Investigation of trends in spectral data in homologous series of carboxylic acids

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) ?

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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:

• Integral spins:  example -  I = 1, 2, 3
• Fractional spins: example - I = 1/2, 3/2, 5/2
• Spinning charges generate magnetic fields. The spin is proportional to the magnetic moment of the resultant spin-magnet.
• In an external magnetic field (B₀), 2 spin states exist:  +1/2 and -1/2.
• The magnetic moment of +1/2 state is aligned with the external field, but that of -1/2 spin state is opposed to the external field. The arrow representing the external field points North.
• The difference in energy between the two spin states is always very small and depends on the external magnetic field strength.
• No spin:  I = 0, example - ¹²C, ¹⁶O, ³²S

• All nuclei are electrically charged and many have spin.
• Transfer of energy is possible from base energy to higher energy levels when an external magnetic field is applied.
• The transfer of energy occurs at a wavelength that coincides with the radio frequency.
• Also, energy is emitted at the same frequency when the spin comes back to its base level.
• Therefore, by measuring the signal which matches this transfer the processing of the NMR spectrum for the concerned nucleus is yield.

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 (B_o) 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 B_o 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:

• Solubility: solubility and sensitivity have a positive correlation.
• Solvent viscosity: lower the sample viscosity the better will be the spectral resolution. Due to better homogenization of the sample.
• Deuterated solvents: hydrogen needs to be replaced by deuterium to minimize the interference that is caused by protons.

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

• Electronegativity: Increase in electronegativity of surrounding groups result in decreased electron density which lead to an increase in chemical shift value due to the shielding of the nucleus.
• Hydrogen bonding: Hydrogen bonding results from the presence of electronegative atoms in neighbourhood of protons .The resulting de-shielding leads to higher values of chemical shifts. This confirms the presence of hydrogen bonding in the molecules.
• Inductive effect: The inductive effect is due to the difference in electronegativity of atoms bonded together. A bond between two atoms is polarized if there is a difference between their electronegativities. It is accepted that after four bonds, this effect is no longer detectable. It may be electron withdrawal (atoms more electronegative than carbon: O, N, F, etc.) or electron repelling (atoms less electronegative than carbon: Mg, Al, etc.) (we are dealing, here, with the bonding of different atoms to carbon).

• Bond order: higher the bond order, larger the band frequency. A C - C triple bond is stronger than a C = C bond, so a C - C triple bond has higher stretching frequency than does a C = C bond. The C - C bonds show stretching vibrations in the region from 1200 - 800 cm^-1 but these vibrations are weak and of little value in identifying compounds. Similarly, a C = O bond stretches at a higher frequency than does a C - O bond and a C - N triple bond stretches at a higher frequency than does a C = N bond which in turn stretches at a higher frequency than does a C - N bond.
• Resonance and inductive electronic effects: Different substituents of carbonyl carbon change the electronegativity of carbonyl group due to the inductive effect which arises due to the different electronegativities of the carbonyl carbon and of the substituent in compounds of the type RCOZ. Electron releasing groups attached to the carbonyl group tend to favour the polar contribution by mesomeric effect and thus lower the bond order of the C = O bond (less double bond character) hence resulting in a decrease of the carbonyl stretching frequency. Electron withdrawing groups suppress the polar contribution with an effective increase in the double bond character and hence resulting in the increase of the frequency of absorption. If this C - O bond is a part of a carboxylic group, the stretching frequency will occur at the higher end of the range. The position of the absorption varies because the bond in a carboxylic acid has partial double bond character that is due to resonance electron donation by OH group in acids.
• Bond angles : Smaller ring requires the use of more p-character to make the internal C - C bonds for the requisite small angles. This gives more s character to the C = O sigma bond which causes the strengthening and stiffening of the exocyclic double bond. The absorption frequency increases.

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.

• Physical Properties of Carboxylic Acids
  • Carboxylic acid molecules are polar due to the presence of two electronegative oxygen atoms.
  • They also participate in hydrogen bonding due to the presence of the carbonyl group (C = O) and the hydroxyl group.
  • These compounds have the ability to donate protons and are therefore Bronsted-Lowry acids.
• Chemical Properties of Carboxylic Acids:
  • These compounds can be converted into amines using the Schmidt reaction.
  • A carboxylic acid can be reduced to an alcohol by treating it with hydrogen to cause a hydrogenation reaction.
  • Upon reaction with alcohols, these compounds yield esters.

• Chain length of carboxylic acid according to number of carbon atoms in the chain  - varied by 1 Carbon atom each time.
• Samples are:-
  • Methanoic acid (CH₂O₂), 1 carbon chain
  • Ethanoic acid (CH₃COOH), 2 carbon chain
  • Propanoic acid (C₂H₅COOH), 3 carbon chain
  • Butanoic acid (C₃H₇COOH), 4 carbon chain
  • Pentanoic acid (C₄H₉COOH), 5 carbon chain.

• 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

• Functional groups - All the samples considered in this investigation are carboxylic acids.
• Sources -  All the data has been extracted from the same data source.

• Spectrabase.com: Description - SpectraBase is a freely available collection of spectra, covering hundreds of thousands of organic, organometallic and inorganic compounds. Spectra include proton NMR, heteronuclear NMR, FTIR, transmission IR, Raman, UV - VIS and mass spectra, plus basic property data, though not all spectra types are available for all compounds. Peak assignments are available for NMR spectra. The collection may be searched by chemical name, CAS Registry Number or InChi key. Only one spectrum may be displayed at a time. Materials Indexed: Datasets, Technical Data Database Type: Data Collection
• PubChem.com: This website has information about chemical compounds, and shows melting points, chemical formulas, and boiling points. PubChem is a governmentally run website, so the information should be reliable.
• ChemicalBook.com: Chemicalbook is a website that can be used to find information about different chemical products and includes suppliers where you can order these products. They launched structural formula search, supplier certifications, secondary screening, suppliers booth and mobile website.
• Webbook.nist.gov: NIST Chemistry WebBook: this is a database that provides thermochemical, thermophysical, and ion energetics. This database allows you to do a general search such as formula, IUPAC identifier or structure as well as an advanced search for more specific properties.

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

• The graph displays how the chemical shift of the OH proton in the COOH group changes as we move down the homologous series from methanoic acid to pentanoic acid.
• The graph displays that as we move from methanoic to ethanoic acid, the chemical shift increases abruptly from 9.22 to 11.65 while after that the value remains almost the same. From ethanoic to propanoic, the chemical shift decreases from 11.65 ppm to 11.64 ppm which is not a significant decrease though. From propanoic to pentanoic, the chemical shift increases from 11.64 ppm to 11.87 ppm. The increase is gradual.
• As we move from methanoic acid to ethanoic acid, the increase in the value of chemical shift may be explained on basis of the presence of the methyl group with the COOH group in ethanoic acid which is not there in methanoic acid. The methyl (CH₃) group in ethanoic acid exerts a positive inductive effect (electron releasing effect) pushing electron towards the H atom in the OH group of COOH. Such effects are not observed in case of methanoic acid due to the absence of methyl group (CH₃).
• The inductive effect exerted by an alkyl group or precisely the electron density pushed by an alkyl group towards the COOH group also depends on the chain length. As the chain length increases, the inductive effect becomes stronger. Thus, as we move from propanoic to pentanoic acid, the alkyl group changes from ethyl (C₂H₅) to butyl (C₄H₉) which in turn increases the electron releasing effect of the group and thus the electron density around the H atom in COOH group increases which eventually increases the chemical shift. However, the increase in positive inductive effect with the increase of chain length is not that significant and thus the change of chemical  shift is also not much.
• There is a sharp increase while moving from methanoic to ethanoic as we are moving from a system with no electron releasing effect towards the COOH group to a system with an alkyl group exerting positive inductive effect. Whereas the change from propanoic to pentanoic is much less as that is simply because of the increase in positive inductive effect due to the increase in chain length.
• The decrease in the value of chemical shift from ethanoic to propanoic can simply be an outcome of an experimental error as the decrease is 11.65 ppm to 11.64 ppm which is by 0.01 units and much lower than the values of absolute errors as presented in Table-1.
• As suggested in the IB Chemistry Data booklet, the acceptable range for the chemical shift of H in R-COOH is 11.00 to 13.00 and thus all values in the list are within the range except that for methanoic acid which is beyond the range.

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.

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) ?

• As we move from methanoic to ethanoic acid, the chemical shift increases abruptly from 9.22 to 11.65 while after that the value remains almost the same. From ethanoic to propanoic, the chemical shift decreases from 11.65 ppm to 11.64 ppm which is not a significant decrease though. From propanoic to pentanoic, the chemical shift increases from 11.64 ppm to 11.87 ppm. The increase is gradual.
• 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.
• 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 spectral data for any compound also depends on the physical state of the compound or the solvent in presence of which it is being measured. It was made sure that the solvent remains the same in all cases. For example, for all data in NMR, the solvent used was TMS. All data for IR was taken using the carboxylic acid in the solid / liquid phase. Thus, difference of solvent or presence of foreign particle as impurity affecting the spectral data has never been a case in anyone of the data tables.
• Though the use of three different sources to procure the data has created a high value of percentage error in certain cases yet the consideration of three different sources can also be looked upon as a factor to strengthen the investigation. Considering data from multiple sources reduces the chances of random error and thus makes the data more precise. However, the values of absolute error in this case limits the precision of the data collected.

• For all the raw data tables, the absolute error has been calculated. The values of absolute error has been obtained using the formula: absolute error = ± (max value – min value)/2. This error actually gives us an idea about the discrepancy or disagreement in the raw data values obtained from the three different sources. The values of absolute error are not negligible and especially if the magnitude of the error is compared with the magnitude of the original data, then in certain cases they questions the accuracy of the result significantly. For example, in Table - 3, where we are comparing the chemical shift (∂) in ppm of the H atom of COOH group for methanoic acid, the absolute error is ± 2.77 and the mean value is 9.22.

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.

• The investigation displayed certain trends that lacks a scientific justification. For example, trend in Graph - 8 the variation of wavenumber of C = O in COOH group with the chain length of carboxylic acid shows certain trends like decrease in value from methanoic acid to ethanoic acid, sharp increase from ethanoic acid to propanoic acid and accordingly was not in agreement with the literature values.

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.

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