Caprio, Vittorio

Mewis, Ryan

Sutcliffe, Oliver B

E-space: Manchester Metropolitan University's Research Repository

Caprio, Vittorio and Sutcliffe, Oliver B and Mewis, Ryan (2018) Analysis ofthe reduction product of 3-nitrobenzaldehyde using Pulsar. Technical Report.Oxford Instruments.Downloaded from: https://e-space.mmu.ac.uk/627992/Version: Published VersionPublisher: Oxford InstrumentsPlease cite the published versionhttps://e-space.mmu.ac.ukPULSAR Technical PaperAnalysis of the reduction product of 3-nitrobenzaldehyde using PulsarDr Vittorio Caprio, Dr Oliver Sutcliffe and Dr Ryan Mewis, Manchester Metropolitan University 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 predict the 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 and 13C{1H} NMR data of their starting material and product and subsequently analyse and interpret it to gain insight in to the reaction conducted. Results1H NMRThe Shoolery predicted aromatic 1H NMR chemical shifts are shown in Table 1. The conversion of the aldehyde group, an electron-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 are such that they will be readily observable in the 1H NMR spectrum. 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 that molecules may be coupled together. An example of a functional group conversion is shown in Scheme 1. The reduction of 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.Substituent  constant for –NO2Substituent  constant for –CHOPredicted δ (ppm)Substituent  constant for –NO2Substituent  constant for -CH2OHδHA0.95 0.58 8.80 0.95 −0.07 8.15HB0.95 0.27 8.49 0.95 −0.07 8.15HC0.17 0.21 7.65 0.17 −0.07 7.37HD0.33 0.58 8.18 0.33 −0.07 7.53Reactant ProductTable 1 : Predicted values for the ring 1H NMR chemical shift environments of the reactant and product using Shoolery’s rules. Shoolery’s rules states that for aromatic protons δ = 7.27 + ΣSi where Si are the substituent constantsThe 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, the chemical shift values deduced from Figure 1 correspond strongly with those calculated using Shoolerys’ rules.The COSY spectrum of 3-nitrobenzaladehyde is shown in Figure 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 δ 8.70 shows minimal interaction with the other peaks whereas the other peaks all display strong coupling to one another. This confirms that one of the aromatic proton nuclei, HA, is isolated from the remainder protons in the aromatic ring.Figure 1 : 1H NMR spectrum of  3-nitrobenzaldehyde in CDCl3 collected at 60 MHz. The 1H NMR spectrum of the product, 3-nitrobenzyl alcohol possesses four protons in the aromatic region (integrated to 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 the reduction of the aldehyde; the peak for the aldehyde proton is no longer visible in the 1H NMR spectrum.Figure 2 : 1H-1H COSY NMR spectrum of 3-nitrobenzaldehyde in CDCl3 collected at 60 MHz. The spectrum has been truncated to focus on only the aromatic and aldehyde regionsThe 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. Figure 3 : 1H NMR spectrum of 3-nitrobenzyl alcohol in CDCl3 collected at 60 MHz. These again compare well against the predicted Shoolery values given in Table 1 and integrate accordingly in the 1H NMR spectrum (Figure 3). This deduction would not have been possible using the 1H NMR spectrum shown in Figure 3 due to the convoluted nature of the aromatic region. Figure 4 : 1H-1H COSY NMR spectrum of 3-nitrobenzyl alcohol in CDCl3 collected at 60 MHz. The spectrum has been truncated to focus on only the aromatic regionFigure 5 shows the overlaid 1H NMR data obtained for the reactant and product. In this figure, the up-field shift of the aromatic protons is visible for the product relative to the reactant. This change is brought about through the conversion of an electron-withdrawing group (aldehyde) to one that is electron donating (CH2OH). The reduction, therefore, not only changes the chemical identity of the molecule but also the electronic effects within the ring system. The signals created and lost as a result of the reduction of the aldehyde to the methyl alcohol are readily apparent, due largely to the significant difference in terms of chemical shift and because the remainder of the 1H nuclei are all aromatic protons.Figure 5 : Overlaid 1H NMR spectra of 3-nitrobenzaldehyde (dark blue) and  3-nitrobenzyl alcohol (light blue) in CDCl3 collected at 60 MHz.4823Product13C NMRThe 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 product13C nucleus Predicted δ / ppm Experimental δ / ppm Predicted δ / ppm Experimental δ / ppm1 138.2 137.6 142.5 143.22 124.3 124.3 122.3 121.33 149.2 145.0 147.4 148.24 129.1 128.6 122.7 122.35 130.4 130.6 128.6 129.56 135.6 134.9 133.6 132.8ReactantTable 2 : Predicted and experimentally determined chemical shift values for the aromatic 13C nuclei of the reactant and product using Shoolery’s rules. Shoolery’s rules states that for aromatic protons δ = 128.4 + ΣSi where Si are the substituent constantsIn general, there is good agreement between the theoretical and experimental chemical shift data for both compounds. As was seen for the 1H nuclei , there is a shift up-field for all the 13C chemical shifts of the product relative to the reactant, with the exception 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. Figure 6: 13C{1H} NMR spectrum of 3-nitrobenzaldehyde collected in CDCl3.For both substances, the quaternary 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 short relaxation delay that is used in the experiment (1 s). The 13C{1H} NMR spectra of the reactant and product are shown in Figures 6 and 7 respectively.PULSARFor more information visit: www.oxinst.com/pulsar or email: magres@oxinst.comThis publication is the copyright of Oxford Instruments and provides outline information only which (unless agreed by the company in writing) may not be used, applied or reproduced for any purpose or form part of any order or contract or be regarded as a representation relating to the products or services concerned. Oxford Instruments’ policy is one of continued improvement. The company reserves the right to alter, without notice, the specification, design or conditions of supply of any product or service. Oxford Instruments acknowledges all trademarks and registrations. © Oxford Instruments plc, 2018. All rights reserved. Part no: MR/201/07184823Figure 7: 13C{1H} NMR spectrum of 3-nitrobenzyl alcohol collected in CDCl3ConclusionThe 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 electron-withdrawing / 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 they can validate the identity of the product produced. 

Analysis of the reduction product of 3-nitrobenzaldehyde using Pulsar

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Analysis of the reduction product of 3-nitrobenzaldehyde using Pulsar

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