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Chemistry Forums for Students => Organic Chemistry Forum => Organic Spectroscopy => Topic started by: ajax0604 on January 20, 2014, 05:50:23 AM

Title: Difference in wave number for the O-H stretch in carboxylic acids and alkanols
Post by: ajax0604 on January 20, 2014, 05:50:23 AM
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? 
Title: Re: Difference in wave number for the O-H stretch in carboxylic acids and alkanols
Post by: Borek on January 20, 2014, 06:09:08 AM
Can you think of any relationship between wavelength and bond energy?
Title: Re: Difference in wave number for the O-H stretch in carboxylic acids and alkanols
Post by: ajax0604 on January 20, 2014, 06:23:41 AM
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?
Title: Re: Difference in wave number for the O-H stretch in carboxylic acids and alkanols
Post by: Borek on January 20, 2014, 07:06:42 AM
What happens to acids in water? What happens to alcohols in water?
Title: Re: Difference in wave number for the O-H stretch in carboxylic acids and alkanols
Post by: ajax0604 on January 20, 2014, 06:34:53 PM
So carboxylic acid will partially ionise whereas alcohol will form hydrogen bonds with water? 
Title: Re: Difference in wave number for the O-H stretch in carboxylic acids and alkanols
Post by: Borek on January 21, 2014, 03:15:39 AM
Yes, acid will ionize, alcohol will NOT ionize.

Do you see a connection with bond strength?
Title: Re: Difference in wave number for the O-H stretch in carboxylic acids and alkanols
Post by: ajax0604 on January 22, 2014, 04:13:49 AM
Yes I get the connection. Why does the proton come off more readily from the carboxylic acid?
Title: Re: Difference in wave number for the O-H stretch in carboxylic acids and alkanols
Post by: Borek on January 22, 2014, 04:20:51 AM
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.
Title: Re: Difference in wave number for the O-H stretch in carboxylic acids and alkanols
Post by: ajax0604 on January 22, 2014, 06:52:16 AM
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.