TBR, Ochem, Structure Elucidation, Passage 4 # 25

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Sammy1024

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Choice A is correct. A broad signal near 2850 cm"1 indicates the hydroxyl group of a carboxylic acid (O-H with hydrogen-bonding). A hydroxyl group exhibits hydrogen-bonding, so the signal is broad. Because the O—H bond of a carboxylic acid is weak, its absorbance is lower than that of standard hydroxyl groups. Of the answer choices, only choice A has a carboxylic acid functionality, let alone a hydroxyl group. Pick A for happiness.

I see the answer, and I suppose it makes sense, but I was wondering how I was supposed to conclude that. The only information from passage about signals below 3000 (except the info about 1715) is the following:

If a carbon has sp3-hybridization, the bonds it forms to hydrogen are found just below 3000 cm'1.

I thought that phrase was talking about a CH3.

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You're right, Sammy. That phrase you mentioned is talking about C-H bonds, and is put in the passage strictly to throw you off.

The "gimmes" in this particular question are:
  • the molecule has oxygens
  • it is exhibiting a broad IR signal around 3000 cm^-1.
If you have these two things in a question (or IR spectra), you can bet your ass it has an -OH group (this is the first thing you learn in IR spectra interpretation). This question is easy because only one of those isomers actually has an -OH group.

Hope this helps!
 
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Oh!! Omg! I should have read the question more carefully! I got all caught up in the phrase in the passage! Thank you!
 
AWESOME insight medic86!!! That is exactly what that question was aiming to extract from test takers. We get caught up in certain facts and overlook something that is crucial. The key thing here was the broad signal. The number can vary from compound to compound, but the broadness is a signature for H-bonding.
 
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