Derivatives of the parent reactant molecules benzaldehyde and acetophenone can include a variety of R, R1 and R2 functional groups, such as methyl (-CH3), methoxy (-OCH3), chloro (-Cl), bromo (-Br), amino (-NH2), hydroxy (-OH), nitrile (-CN), etc. and in various positions around the phenyl ring. Due to intermolecular hydrogen bonding, the hydroxy group will show temperature, concentration and solvent polarity dependence, which complicates interpretation. Similarly, complications will arise in the spectrum of the amino group due to spin coupling of the proton with the nitrogen nucleus, the variable rate of exchange of the labile amino proton and the electric quadrupole moment of the 14N nucleus. Halogen substitution does not introduce a new peak in a proton NMR spectrum but will affect peak positions and splitting patterns on the aromatic protons. Only the methyl and methoxy groups introduce new, non-overlapping resonance lines in the parent molecule spectrum, which affords straightforward interpretation.
1H NMR – A Qualitative Lesson
It is easy to identify the prominent peaks in the 1H NMR spectra of the reactants in the aldol reactions of Figures 5 and 6. Both the aldehyde proton and methyl-ketone protons produce only one resonance line each, thus making them simple to monitor. NMR spectra can be measured on pure samples or using an aliquot of the reaction mixture.
The picoSpin 45 NMR spectrometer is ideal for preliminary qualitative analysis of neat samples and mixtures, giving students the opportunity to determine the quality of their starting materials, establish the stoichiometry of their mixture, identify the predominant functional groups which undergo chemical transformation, and gain valuable hands-on experience using NMR in the lab. Because the capillary cartridge can be quickly flushed and refilled, it can be used during a 3–4 hour organic chemistry lab experiment. Students can sample their reaction mixture multiple times at various stages throughout their experiment, obtaining spectra as the reaction proceeds and measuring a spectrum of isolated product. 1H NMR spectra shown here were acquired with a 90-degree pulse angle, 750 ms acquisition time, 10–20 s recovery delay, and are an average of 9 or 49 scans.
Spectra of reactants 4-methoxybenzaldehyde and 4-methylbenzaldehyde are shown in Figures 7 and 8. The aldehydic proton produces a lone signal due to a lack of neighboring proton. Diamagnetic anisotropy arises from circulating π electrons of the carbonyl (C=O) bond induced along the transverse axis by the applied field, giving rise to strong deshielding of this lone proton. In the spectrum of 4-methoxybenzaldehyde, the aldehyde proton appears as a singlet at 9.90 ppm, while for 4-methylbenzaldehyde this same signal is at 10.08 ppm.