore shows a larger +I effect. If you look at charge radii for H vs D you can see: proton = 0.8768(69) fm, deuteron = 2.1402(28) fm. Since deuterium is not charged, you can understand this better by understanding vibrations, ZPE, and force constants, etc. "D prefers the largest force constant". You will probably have to look at the entire reaction coordinate instead of just the first bit.