TBR ochem question

[greenseeking]

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Hi this is the TBR Ochem Book I Pg. 71, passage XI #71.

Please refer to the attached chart that I have uploaded.

Q: Which of the following compounds can deprotonate C5H6?

A: H3CCOCH3
B. H3CCH2SH
C. H3CO-
D. H2CNO2-

The answer is C. I was debating between C and A. Can anyone please explain to me how I'm supposed to figure out the keq's and why compound A is not a base?

The back of the book said that it can't be A because it is an Acid, not a base... but can anyone tell me why? Isn't it a ketone? besides the Keq for H3CCOCH3 as an organic acid was 2.0X 10^-4 which was very low and unfavorable.

 

[hiaips]

Hi this is the TBR Ochem Book I Pg. 71, passage XI #71.

Please refer to the attached chart that I have uploaded.

Q: Which of the following compounds can deprotonate C5H6?

A: H3CCOCH3
B. H3CCH2SH
C. H3CO-
D. H2CNO2-

The answer is C. I was debating between C and A. Can anyone please explain to me how I'm supposed to figure out the keq's and why compound A is not a base?

The back of the book said that it can't be A because it is an Acid, not a base... but can anyone tell me why? Isn't it a ketone? besides the Keq for H3CCOCH3 as an organic acid was 2.0X 10^-4 which was very low and unfavorable.

By C5H6, do they mean cyclopentadiene? And I don't see which reaction this Keq pertains to, but maybe I'm missing something obvious here.

In any case, hydrogens on the alpha carbon of acetone (choice A) will be weakly acidic due to resonance in the conjugate base. That's where the claim of "acidity" comes from.

Next, keep in mind that the carbonyl oxygen of an aldehyde or ketone (the only site on acetone that could be protonated) is not particularly basic. (I *think* that the pKa of the conjugate acid is like -7 or so. In any case, it's definitely negative.) The reason for this, if I recall, is that we'd a positive charge on an oxygen that is adjacent to a carbonyl carbon.

 

[Silo004]

Draw out the molecules and think pka's.

You want the strongest base i.e. the most nucleophilic compound/ion. i.e. the thing that wants protons.

An ethoxy group is extremely basic b/c ethanol has a pka around 15, so C is the best answer.

The Ketone is neutral. If the Sulfur were also negatively charged, I might pick that one, but it's not. Also, the nitrate appears to be a zwitter ion. No way that's nucleophilic

 

[greenseeking]

ok thanks!