Is there any trend in the spectral data for – Chemical shift (∂) in ppm of the OH proton of carboxylic acids in 1H – NMR spectroscopy, Chemical shift (∂) in ppm of the C atom of COOH group of carboxylic acids in 13 C – NMR spectroscopy, wavenumber of the C = O of the COOH group in the IR spectroscopy down the homologous series of aliphatic mono carboxylic from-methanoic acid (HCOOH), ethanoic acid (CH3COOH), propanoic acid (C2H5COOH), butanoic acid (C3H7COOH) and pentanoic acid (C4H9COOH) ?
Being an inquirer and a creative thinker, I always wanted to learn beyond the restrictions of curriculum. Though the process was full of hurdles but the zeal of the process has enabled me to get the true essence of education. This behavior of mine has landed me to several questions of which some were answered in the past and some left unanswered still today. I am exploring on this topic due to the same reason. During the COVID – 19 pandemics, like others, I had immense interest in going through the COVID – 19 vaccines and medicine making procedures. Thus, I always researched about the same in the internet. There are few questions that has triggered me which are not directly related to COVID – 19 vaccine making but they are aligned with the same field. How does the researchers and scientists get assured that the compound they are wanting to make in the medicine has been completely and without any error formed within the medicine? This is because, a minute error in this case may lead to another deadly pandemic. This question has pushed me to do thorough research on procedures of identification of compound. I have read several research papers and journals and understood that there are few techniques which are used to the molecular structure of the compound. This has landed me to work on the above mentioned research question.
NMR Spectroscopy is the abbreviation for Nuclear Magnetic Resonance spectroscopy.Nuclear magnetic resonance (NMR) spectroscopy is the study of molecules by recording the interaction of radiofrequency (Rf) electromagnetic radiations with the nuclei of molecules placed in a strong magnetic field. It determines the physical and chemical properties of atoms and molecules mainly relying on the phenomenon of nuclear magnetic resonance.
The nuclei of many elemental isotopes have a characteristic spin:
Chemical shift is the difference between the resonant frequency of the spinning protons and the signal of the reference molecule. Modern NMR spectrometers use powerful magnets having fields of 1 to 20 T(Tesla). Even with these high fields, the energy difference between the two spin states is less than 0.1 cal/mole.
Different nuclei that can be detected by NMR spectroscopy , 1H and 13C are the most widely used. In NMR, when we reach the radio frequency (Rf) radiation nucleus, it causes the nucleus and its magnetic field to turn (or it causes the nuclear magnet to pulse, thus the term NMR).
The NMR spectrometer must be tuned to a specific nucleus, in this case the proton. The simplest procedure for obtaining spectrum is referred to as the continuous wave (CW) method. An NMR spectrum is acquired by observing Rf signal from the sample. Since protons all have the same magnetic moment, we might expect all hydrogen atoms to give resonance signals at the same field / frequency values. Fortunately for chemistry applications, this is not true. A number of representative proton signals will be displayed over the same magnetic field range. It is not possible to examine isolated protons in the spectrometer described above. Since electrons are charged particles, they move in response to the external magnetic field (Bo) so as to generate a secondary field that opposes the much stronger applied field. This secondary field shields the nucleus from the applied field, so Bo must be increased in order to achieve resonance (absorption of Rf energy). Most organic compounds exhibit proton resonances that fall within a 12 ppm range (the shaded area), and it is therefore necessary to use very sensitive and precise spectrometers to resolve structurally distinct sets of hydrogen atoms within this narrow range.
Solvents are used since the dissolving process is essential for the homogenized distribution of sample molecules through the observation volume.
The characteristics to keep in mind while picking a solvent are the following:
Example of used solvent: CDCl3 - deuterated chloroform. It low priced and small peaks can be observed easily.
The power and usefulness of 1H NMR spectroscopy as a tool for structural analysis should be evident from the past discussion. Unfortunately, when significant portions of a molecule lack C-H bonds, no information is forthcoming. These difficulties would be largely resolved if the carbon atoms of a molecule could be probed by NMR in the same fashion as the hydrogen atoms. Since the major isotope of carbon (12C) has no spin, this option seems unrealistic. Fortunately, 1.1% of elemental carbon is the 13C isotope, which has a spin I = 1/2, so in principle it should be possible to conduct a carbon nmr experiment. The carbon NMR spectrum of a compound displays a single sharp signal for each structurally distinct carbon atom in a molecule.
Infrared Spectroscopy (IR) generally refers to the analysis of the interaction of a molecule with infrared light. The IR spectroscopy concept can generally be analysed in three ways: by measuring reflection, emission, and absorption. The major use of infrared spectroscopy is to determine the functional groups of molecules, relevant to both organic and inorganic chemistryAn IR spectrum is essentially a graph plotted with the infrared light absorbed on the Y-axis against. frequency or wavelength on the X-axis. A bond will only interact with the electromagnetic infrared radiation if it is polar. The presence of areas of partial positive and negative charge in a molecule allows the electric field component of the electromagnetic wave to excite the vibrational energy of the molecule. The intensity of the absorption depends on the polarity of the bond.
The IR spectroscopy theory utilizes the concept that molecules tend to absorb specific frequencies of light that are characteristic of the corresponding structure of the molecules. The energies are reliant on the shape of the molecular surfaces, the associated vibrionic coupling, and the mass corresponding to the atoms
A Carboxylic Acid is an organic compound containing a carboxyl functional group The carboxylic acids are the most important functional group that present C = O.
The general formula of a carboxylic acid is R - COOH, were COOH refers to the carboxyl group, and R refers to the rest of the molecule to which this group is attached.
Trivial name ends with the suffix “-ic acid”. An example of a trivial name for a carboxylic acid is acetic acid (CH3COOH). In the IUPAC nomenclature of these compounds, the suffix “-oic acid” is assigned.
Crotonic acid, CH3CH=CH-COOH
Carbonic acid, OH-COOH
Butyric acid, CH3(CH2)2COOH
Figure 3 - Table On Chemical Shift (∂) In ppm Of The OH Proton Of Carboxylic Acids in 1H – NMR Against Chain Length
Formulas used:
Mean = \(\frac{Sum\ of\ values\ from\ Source-1,\ Source-2\ and\ Source-3}{3}\)
Absolute error = \(\frac{± (maximum\ value-minimum\ value)}{2}\)
Percentage error = ± \(\frac{absolute\ error}{mean\ value}\)× 100
Figure 4 - Chemical Shift (∂) In ppm Of The OH Proton Of Carboxylic Acids In 1H – NMR Against Chain Length
The graph depicts the variation of chemical shift (∂) in ppm of the C atom in the COOH group in 13C - NMR spectroscopy against the chain length of the carboxylic acid. From methanoic acid to propanoic acid, the chemical shift (∂) in ppm of the C atom in the COOH group in 13C - NMR increases from 167.71 ppm to 184.72 ppm. This is mainly due to the fact that as we move from methanoic acid to propanoic acid, the size of the alkyl group is changing from H in methanoic acid to C2H5 (ethyl) in propanoic acid, the positive inductive effect or the electron releasing effect is increasing, thus the electron density at the C atom in the COOH group increases, chemical shift increases. The significant change is From propanoic acid to pentanoic acid, the chemical shift (∂) in ppm of the C atom in the COOH group in 13C - NMR increases from 184.72 ppm to 179.95 ppm. This can be related to the steric hindrance provided by the larger alkyl size of butanoic acid and pentanoic acid.
Figure 7 - Table On Variations In Wavenumber Of C = O Of COOH In cm-1 versus the chain length
Figure 8 - Variations In Wavenumber Of C = O Of COOH In cm-1 Versus The Ehain Length
As indicated in the graph above, the wavenumber of C = O of COOH group in all the carboxylic acids ranges from 1700 cm-1 to 1800 cm-1. From methanoic acid to ethanoic acid, the value is found to decrease from 1722.00 cm-1 to 1714.33 cm-1. This is followed by a sharp increase in value from 1714 cm-1 to 1782 cm-1 as we move from ethanoic to propanoic acid. Followed by this, there is not much variation. The wavenumber remains within the range 1782 cm-1 to 1783 cm-1. The increase from methanoic acid to ethanoic acid cannot be justified using the concept of electronic effects. The increase from ethanoic acid to propanoic acid may be a result of the increase in the positive inductive effect or electron releasing effect as we are moving from the methyl group in ethanoic acid to the ethyl group in propanoic acid. However, as we are moving from propanoic to pentanoic acid, the alkyl groups are changing from ethyl in propanoic acid to butyl in pentanoic acid and this does not create any significant differences in the positive inductive effect. This may be considered as a reason behind the less significant difference between the values of wavenumbers in the values of wavenumber.
As reported in the literature - “THE EFFECTS OF CHAIN LENGTH ON THE INFRARED SPECTRA OF FATTY ACIDS AND METHYL ESTERS” by R Norman Jones, the intensity and the wavenumber of the peak for C = O in COOH does not depend on the chain length at all. In the above graph, if the points for methanoic acid and ethanoic acid are ignored, the other three data points – propanoic acid, butanoic acid and pentanoic acid conforms this. As also reported in the literature, there is a difference between methanoic acid and ethanoic acid as ethanoic acid has a methyl group showing positive inductive effect unlike the methanoic acid which is not significantly observed here. Moreover, as per the literature, the sharp increase in the value of wavenumber from ethanoic to propanoic is not in agreement as per the result observed in the literature mentioned here.
Various factors like conjugation, resonance, presence of halogen atom, formation of H bonding also plays a role in impacting the values of wavenumber of C = O of COOH in IR spectra. None of the carboxylic acids can show resonance unless they lose a proton to form the conjugate base which may show resonance. They all contain only one C = O as an unsaturation within the same molecule. Thus, there is no question of alternate single and double bonds in the molecule. Thus, conjugation cannot be used to explain the variations in wavenumber. None of the acids have any substituents like halogen or nitro that may act as a electron withdrawing group affecting the electron density of the C = O in the carboxylic acid group. The only two factors that may be used here are formation of intermolecular H bonding between carboxylic acid and the increase in positive inductive effect or electron releasing effect of the alkyl groups in the carboxylic acids. The intensity of a peak in IR spectrum depends on the extent of intermolecular attraction. Stronger the intermolecular attraction between the molecules, broader the peak. Thus, if IR spectrum of carboxylic acids are recorded in solid phase or liquid phase, the existence of inter molecular H bonding associates the molecules, cause them to exist as a dimer and thus offers a broad peak rather than a narrow peak. However, it must be noted that here, the investigation compares the values of wavenumber which is in no way dependent on the intermolecular attraction and thus the presence of H bonding between carboxylic acids cannot be a driving factor behind the wavenumber of the C = O signals.
Is there any trend in the spectral data for – Chemical shift (∂) in ppm of the OH proton of carboxylic acids in 1H – NMR spectroscopy, Chemical shift (∂) in ppm of the C atom of COOH group of carboxylic acids in 13 C – NMR spectroscopy, wavenumber of the C = O of the COOH group in the IR spectroscopy down the homologous series of aliphatic mono carboxylic from-methanoic acid (HCOOH), ethanoic acid (CH3COOH), propanoic acid (C2H5COOH), butanoic acid (C3H7COOH) and pentanoic acid (C4H9COOH) ?
This means that the percentage error is \(\frac{±2.77}{9.22}\)× 100 = ± 30.04
This is quite a huge amount of percentage error and indicates a major systematic error in the experimental designing. Since this experiment is based on database, thus the only source of error can be the differences in the data procured from the three different sources indicating that that there is a challenge with the authenticity of the data sources. This limits the accuracy of the result and the reliability on the trend that is predicted from the graphs plotted.
A possible extension of this investigation could be measuring the same pattern in the homologous series of alcohols as done for the carboxylic acids. Moreover, to understand the effect of other factors like presence of halogen we can also do similar studies for chloro ethanoic acid, dichloro ethanoic acid and trichloro ethanoic acid; this will allow us to interpret how the increase in number of electronegative atom and thus the electron withdrawing effect may affect the magnitude of the spectral data like chemical shift, wavenumber and so on. Similarly, a study with chloro ethanoic acid, bromo ethanoic acid and iodo ethanoic acid can also predict how the increase in electronegativity may affect the spectral data. Conjugation and ring strain also plays a major role in affecting the electron density of a particular bond in the molecule and thus the magnitude of the spectral data associated with it. Thus, if an investigation is carried out choosing cyclopropanoic acid, cyclobutanoic acid, cyclopentanoic acid, cyclohexanoic acid; the effect of increase in ring size on the same set of spectral data can be observed.