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Figure 1: Molecular structure of Methyl Propanoate, C3H8O2


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Figure 2: Complete H NMR spectrum for methyl propanoate, C3H8O2.

The H NRM spectra of methyl propanoate, C3H8O2, is seen in the above image. Formed from the Fisher esterification of methanol and propanoic acid, this ester is a four-carbon molecule with a characteristic carbonyl group adjacent to an oxygen atom within the main carbon chain. This H NMR graph shows four main signals, the first of which is located very close to 0.000 ppm. This peak is actually due to the addition of tetramethylsilane to the sample. Due to the great shielding of the TMS protons, this signal is the zero standard from the chemical shift and degree of deshielding of the other signals are measured. The three signals that directly illuminate the structure of methyl propanoate are the triplet centered at 1.090 ppm (3 H), the quartet centered at 2.280 ppm (2 H), and the singlet ceneterd at 3.620 ppm (3 H). The presence of these three signals confirm that there are three different groups of nonequivalent hydrogens in methyl propanoate.



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Figure 3: An enlarged view of the triplet centered at 1.090 ppm for the alkane hydrogens.

The image above allows for a more thorough view of the signal that occurs at around 1.090 ppm in methyl propanoate. The number and 1:2:1 relative heights of the three peaks within this signal confirm the presence of a triplet. This triplet represents the hydrogen molecules that are found in the –CH3 methyl group at the left end of the carbon chain. The expected H NMR range for such an alkyl hydrogen is within 0.9 to 1.5 ppm, a range which included the 1.090 ppm value actually observed. Since this methyl group is not in contact with any strongly electronegative groups, the amount of deshielding is the lowest for these hydrogens than any other hydrogen type in methyl propanoate. This signal is split into three peaks, indicating that the adjacent carbon only has two hydrogens bonded to it, as is true of the adjacent –CH2 group that is only close enough to affect the absorbance of the three alkyl hrdrogens under study.



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Figure 4: A zoomed in depiction of the signal centered at 2.280 ppm, representative of the –CH2 hydrogen’s adjacent to the carbonyl group.

This second type of hydrogen shows itself in the H NMR spectrum of methyl propanoate further downstream than the alkyl hydrogen’s already analyzed. This is consistent with the more electronegative chemical environment around this hydrogen type, which results in a greater degree of deshielding. For any hydrogen’s bound to a carbon adjacent to a carbonyl group, the predicted H NMR signal is around 2.1 to 2.2 ppm. The value observed in the spectrum above is just slightly higher than the accepted chemical shift, which can be attributed to the slight electronegative, deshielding effect of the oxygen atom further down the carbon chain. The carbon adjacent to the –CH2 group, which is the methyl group at the left end of the chain, contains three hydrogens. The value of N in this case is assigned to be 3 adjacent equivalent protons, so that the –CH2 hydrogen signal will be split into a quartet in accordance with the N + 1 rule. This quartet also follows the 1:3:3:1 area ratio predicted by Pascal’s triangle.


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Figure 5: An expanded view of the signal centered at 3.620 ppm, representing the hydrogen’s bound to the carbon adjacent to an oxygen atom.

This final signal is the furthest downstream, and thus the most deshielded hydrogen type, in the H NMR spectrum of methyl propanoate. The oxygen adjacent to this left end –CH3 group creates the most electronegative chemical environment, typically showing a signal in the 3 to 4 ppm range. As seen above, the 3.620 ppm chemical shift value is consistent with this accepted range. Its place at the higher end of this range may be due to the slight deshielding that possibly results from the carbonyl group two positions to the right on the main carbon chain. Furthermore, this signal is a singlet with only one peak, meaning that signal splitting does not occur. After all, this hydrogen type is too great a distance from and is, thus, not coupled with any other nonequivalent hydrogens.

[looks good - note that you should use the term downfield, not downstream JCB]

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