By itself, Infrared (IR) spectroscopy isn’t a great technique for solving the structure of an unknown molecule. However, we’ve seen that IR spectroscopy can a great technique for identifying certain functional groups in an unknown molecule – especially functional groups containing OH or C=O.

You are watching: Given the structure below, which ir absorption band is inconsistant with the structure?

For instance, in an earlier post on the structure determination of deer tarsal gland pheromone, we saw how the authors of the study used IR spectroscopy to identify the presence of a lactone functional group (i.e. a cyclic ester) by its characteristic absorbance at 1775 cm-1.

Additionally, if you’ve narrowed down a structure to several possibilities, it can be very helpful in ruling various possibilities out.

In this post we’re going to go through four (simple) practice problems where you’ll be provided with an IR spectrum and the molecular formula, and are then charged with the task of figuring out which molecule best fits the spectrum.

Everything you need to know about IR in order to solve the problems below was presented in the previous post on how to do quick analyses of IR spectra, so go back and read that if you haven’t done so already.

Let’s begin.

(answers to each problem, along with analysis, are at the bottom of the post. Don’t peek until you’ve given each problem a thorough attempt).

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Table of Contents

Problem #1: Unknown molecule with molecular formula C5H10O. Which of these five molecules is it most likely to be?

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There’s likely room for a third post on IR spectroscopy covering some more rarely encountered functional groups and other minutiae. But for now we’re going to move on to the next technique, mass spectrometry (MS) in the next post in this series.