Chem 112B W12 Problem Set 1 Problem Set 1 Answer Key –Infrared and Ultraviolet Spectroscopy Question 1. Describe how you would use IR spectroscopy to distinguish the following pairs of molecules: NOTE –all the molecules will have C-H stretches ~2950 cm-1, so you can’t use that to distinguish them. a)Explanation: Look for the N-H vs O-H stretch –O-H absorbs ~3400 cm-1, N-H~3200 cm-1. Also, the NH2will give TWO peaks from the symmetric and antisymmetric stretch, O-H only one. b)Explanation: One is an alcohol and has an O-H stretch (~3400 cm-1), one has none.c)Explanation: The keto-alcohol will have stretches at ~3400 cm-1for the OH and ~1725 cm-1for the ketone, whereas the ester will have a stretch at 1740 cm-1and no otherd)Explanation: Ester vs ketone –the ester (left molecule) will have a higher absorbance (although not by much).e)Explanation: The terminal alkyne will show a C-H stretch at 3300 cm-1, the internal alkyne will only show C-H stretches (as it is symmetrical, no C≡C stretch will be observed) f)Explanation: tert-butane is symmetrical, and so there will be no C-C stretches (although you should see weak C-H stretches). n-Pentane will have C-C and C-H stetches
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