PULSARTechnical Paper

Analysis of the reduction product of 3-nitrobenzaldehyde using Pulsar
Dr Vittorio Caprio, Dr Oliver Sutcliffe and Dr Ryan Mewis, Manchester Metropolitan University

Introduction
Conversion of chemical functional groups from one to another is a mainstay of organic chemistry. It enables molecules
to be produced that are more reactive for future reactions, for example, or to provide suitable functional groups so thatmolecules may be coupled together. An example of a functional group conversion is shown in Scheme 1. The reductionof the aldehyde group using sodium borohydride results in the production of an alcohol-containing product. The nitro
functional group is unchanged during this chemical process.

Scheme 1 : Conversion of 3-nitrobenzaldehyde to 3-nitrobenzyl alcohol using sodium borohydride.

NMR spectroscopy is one of the main techniques used to confirm the conversion of the functional group. The collection of
NMR spectra can also aid in the assessment of the purity of the isolated material by highlighting the presence of starting
material and / or other products as a result of the reaction. For aromatic systems, Shoolery’s rules can be used to predictthe relative chemical shifts of 1H and 13C nuclei of the ring, provided that substituent groups are not located ortho to one
another. These values are tabulated and readily available for aromatic systems as well as others. Thus, the spectroscopist
can begin to construct an idea of how the resulting spectrum will present itself prior to collecting it.
This application note is concerned with analysing the reactant and product as shown in Scheme 1 using 1H and 13C{1H}
NMR spectroscopy. Shoolery’s rules are employed to predict the appearance of the resulting 1H and 13C NMR spectra. The
reaction represents a typical undergraduate experiment and as such, exemplifies that students can collect their own 1H andC{1H} NMR data of their starting material and product and subsequently analyse and interpret it to gain insight in to the13reaction conducted.

Results
H NMR

1

The Shoolery predicted aromatic 1H NMR chemical shifts are shown in Table 1. The conversion of the aldehyde group, anelectron-withdrawing group, to a methyl alcohol, an electron-donating group, will result in distinct changes in the 1H NMR
spectrum. Consequently, the chemical shifts of the aromatic protons are up-field shifted relative to the reactant. The shifts aresuch that they will be readily observable in the 1H NMR spectrum.

ReactantProduct

Substituent
constant for –NO2

Substituent
constant for –CHO

Predicted
δ (ppm)Substituent
constant for –NO2

Substituent
constant for -CH2OH

δHA

0.95

0.58

8.80

0.95

−0.07

8.15

HB

0.95

0.27

8.49

0.95

−0.07

8.15

HC

0.17

0.21

7.65

0.17

−0.07

7.37

HD

0.33

0.58

8.18

0.33

−0.07

7.53

Table 1 : Predicted values for the ring 1H NMR chemical shift environments of the reactant and product using Shoolery’s rules. Shoolery’srules states that for aromatic protons δ = 7.27 + ΣSi where Si are the substituent constants

The 1H NMR spectrum of the reactant, 3-nitrobenzaldehyde, is shown in Figure 1. The aldehyde proton is present at δ 10.14. Despite
the aromatic region being crowded, it is possible to integrate the region to give a 3 : 1 ratio (regions 8.57 – 8.20 ppm and 7.91 –7.65 ppm respectively). In comparison to the predicted chemical shifts as listed in Table 1, this integral ratio corresponds accordingly.
The peak that presents as a triplet at δ 7.78 has a coupling constant of 7.8 Hz. It also shows strong second order effects. A further
two doublets, which both show second order coupling, are observed at δ 8.50 and 8.27; they possess J-coupling of 8.4 and 7.8 Hz
respectively. This is of the same magnitude as the peak at δ 7.78 and therefore, these peaks are coupled with one another. Thus,
these peaks are assigned as being HB (δ 8.50) and HD (δ 8.27). This leaves a singlet at δ 8.70. The absence of coupling and that it is
the most down-shifted aromatic peak leads to the conclusion that this peak is HA in the starting material. By reference to Table 1, thechemical shift values deduced from Figure 1 correspond strongly with those calculated using Shoolerys’ rules.
The COSY spectrum of
3-nitrobenzaladehyde is shown inFigure 2. This shows the complex
coupling that is present between
the four aromatic 1H nuclei.
However, upon closer inspection
it is noticeable that the peak at δ8.70 shows minimal interaction
with the other peaks whereas
the other peaks all display strong
coupling to one another. Thisconfirms that one of the aromatic
proton nuclei, HA, is isolated fromthe remainder protons in thearomatic ring.
Figure 1 : 1H NMR spectrum of
3-nitrobenzaldehyde in CDCl3
collected at 60 MHz.

The 1H NMR spectrum of theproduct, 3-nitrobenzyl alcohol
possesses four protons in thearomatic region (integratedto give a 2 : 2 ratio), a CH2
peak at 4.81 ppm and an OH
peak at 2.41 ppm. The latter
two peaks result due to thereduction of the aldehyde; thepeak for the aldehyde proton is
no longer visible in the 1H NMRspectrum.
Figure 2 : 1H-1H COSY NMR
spectrum of 3-nitrobenzaldehyde
in CDCl3 collected at 60 MHz. The
spectrum has been truncated tofocus on only the aromatic andaldehyde regions

The 1H-1H COSY spectrum of 3-nitrobenzyl alcohol provides good insight in to the coupling between proton nuclei. In
particular, there are COSY interactions between the peak centred at 8.21 and 7.63 ppm, and also between peaks at 7.63
and 7.50 ppm. Because of the connectivity that the peak at 7.63 ppm shows, this must be the HC-resonance. The peaks
at 8.21 and 7.63 are HB and HD respectively. The peak at 8.07 ppm shows no COSY interaction and is, therefore, the peak
responsible for HA. Thus, the values obtained for the 1H NMR chemical shifts for the aromatic protons are δ 8.07, 8.21, 7.63
and 7.50 for HA-HD respectively.
These again compare wellagainst the predicted Shoolery
values given in Table 1 andintegrate accordingly in theH NMR spectrum (Figure 3).

1This deduction would not havebeen possible using the 1H NMR
spectrum shown in Figure 3 dueto the convoluted nature of thearomatic region.
Figure 3 : 1H NMR spectrum of3-nitrobenzyl alcohol in CDCl3
collected at 60 MHz.

Figure 4 : 1H-1H COSY NMR
spectrum of 3-nitrobenzyl
alcohol in CDCl3 collected at 60MHz. The spectrum has beentruncated to focus on only thearomatic region

Figure 5 shows the overlaid 1H NMR data obtained for the reactant and product. In this figure, the up-field shift of the aromaticprotons is visible for the product relative to the reactant. This change is brought about through the conversion of an electronwithdrawing group (aldehyde) to one that is electron donating (CH2OH). The reduction, therefore, not only changes thechemical identity of the molecule but also the electronic effects within the ring system. The signals created and lost as a resultof the reduction of the aldehyde to the methyl alcohol are readily apparent, due largely to the significant difference in terms ofchemical shift and because the remainder of the 1H nuclei are all aromatic protons.
Figure 5 : Overlaid 1H NMR spectra of3-nitrobenzaldehyde (dark blue) and3-nitrobenzyl alcohol (light blue) inCDCl3 collected at 60 MHz.

C NMR13

The chemical shifts of the aromatic 13C nuclei can be predicted similarly using Shoolery’s rules. Table 2 lists the predicted
and experimentally determined 13C NMR peaks for the aromatic 13C nuclei of the reactant and product

Reactant

Product

Predicted δ / ppm

Experimental δ / ppm

Predicted δ / ppm

Experimental δ / ppm

1

138.2

137.6

142.5

143.2

2

124.3

124.3

122.3

121.3

3

149.2

145.0

147.4

148.2

4

129.1

128.6122.7

122.3

5

130.4

130.6

128.6

129.5

6

135.6

134.9

133.6

132.8

C nucleus

13

Table 2 : Predicted and experimentally determined chemical shift values for the aromatic 13C nuclei of the reactant and product using Shoolery’srules. Shoolery’s rules states that for aromatic protons δ = 128.4 + ΣSi where Si are the substituent constants

In general, there is good agreement between the theoretical and experimental chemical shift data for both compounds. As wasseen for the 1H nuclei , there is a shift up-field for all the 13C chemical shifts of the product relative to the reactant, with theexception of carbon 1, which displays a down-field shift. In addition to the data contained within Table 2, the reactant possesses
a peak at δ 189.9 for the aldehyde carbon and the product has a peak at δ 63.4 for the CH2 carbon.
For both substances, thequaternary carbons display
as peaks with a low intensity;
this is due to the elongated T1
values that they possess relative
to the other 13C environments
coupled with the relatively shortrelaxation delay that is used in
the experiment (1 s). The 13C{1H}
NMR spectra of the reactant andproduct are shown in Figures 6and 7 respectively.
Figure 6: 13C{1H} NMR spectrumof 3-nitrobenzaldehyde collectedin CDCl3.

Figure 7: 13C{1H} NMR spectrumof 3-nitrobenzyl alcohol collectedin CDCl3

Conclusion
The Pulsar system is capable of being used to track an organic reaction, for example, as detailed herein, a hydride
mediated reduction. The spectra that are obtained contain sufficient information to be used by undergraduate students
to elucidate the chemical structure of their reaction product by reference to the starting material. Laboratory instructors
can probe students understanding, through reference to the 1H and 13C{1H} NMR spectra collected, of the electronwithdrawing / electron-donating effects upon the shielding experienced by the aromatic proton nuclei. The utilisation of
Shoolerys’ rules enable students to predict the appearance of the resulting 1H and 13C{1H} NMR spectra, from which theycan validate the identity of the product produced.

For more information visit: www.oxinst.com/pulsar or email: magres@oxinst.com

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