Introduction:
Three certainties in life are death, taxes, and an NMR spectroscopy question on your A-Level Chemistry exam paper! While I can’t help with the first two, this article will provide a comprehensive guide on how to decipher NMR spectra, breaking down the process into manageable steps for any NMR question.
Exam boards may have varying expectations about NMR process mechanics, but this article will focus specifically on interpreting the spectra. You’ll encounter two types of NMR spectra: 13C NMR and 1H NMR, which we’ll explore one by one.
1. 13C NMR Spectroscopy:
13C NMR is the more straightforward of the two NMR types. Each peak in the spectrum represents a different carbon environment. Let’s take propan-2-ol as an example.
Propan-2-ol contains three carbon atoms, but due to the molecule’s symmetry, carbon atoms 1 and 3 are identical. Thus, we have two unique carbon environments, leading to two peaks in the 13C NMR spectrum.
Now, let’s identify the peaks: the distance from 0 on the x-axis, called the chemical shift, helps us do that. Exam boards provide a data sheet with chemical shift values. Below is a section from the AQA Chemistry A-Level data sheet:
Carbon atoms 1 and 3 are bonded to carbon and hydrogen atoms, so we would expect a peak in the 5-40 ppm region. Carbon atom 2 is bonded to an oxygen atom, and we can expect a peak between 50-90 ppm.
In the spectrum, we can see a peak around 25 ppm (peak A) and another around 65 ppm (peak B). Peak A corresponds to carbon atoms 1 and 3, while peak B corresponds to carbon 2.
Note that the scale is relative to a standard chemical, tetramethylsilane (TMS), which gives a single peak at 0 ppm for both 13C NMR and 1H NMR.
The data sheets provided by different exam boards may differ in appearance, but the information given is consistent. Familiarize yourself with your specific data sheet to avoid wasting precious exam time searching for necessary information.
2. 1H NMR Spectroscopy:
1H NMR provides more information than 13C NMR. There are three key features to look at:
This is similar to 13C NMR, where you look at the data sheet to find the range in chemical shift. However, this time you’re looking at hydrogen atoms that share a carbon atom with various groups. A section of the OCR (A) data sheet is shown below:
Let’s revisit propan-2-ol.
We need to consider the number of different hydrogen environments. The molecule has two equivalent CH3 groups, one CH group, and one OH group. Therefore, we would expect to see three distinct peaks in the 1H NMR spectrum.
Looking at the data sheet, the CH3 groups are connected to a carbon atom, so we would expect a peak in the range 0.5-2.0 ppm. The CH group is also bonded to an alcohol group, and the data sheet tells us to expect a peak between 3.0-4.2 ppm. Finally, the OH group could appear anywhere between 1.0-12.0 ppm.
This is what we see in the spectrum. By measuring the area under each peak, you can determine the relative number of hydrogen atoms represented by each peak. In the exam, you’ll be given integration numbers. It’s important to remember that these are simplified ratios – your actual molecule could have multiples of these.
Example:
Chloroethane has two different hydrogen environments: a CH2Cl group and a CH3 group. Therefore, we would expect the integration values to have a 2:3 ratio, which is exactly what we observe in the spectrum.
The numbers above each peak represent the integration values for each peak. They give the relative ratio of hydrogen atoms in each environment.
However, beware! Other molecules may have the same integration values. For instance, pentan-3-one has two different hydrogen environments, but the molecule may have the same integration values.
In conclusion, mastering NMR spectroscopy is an essential skill for acing your A-Level Chemistry exam. Practice interpreting spectra using the data sheets provided by your exam board, and familiarize yourself with the key features to look out for in both 13C NMR and 1H NMR spectra. Good luck!
