Organic Chemistry 2- NMR

Question

Assign each of the peaks.

Answer 1

Let's approach this problem stepwise.

1) The unknown has four different carbons.

2) One is a carbonyl carbon (170 ppm), one is attached to oxygen (60 ppm), the other two are aliphatic.

3) So at this point, without further information, I would draw the two possible options as the structural

isomers

methyl propanoate CH₃OC(O)CH₂CH₃ and

ethyl ethanoate CH₃C(O)OCH₂CH₃

The best fit is ethyl ethanoate with the OCH₂ carbon downfield at 60 ppm and the methyl CH₃C=O carbon deshielded by the carbonyl at 21 ppm. The CH₃ carbon at 14 ppm is a bit downfield due to the effect of the oxygen. In methyl propanoate the CH₃O carbon would be expected around 50 ppm.

Note that the real spectrum would not have all four peaks at equal height, The sp² carbonyl carbon is actually quite small since it is not attached to any hydrogens and, being attached to three hydrogens, the CH₃ signals are larger than the CH₂ signals. Remember, the more hydrogens attached to a carbon, the faster the carbon relaxes and the more intense is the resonance signal.