Valine


Figure A:
Valine [(2S)-2-amino-3-methyl-butanoic acid]
B3.jpg
Figure B:
This figure shows the different protons which cause peaks in the H-NMR. The proton indicated by the line underline interacts with the protons indicated by the dot of the same colour. The peaks are analysed in the figure indicated by the number.

The the H of the hydroxyl group is not seen on the spectrum, Figure C. The H of the hydroxyl group is highly deshielded and the the expected range is 10 - 12 ppm. However, it is not seen on the spectrum, which ends at around 10.8 ppm.
Also, the two Hydrogens on the Amine group do not form peaks on the spectrum because they are exchanged in the solvent.

C.jpg
Figure C:
H-NMR of Valine [(2S)-2-amino-3-methyl-butanoic acid]

D.jpg

Figure D:
This peak at 4.80 ppm represents the solvent, Deuterium Oxide, also known as Heavy Water, which was used to dissolve the Valine in.

1.jpg
Figure 1:
This multiplet is caused by both spin-spin splitting and coupling. Looking at the H on Carbon 3, we see that there are 2 methyl groups attached. The two methyl groups are not equivalent to each other, but the three hydrogen atoms on each methyl group are equivalent. So, we would expect the three hydrogen atoms on each of the methyl groups couple with the H on Carbon 3, and cause a quartet of quartets. However, there is also an H on Carbon 2 which couples with the H on Carbon 3, and this would lead us to expect the presence of a quartet of quartet of doublets, for a total of 32 peaks.
According to Pascal’s triangle, the areas of the peaks should be in the ratio (for quartets) 1:3:3:1. This suggests that in the multiplet, there should be eight relatively smaller peaks at the ends, with sixteen peaks, three times as large in the middle.
However, looking at the spectrum, there appear to be two peaks, falling at 2.26 and 2.27 which are higher than the surrounding peaks. There are four peaks of intermediate height which appear at 2.29, 2.28, 2.25 and 2.24, and there are four peaks which are small, which appear at 2.30, 2.29, 2.23 and 2.22.
Only ten peaks are visible. This is because of overlap of peaks caused by small coupling constants.


2.jpg
Figure 2:
A doublet is observed at 3.60 and 3.59 ppm. This corresponds to the Hydrogen atom found on Carbon 2 which couples with the Hydrogen atom on Carbon 3, resulting in this doublet. The peaks are shifted to the left because this Hydrogen is deshielded due to its close affinity to both the N of the amino group and the O of the carboxylic acid group.

3.jpg
Figure 3:
This part of the spectrum shows four peaks. But these peaks are not doublets of doublets. Instead they are two distinct sets of doublets. There are two methyl groups attached to Carbon 3. The methyl groups are not equivalent because they are both attached to Carbon 3, which is attached to Carbon 2, which is a chiral carbon. They cannot influence each other, but they are both influenced by the Hydrogen on Carbon 3, resulting in the two pairs of doublets at 1.04 and 1.02 ppm (one pair) and 0.99 and 0.97 ppm (the other pair). The integral height of each peak corresponds to 3 H.

[Good JCB]

tag loadtochemspider