Acidic Deprotection of an Alcohol

[Monkeydfdg64]

I have a question that states when chromic acid (CrO3, H2SO4) is added to a protected alcohol it won't react because it is protected. However, my flashcard states that protected alcohols are deprotected in acidic conditions. Wouldn't the acidity deprotect and cause the alcohol to be consequently oxidized?

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[deleted936470]

what's the alcohol's structure in particular? chromic might not be acidic enough, it oxidizes primary alcohols to carboxylic acids and it oxidizes secondary alcohols to ketones but tertiary alcohols do not react with chromic acid under most conditions..

 

[BerkReviewTeach]

Depending on what group is being used to protect the alcohol group, it is highly likely that it can (and will) be removed under acidic conditions. Take a silyl protecting group for instance, it can be removed using acid or F-. Perhaps the question was talking about using an alcohol to protect a carbonyl group as a ketal (or acetal). Under acidic and aqueous conditions, a ketal (or acetal) protecting group will be hydrolyzed and the compound will be converted into the original carbonyl. You are correct in your reasoning to think that acid will cleave the protecting group, but the key here is whether there is water present with the acid.

Can you post the question, because it is highly likely that you are correct, but they might have used a specialty reagent to keep the mixture free of water. I doubt that's the case, given that H2SO4 is extremely hygroscopic, but it would be good if you could post it to see for sure.

 

[Monkeydfdg64]

An unknown compound containing both a secondary alcohol and an aldehyde in its structure was given to a student. Suppose the Lucas test confirmed there was an alcohol present. If a protecting group were added to the alcohol, would a positive result from the chromic acid test be indicative of only the presence of the aldehyde?

A) Yes, because an aldehyde is unreactive under oxidizing conditions
B) No, because an aldehyde is reactive under oxidizing conditions
C) Yes, because a protected alcohol group is unreactive under oxidizing conditions.
D) No, because a protected alcohol group is reactive under oxidizing conditions

I said D. Answer was C

From passage:
Lucas Test
R-OH + ZnCl2 --HCl--> R-Cl + (HO-ZnCl2)-
Chromic Acid test
R-OH + CrO3 --H2SO4--> R=O + Cr2(SO4)3 + H2O

 

[BerkReviewTeach]

As that question is written, you are correct that the best answer is D. No answer is "correct", but the answer you chose is the best. I get what they were trying to ask and feel with a little editing, that could be a very good question and one that is in the style of the AAMC test writers. But the combination of CrO3 and H2SO4 must include H20, which over time would remove the protecting group from the alcohol and expose it to oxidative conditions. Even if they would have chosen PCC (CrO3, HCl, and pyridine with no water present), the aldehyde would not have been oxidized under those conditions, and no answer would have been right.

Excellent work recognizing that error. I'm not sure what the source of that question is, but the style of the question is very good and thankfully you have a strong working knowledge of organic chemistry to catch any of the minor errors they make on other questions.