December 09, 2019, 09:41:19 PM
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Topic: Difference in wave number for the O-H stretch in carboxylic acids and alkanols  (Read 4452 times)

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Offline ajax0604

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Could someone explain why the wave number associated with the O-H stretch in carboxylic acids is lower at 2500-3300, while the O-H stretch in alcohols is higher at 3230-3550? 

Offline Borek

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Can you think of any relationship between wavelength and bond energy?
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Offline ajax0604

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The wave number for the OH stretch in carboxylic acid is lower which means that the bond energy isn't as great as it is in an alcohol? Does this have something to do with the C=O group?

Offline Borek

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What happens to acids in water? What happens to alcohols in water?
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Offline ajax0604

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So carboxylic acid will partially ionise whereas alcohol will form hydrogen bonds with water? 

Offline Borek

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Yes, acid will ionize, alcohol will NOT ionize.

Do you see a connection with bond strength?
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Offline ajax0604

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Yes I get the connection. Why does the proton come off more readily from the carboxylic acid?

Offline Borek

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One answer you already know: because its bonding energy is lower.

Second answer (which is generally the same, just makes a handwavy argument about it): -COO- is stabilized by resonance.
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Offline ajax0604

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Thanks. It's your second answer I didn't think of until you mentioned it - the -COO- is stabilized by resonance whereas alcohols have no such resonance forms.

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