### Example 6

C5H10O

MW 86

Calculate the degree of unsaturation: the answer is 1; it can have a carbon-carbon double bond, a carbonyl, or a ring.

### IR Spectrum

Since the degree of unsaturation indicates that the compound could have a carbonyl, let's look for that first. There is a band at 1726, suggesting a saturated aldehyde (1740-1720). Whenever you suspect a compound is an aldehyde, look for bands in the region 2830-2695. They often appear as shoulder-peaks, just to the right of the C-H alkane stretch peaks. In the spectrum below, note the two bands in this region, suggesting that the compound is indeed an aldehyde.

### Proton NMR Spectrum

Since the IR spectrum indicates an aldehyde, look for this functionality in the NMR spectrum. The aldehydic proton appears in the NMR from 9-10, usually as a small singlet.

The spectrum above shows a small singlet corresponding to one proton at 9.2 ppm, confirming that the compound is an aldehyde. Protons on the carbon adjacent to the aldehyde carbonyl will show up at 2-2.7 ppm; this is the triplet peak of 2 protons at 2.4 ppm on the above spectrum. Thus, so far we know that there is an aldehyde group next to a methylene group which is next to a carbon that has two hydrogens:

This accounts for 3 of the 5 carbons in the molecule. The un-colored hydrogens in the above structure could correspond to the peak of 2 hydrogens centered at 1.6 ppm; this peak is a pentet indicating that these protons are adjacent to carbons with a total of 4 hydrogens. The peak centered at 1.35 ppm has two hydrogens and is a sextet, indicating it is next to carbons that have a total of 5 hydrogens. Finally, the peak at 0.9 ppm has 3 hydrogens and is a triplet, indicating it is a methyl group adjacent to a carbon that has 2 hydrogens. Therefore, it looks like the molecule is a straight-chain of 5 carbons with the aldehyde group at one end:

Note that the closer a group is to the carbonyl function, the further downfield it is shifted. Here is how the NMR correlates to the structure:

### Summary

Example 6 is 1-pentanal:

1-pentanal