NMR Practice Problems
In the following example, we learn how to do the NMR application problem. Step by step in the 100-minute video solution required to detect organic structure.
The 1H proton focuses on NMR and most of the questions are based on its fundamental understanding of the number of NMR signals. Integration, differentiation of signals, and of course techniques to integrate them. It has the right structure
By solving 1H NMR and determining the Hydrogen Deficiency Index (HDI). We solve the 13C NMR, DEPT, and IR problems that occur with 1H NMR spectroscopy.
These questions were chosen to describe the most common methods on 1H NMR spectra and the possibility of overlapping signals. Where the absolute value of integration should be considered incorrect because the instrument only measures the relative field of each peak. An example of a spectrum with a lower signal is produced by larger molecules due to symmetry factors. We discuss the purpose of stirring the sample with deuterated solvents.
The structure of an unknown compound
Two things you should always do:
HDI – In general, most problems give you an unknown molecular formula and you need to know how to determine the hydrogen deficiency index. Here is its formula:
But you can find it without a formula (read about the hydrogen deficiency index here).
IR – Useful for remembering key signals in the IR spectrum so you can identify functional groups. Troubleshoot IR – here.
Before we get into the first issue or problem, let’s summarize the most common patterns you need to identify.
Patterns in NMR spectroscopy
The aliphatic region is analyzed first. Many molecules have at least one methyl group and common cleavage patterns can be observed depending on the environment.
Triplets with triple integrals represent methyl groups with adjacent CH2 groups. Therefore, look for triples and quaternions with primes 3 and 2 denoting the ethyl group.
If left with only 3 prime numbers, it represents the isolated methyl group (b). On the other hand, if the singlet is composed of 9 protons, it represents the tert-butyl group (C).
Another common piece is the combination of double (6H) and seventh (d). This represents the isopropyl group, where the methyl proton is split by the adjacent proton, and the six equivalent methyl protons split the signal on the seventh.
If you look at a set of triplets, each of which is bound to two protons, consider ethylene (e) -CH2-CH2-.
Other Regions and Splitting Patterns:
Olefin protons are produced at ~5-6 ppm and you can identify them by small prime numbers (1 or 2) and complex fission. This is because all the protons in the double bonds can be separated from each other and their binding constants are in the range of 5 to 18 Hz.
Aromatic protons occur in the region of ~7-8 ppm. If there is a code, first check how many groups (or how many protons) are on the aromatic ring. This is done by looking at the integration:
The most common pattern of cleavage in aromatic regions is the presence of two dyads, each consisting of two protons. Represents the substitution ring symmetrically with two groups (1, 4, or para substituents).
Most of the other alternative models offer a complex section where you can easily see the total premium of all scent cues.
The next functional group identified in the 1H NMR spectrum is the 1010 ppm aldehyde signal. It usually comes as a singleton, but it’s not uncommon to split with adjacent protons:
Another method is to identify aldehydes and ketones and monitor ~200 ppm signal in 13C NMR. Esters and carboxylic acids appear with a low tone: 160-180 ppm.
If you look at the broad signal at 12 ppm, it tells you about 90% carboxylic acid.
In the following NMR application questions, we review the best techniques that can be used to determine the structure of an unknown compound. As a core member of Chemistry Stages, you can access the Spectral Summary page, along with these 100-minute videos on how to solve NMR problems.