It says in the TBR Ochem book Ex.5.3 that UV absorbances of ketones and aldehydes are different because they have different pi bonds. But I don't understand why that is because they have the same carbonyl pi bond. Please help!!
Aldehyde and Ketone UV absorbances
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Is it because ketones may have more electron donating groups attached???
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Also my guess.
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This would be similar to the variation in carbonyl stretch frequency in ketones (1705-1725 )vs. aldehydes (1740-1720). The reason for the difference being related to the relative contribution of the single-bond C-O resonance structure of each.
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For what it's worth, Question 5.3 in the organic book (the current one at least) involves the conversion of an aldehyde into a acetal using excess ethanol under acidic conditions.
The answer explanation to Question 5.2 points out that the pi-bonds of an aldehyde and ketone can be distinguished by their different IR absorbances (as Bru pointed out) and their different UV absorances. This is because R-groups are electron-donating and thus make the pi-bond of a ketone slightly weaker than the pi-bond of an aldehyde. This explains why the IR absorbance (energy to stretch the pi-bond) is slightly lower for a ketone than an aldehyde.
Pages 111-112 in Book 1 address this. You'll also run into it again in one of the passages from the third homework set of that chapter.
Hey BerkReviewTech, thank you for your reply. I guess I have the old version of the book. The exact questions is this:
Pentanal can be distinguished from 3-pentanone by all the of the following EXCEPT:
A. A signal at 9-10ppm in the HNMR
B. five signals in the HNMR rather than two signals
C. an IR absorbance at 1826cm^-1
D. an ultraviolet absorbance at 230nm instead of 240nm.
The answer is C.
This implies that the aldehyde and ketone can't be distinguished by their IR absorbances but they can by their UV absorbances, which is contrary to what you're saying. That's why I'm confused as well! If you know why the book is saying this, please let me know. Thank you so much!
Pentanal can be distinguished from 3-pentanone by all the of the following EXCEPT:
A. A signal at 9-10ppm in the HNMR
B. five signals in the HNMR rather than two signals
C. an IR absorbance at 1826cm^-1
D. an ultraviolet absorbance at 230nm instead of 240nm.
The answer is C.
This implies that the aldehyde and ketone can't be distinguished by their IR absorbances but they can by their UV absorbances, which is contrary to what you're saying. That's why I'm confused as well! If you know why the book is saying this, please let me know. Thank you so much!
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I wouldn't worry too much about this for the MCAT. They usually group all carbonyl groups into the 1700 range. Still nice to know though
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The issue is not whether aldehydes and ketones can be distinguished by IR, but rather, can they distinguished by an absorbance of 1826 cm-1. AAMC expects you to know common IR absorbaces, so if you keep in mind that carbonyls are generally found around 1700 cm-1, then what makes Choice C an invalid statement is the numerical value for the absorbance, not the ability or inability to distinguish a ketone from an aldehyde using IR.
