2 BUTANONE .....SPECTROSCOPY PROBLEM

Butanone, also known as methyl ethyl ketone or MEK, is an organic compound with the formula CH₃C(O)CH₂CH₃. This colorless liquid ketone has a sharp, sweet odor reminiscent of butterscotch and acetone. It is produced industrially on a large scale, and also occurs in trace amounts in nature. It is soluble in water and is commonly used as an industrial solvent

Example 

C₄H₈O
MW 72
First calculate the degree of unsaturation: the answer is 1. This means that the compound has four carbons and an oxygen, it can have a carbon-carbon double bond, a carbon-oxygen double bond - a carbonyl, or a ring.

IR Spectrum

A table of characteristic IR absorptions is available online: click on the link below. Note that this chart is also linked to in the frame to the left.
The IR spectrum for Example 1 is below. Since the degree of unsaturation indicates that the compound could have a carbonyl, let's look for that first, since carbonyl bands are strong and distinct. Carbonyls show up in the region 1760-1665, and specifically, saturated aliphatic ketone close to 1715. Sure enough, there is a band at 1718 indicating a saturated aliphatic ketone.

Think of possible structures

Now we know that the compound has a carbon-oxygen double bond, but there are still a few ways that this four-carbon molecule could be put together. Examples are below:

The second and third structures above are saturated aliphatic aldehydes, which show up at 1740-1720. While in the above IR spectrum the band at 1715 might be close to the range of saturated aliphatic aldehydes, an aldehyde would also show a distinct band for H-C=O stretch in the region 2830-2695, so it is not likely that it is one of these structures. That leaves the first compound, which is 2-butanone.

Proton NMR Spectrum

A table of characteristic NMR shifts is available online: click on the link below. Note that this chart is also linked to in the frame to the left.

• Before you look at the NMR spectrum, think about what the spectrum of 2-butanone should look like. There are three different types of protons:

The 3 protons in green will be a singlet and show up from 2-2.7 ppm. The 2 protons in blue will be split to a quartet by the protons in red; they will show up from 2-2.7 ppm. They will be further downfield (have a higher ppm value) than the protons in green because they are shielded both by the carbonyl and by the red methyl group. The 3 red protons are the farthest from the carbonyl and are split into a triplet by the blue protons. Let's look at the NMR and see if this is what we see.

Sure enough, here is how they correlate with the structure:

Note the pattern of the ethyl group -CH₂CH₃ in the above NMR spectrum. Whenever you suspect an ethyl group in a molecule, look for a quartet of 3 protons and a triplet of 2 protons, with the methylene (-CH₂-) group further downfield than the methyl group (-CH₃).

Summary

Example 1 is 2-butanone:

COSY