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I was reading MECHANISM OF α-HALOGENATION OF ALDEHYDES AND KETONES:

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As you can see in the third step. Bromide ion is picking up a base from protonated acetone. Doesn't that step mean that protonated acetone is stronger acid than Hydrobromic acid? Is that even possible?

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Come on, you can figure out this one on your own! How do you compare acidities? Here's a helpful hint.

Armed with that information, I have to add something. I believe the mechanism as drawn above is not universal: it sure depends on the conditions and solvent used. Some other proposed mechanisms involve the enol, rather than the protonated ketone, which might be more plausible in mild conditions. In water, anyway, HBr is fully dissociated and it shouldn't be “reformed” like that. The proton will return to be solvated in the solution.

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  • $\begingroup$ I did looked up for pKa values and compared. According to the pKa values that I've seen, HBr was 1000 times stronger acid than protonated acetone. That's why the question. The pKa values given in your chart are different from what I've in my hand. $\endgroup$
    – claws
    Jan 18, 2013 at 21:08
  • $\begingroup$ @claws okay, that's weird, but evaluation of pKa values outside 0–14 is tricky business, so why not. In that case, it would have helped to include that information in your question, so we know what you have tried and not. In any case, I do agree that Br– picking up a stray H+ looks weird. I wouldn't write it that way. $\endgroup$
    – F'x
    Jan 18, 2013 at 21:10

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