Question

What are the IR and NMR spectral features of isoamyl acetate?

Answer 1

First calculate the degree of unsaturation: the result is 1. The compound will have a ring or a double bond.

IR Spectrum

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 esters from 1750-1735. Sure enough, there is a band at 1743 indicating a saturated aliphatic ester. There is a possibility that the compound is has both a ketone and an ether group instead of an ester, however, the band at 1743 indicates an ester instead of a saturated aliphatic ketone. You can also see bands in the region 1300-1000 which can indicate the C-O stretch.

We think that the compound is an ester, having the functional group -O-(C=O)-. Let's look at the NMR to get an idea of how many different hydrogens are in the molecule so that we can narrow down the number of possible structures.

The NMR indicates that there are 5 different types of protons in the molecule. The 6 protons at 0.9-1.0 ppm indicate two methyl (-CH₃) groups, each adjacent to a carbon which has 1 hydrogen:

The peak at 4.0-4.2 ppm has two protons, so it is a methylene group. Since it's in the range 3.7-4.1 ppm, it is next to an oxygen, and since we expect the compound to be an ester, this indicates the sub-structure below:

The peak at 2.0-2.1 ppm has three protons, so it is a methyl group. In an ester, hydrogens on a carbon adjacent to the carbonyl are from 2.0-2.2 ppm. This leads to the sub-structure below. Since there are no hydrogens on the adjacent carbon, the peak is a singlet.

We can go ahead and put together the two groups on the ester:

f we add up the carbons in the substructures above, we get 8, one more than the number of carbons in the molecule. So they must join, so let's do that, and fill in the proper number of hydrogens per carbon:

There are 5 different types of protons in this structure, as indicated by the NMR. The 2 protons in turquoise are adjacent to carbons with a total of 3 hydrogens, corresponding to the quartet at about 1.5 ppm (this quartet runs into the peak next to it and is kind of hard to decipher). The proton in purple is adjacent to carbons that have 8 hydrogens, this corresponds to the multiplet at about 1.7 ppm.

This is how the structure correlates with the NMR: