Ordering Carbonyl Group Wavenumbers in NMR

In summary: Those are all straight-chain molecules. Let's take a look at a molecule with a double bond in it, ethyl acetate.It has a peak at about 1610, which is due to the stretching of the C-C bond.
  • #1
Amy B
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Homework Statement


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I want to put these in order of increasing wavenumber for the absorptions of the carbonyl group in an nmr spectrum

Homework Equations

The Attempt at a Solution


I know that the third one is the highest, the NH ketoamine is the lowest and last one is the second lowest but I'm unsure about the first. Which one has higher absorption?
 
  • #3
Recall that IR works by exciting bonding electrons (for lack of a better term) and this causes an increase in rotational and vibrational energy, as well as bond stretching and bending. Also recall that electronegative atoms tend to shift things downfield, the more electronegative something is, the farther downfield it is shifted. So looking at what you've posted we have (from left to right) cyclohexanone, 3-cyclohexen-1-one, δ-valerolacetone, δ-valerolactam, and 2-cyclohexen-1-one (I'm not 100% on this, but I think we can call 3-cyclohexen-1-one just 3-cyclohexenone since the carbonyl carbon is always the first carbon in these molecules, maybe someone with a stronger knowledge of IUPAC nomenclature can verify this). The links are links to the NIST website with the relevant IR Spectra.

Right away you should see that the cyclic ether and cyclic amide (lactones and lactams respectively) will be shifted further than the usubstituted cyclohexanone. Typically carbonyl C=O stretches occur in the range of 1670 to 1820 or there about. Cyclohexanone has a peak at about 1710. Now valerolacetone has a broad peak about 1730 to about 1750. This is due to the electronegative oxygen atoms. Valerolactam has a forked peak at about 1710 and 1720. As you can see the more electronegative oxygen shifts this peak further than nitrogen does.

These are the kinds of things you should be thinking about when you look at IR stretches, what kinds of substitutions are going on and how they will move that peak away from your unsubstituted molecule (cyclohexanone in this case). Also you should consider the types of motions available to your molecule. If the bond can rotate, vibrate, and stretch/bend then it will be able to dissipate that energy better than a bond that cannot rotate or bend and that will affect how far something is shifted. For instance cyclohexanone shows a peak about 1710, 2-hexanone has a peak around 1720, and hexanal has a peak around 1750.
 
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1. Why is the ordering of carbonyl group wavenumbers important in NMR analysis?

The ordering of carbonyl group wavenumbers in NMR is important because it provides valuable information about the chemical environment and structure of a molecule. The wavenumbers can indicate the type of carbonyl group present (aldehyde, ketone, ester, etc.), the electronic nature of the group, and its proximity to other functional groups. This information is crucial in identifying and characterizing compounds.

2. How are carbonyl group wavenumbers ordered in NMR spectra?

Carbonyl group wavenumbers are typically ordered from higher to lower values in NMR spectra. This is known as the downfield region, and it is represented on the left side of the spectrum. The higher wavenumbers correspond to the more deshielded carbonyl groups, meaning they are more affected by nearby electron-withdrawing groups. The lower values, or upfield region, correspond to more shielded carbonyl groups, which are less affected by nearby groups.

3. Can the ordering of carbonyl group wavenumbers change in different solvents?

Yes, the ordering of carbonyl group wavenumbers can change in different solvents. This is because the polarity and electronic environment of the solvent can influence the chemical shifts of the carbonyl groups. For example, a more polar solvent may cause the carbonyl groups to be more deshielded, resulting in a shift to higher wavenumbers in the downfield region.

4. Are there any exceptions to the downfield ordering of carbonyl group wavenumbers?

Yes, there are some exceptions to the downfield ordering of carbonyl group wavenumbers. One example is the carbonyl group in carboxylic acids, which can appear at a lower wavenumber in the upfield region due to hydrogen bonding. Another exception is when a carbonyl group is part of a conjugated system, which can lead to a shift to lower wavenumbers in the upfield region.

5. How can the ordering of carbonyl group wavenumbers be used in NMR analysis?

The ordering of carbonyl group wavenumbers can be used in NMR analysis to determine the structure and chemical environment of a molecule. By comparing the wavenumbers to known values for different types of carbonyl groups, the presence and location of specific functional groups can be identified. Additionally, changes in the ordering can provide insights into the electronic nature and interactions of the carbonyl groups in a molecule.

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