Deepak Singh

Propanal (CH3CH2CHO)


Figure 1) Full Spectrum of Propanal
This is the complete proton NMR spectra of propanal. There are three major absorption signals which will be discussed below.

Figure 2) Zero Signal from TMS

Notice that there is a small signal, at 0ppm. This likely corresponds to tetramethylsilane (TMS), which, because of its highly shielded protons has been defined as the zero value for most proton NMR spectra.

Figure3) Absorption of Methyl Group

This absorption signal, located between 1.08 and 1.12 ppm, is the farthest upfield of the three proton signals seen in the full spectra. This corresponds to the --CH3­­ methyl group of propanal, which are expected to absorb in the range of 0.9 and 1.5ppm, which is seen. Furthermore, because of the nonequivalence with the two protons on the adjacent –CH2 group, a triplet is seen according to the N+1 rule.

Figure 4) Absorption of --CH2 group

Slightly further downfield, a quartet of doublets between 2.40 and 2.52ppm is seen. This corresponds to the –CH2 group of propanal. Because it lies adjacent to the carbonyl group, it is expected to absorb at between 2 and 2.5ppm. The coupling occurs because nonequivalence with the 3 protons of the adjacent –CH3 methyl group, the quartet, as well as the single proton on the other side of the carbonyl group, the doublet, again according to the N+1 rule.

Figure 5) Absorption of proton adjacent to the Carbonyl group

Lastly, the signal furthest downfield corresponds to the single proton adjacent to the carbonyl group of propanal. This is expected to absorb at between 9 and 10ppm as it is the proton of an aldehyde, and thus a signal is seen between 9.775 and 9.80ppm. A triplet coupling occurs because of its nonequivalence with the two protons of the –CH2 group on the other side of the carbonyl group.