A student synthesizing acetone from propan-2-ol used IR spectroscopy to monitor the progress of the reaction. Which of the following would have indicated the reaction was progressing? A) A strengthening absorption in the 3300 to 3400 centimetres to the minus one range. B) A weakening absorption in the 1700 to 1720 centimetres to the minus one range. C) A strengthening absorption in the 1700 to 1720 centimetres to the minus one range. D) A weakening absorption in the 1600 to 1650 centimetres to the minus one range. Or E) a strengthening absorption in the 1600 to 1650 centimetres to the minus one range.
Let’s begin by pulling the most important information out of the question. We’re told that this student is using infrared spectroscopy to monitor the progress of the reaction. So let’s remind ourselves what infrared spectroscopy actually tells us about molecules.
IR spectroscopy can give us information about the bonds in a molecule. It can also help us to identify particular functional groups, for example, an OH. This means it can be helpful to use IR spectroscopy to monitor reactions where the reactant and the product have different functional groups. So let’s take a closer look at the reaction in this question.
The student is synthesizing acetone, which will be our product, from propan-2-ol, which is our reactant. Here, we have the molecular structures of our reactant, propan-2-ol, and our product, acetone. Let’s look closer at both of these, in particular, focusing on the functional groups. We can see that the three-carbon backbone in each of our molecules is the same. This means that these peaks in our IR spectrum are unlikely to change very much. This leaves us with the oxygen containing part of the molecule. In our reactant, we have an alcohol hydroxyl group. Whereas in our product, we have a ketone carbonyl.
Since these are the main changes in our molecule during this reaction, we should focus on the peaks that these bonds are responsible for. The OH peak in our infrared spectrum will be fairly obvious. It will appear at around 3400 centimetres to the minus one. And it will be very broad. We can also refer to these units as wave numbers. This OH peak is very broad due to hydrogen bonding. The carbon–oxygen single bond in propan-2-ol will appear at approximately 1000 wave numbers. We can compare this to the carbon=oxygen double bond found in our carbonyl. This carbonyl group appears at around 1700 wave numbers and it’s very strong. It’s often the strongest peak in an IR spectrum.
Now, we just need to work out what’s going to happen to these peaks. As we convert propan-2-ol into acetone, the peaks coming from propan-2-ol bonds will weaken. As the reaction progresses and we create more acetone, the peaks in the acetone spectrum will increase or strengthen. So the broad OH peak at 3400 wave numbers and the sharp CO peak at 1000 wave numbers will both weaken. At the same time, the carbonyl peak at 1700 wave numbers will become stronger, as we create more acetone and therefore more of these carbonyl bonds.
Let’s now compare this information to the possible answers. Answer A mentions a peak in the 3300 to 3400 wave number range. This corresponds to the OH peak of our reactant. The answer suggests a strengthening of the peak from our reactant, which is not correct. The peaks from our reactant will weaken as the reaction progresses. So this is not the right answer. B suggests a peak in the 1700 to 1720 wave number range. Answer C also discusses the same range. This range is approximately where our carbonyl peak will appear. The carbonyl is, of course, in our product. So as the reaction progresses, we’ll generate more product and this peak will get stronger. So answer B is incorrect. However, answer C does predict a strengthening in this carbonyl range. So this is a correct answer.
Let’s check the last two just to be safe. Answers D and E discuss a peak in the range of 1600 to 1650 wave numbers. None of the key functional group peaks in our two molecules appear in this range. Remember that we can ignore any peaks coming from CH bonds since these are in both the reactant and the products. So we can rule out both of these answers.
So when we monitor the oxidation of propan-2-ol to form acetone using infrared spectroscopy, we will notice a strengthening absorption in the 1700 till 1720 wave number range.