Question 35

[Guest]

Although I got this right by process of elimination, I have to mention that this question was frustrating, because carbonyl groups show at a range of 1780-1720 (at least from what I learned, as a chemistry major). 1680-1620 are more like alkenes, which both Cortisone and cholesterol have. Just a thought.

[mcat_premed3832]

You are correct to suggest that it is a little low but it is still consistent with a carbonyl group.

Carbonyl in an amide has C=O stretch between 1690 - 1630 cm-1.
http://www.chem.ucla.edu/~webspectra/irtable.html

If you had to give just one number to memorize, I suppose you would have to go with 1700 cm-1 but stay flexible.
http://www.800mainstreet.com/irsp/eir.html

Wikipedia: the C=O double bond absorbs infrared light at wavenumbers between approximately 1600–1900 cm−1; http://en.wikipedia.org/wiki/Carbonyl

In the end, the MCAT usually goes with either 1700 for carbonyl or 3300 for -OH. It would be enormously rare (!!!) for them to ask anything else from your IR spectra toolbox. Cheers for using the process of elimination! Did you also use the cross out feature??