#47 Ch. 7 Orgo: I don't like answer a because it's saying that the amine group is more acidic than CF2OH which doesn't make sense. Isn't the pKa of NH3+ around 10? CF2OH should be much less than that. Answer A is correct in that the carboxylic acid is listed as the most acidic, but it seems somewhat strange to list c (R-NH3+) as being more acidic than R-CF2OH. It seems like a question asking order of relative acidity/basicity/nucleophilicity/et al should have the entire sequence be correct not just the first or last.
TBR Orgo Ch. 7 #47
- Thread starter dougkaye
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It makes perfect sense guy, look at your problem again RCF2OH. Draw this structure out there is NO HYDROGEN connected to the Carbon thus it would not even be considered acidic. I want you to ask yourself a question! what is an acidic compound? Something that donated Hydrogens right? (well thats the Bronsted lowry definition of an acid 😉 but guess what!!!! RCf2OH doesn't even have H connected to the C to donate! Had it been (H)CF2OH than YES YOU would have been right that it's more acidic than R-NH3+. Just have to pay attention to the minute details champ good luck!
Um... What? Why do you need an H connected to a C for it to be an acid? CF2OH can donate an H, although it is not as acidic as R-NH3+. HCF2OH would not be more acidic than R-NH3+. The H that would even be donated is not the one bonded to the C but the one bonded to the O. Is HCl not an acid because there is no C that the H is bonded to?
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R-NH2 and R-NH3+ are actually a conjugate pair where both are weak. R-NH2 has a pKb = about 4-5, and R-NH3+ has a corresponding pKa = 9-10
That's just a side note, I don't really have the greatest explanation for WHY R-NH3+ is more acidic than R-CF2OH, except that alcohols tend to be very weak acids, i.e., pKa > 14.
The fluorines, being more electronegative than oxygen, actually withdraw electron density from that -OH group, making it more acidic. That's the inductive effect, and might lead you to believe, "hey, here's an extra good alcohol, as far as acidity goes." The solution to 47 alludes to this a little, but you seem to be pretty clear on the -COOH vs. -CF2OH comparison, so no need to beat that point any more. I guess the lesson here is that the inductive effect of two fluorines isn't strong enough to boost the very weak alcohol into the pKa lower than 10 range, where it would beat the NH3+.
Whole other story here, sort of, but hydrogens bonded to carbons aren't usually acidic at all, except for special cases like alpha-carbonyl carbons, and those are still very weak (pKa = 19 or so) so flodhi1's post is a little confusing. If you compare an alkane and an alcohol, the alcohol, weak as it is, is way more acidic than random hydrogens in the alkane.
