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

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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 -\( I = \frac{1}{2},\frac{3}{2},\frac{5}{2}\) \(\) \(\)
• 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:  \(+\frac{1}{2}\)and \(-\frac{1}{2}\).
• The magnetic moment of +1/2 state is aligned with the external field, but that of \(-\frac{1}{2}\) 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.