How does the chemical shift of the methyl protons in 1-H NMR spectrum of ring substituted methyl benzene depend on the type (methyl, hydroxy, chloro, carboxylic acid & nitro) and position (ortho, meta and para) of the substituent?

Being an inquirer and a creative thinker, the real life significance of learning a new concept has always intrigued me. Though performing an investigation in the laboratory has always been a fascination for me yet, the global pandemic situation has limited that possibility. Thus the context of doing a research which is entirely based on theoretical data made the situation more challenging as well as interesting. It was during my classes of Topic – 10 Organic Chemistry where I got to know about the term synthetic routes but this brought me to a fundamental question – Using the knowledge of organic reactions and reactivity of molecules a pathway to synthesize a particular organic product can be designed and executed. But how do we ensure that the product prepared is actually the one which we intend to prepare? This is only possible when we have evidences that provide information about structural formula of the product. Spectroscopy is invariably the best analytical tool to do this. Knowledge gained from Topic 11.3 made this idea more clear to me. However, while going through the data table for NMR given in the IB Chemistry data booklet and the other NMR Samples, some other questions popped up in my mind. What factors influence the values of chemical shift? Analysing a couple of spectra of some simple compounds made it clear that the electronic environment has an impact on the value of chemical shift for that atom. Molecules which are structural isomers of each other displays differences in the position of functional group in their structure. Will this have an impact on the chemical shift values of a molecule or should the values of chemical shift be same for the set of molecules which are structural isomers of each other? This inquiry led me to the research question stated below.

Nuclear Magnetic Resonance Spectroscopy is one of the latest spectroscopic technique to elucidate the structural feature of a molecule. In short, radio-wave is sent to the nucleus which allows it to transit from a low energy state to a high energy state. The chemical shift of the samples are measured in ppm and represented by the symbol δ. Tetramethyl silane (TMS) is used as a solvent because it is highly volatile and inert in nature. Moreover, all the hydrogen in this compound are in the same chemical environment. NMR Spectroscopy can be done for both ¹H and ¹³C because they have an odd value of spin of nucleus. The NMR Spectrum of a particular sample gives information in multiple aspects – The number of peaks indicates the type of chemical environment in the molecule, the integral ratio indicates the number of H atoms belonging to each of this category of chemical environment and the values of chemical shift when compared to a literature data table indicates the type of atoms that a particular hydrogen atom is surrounded by.

A benzene ring is symmetrical in structure. However, if any hydrogen atom attached with the carbon atoms of the benzene ring is substituted by any group (Gr), then its symmetric nature is no longer existent. As a result, three different positions are created with varying characteristics. This is indicated in the diagram on the left.

According to the above figure, if any group (Gr) is attached with the benzene ring, then according to the IUPAC Nomenclature of organic compounds, the numbering will start from the Carbon atom with which the group is attached.

Let us consider the numbering of the carbon atoms of the benzene ring as shown above. Then, the following positions are named as follows:

• Position 2 and Position 6 are known as ortho position.
• Position 3 and Position 5 are known as meta position.
• Position 4 is known as para position.

Inductive Effect may be defined as a permanent effect which is executed by any group in an organic compound by pulling the bonded electron cloud towards itself or pushing the bonded electron cloud away from itself. There are two types of inductive effect. They are: + I Effect and – I Effect.

+I Effect: The phenomenon in which any group of an organic compound, pushes the electron cloud (electron releasing effect) from itself towards the other groups or atoms present at its vicinity is known as + I Effect. Alkyl group denoted by − R shows + I Effect. For example, − CH₃ (methyl), − C₂H₅ (ethyl) and many more.

-I Effect: The phenomenon in which any group of an organic compound, pulls the electron cloud (electron withdrawing effect) towards itself from the other groups or atoms present at its vicinity is known as - I Effect. Halogens denoted by − X shows – I Effect. For example − Cl (chloro), − Br (bromo) and many more.

Resonance effect may be defined as induction of a polarity on an atom of an organic compound due to presence of groups having lone pair or unsaturated bonds (double or triple bonds) at its vicinity. It happens when pi electrons from one atom transfers to nearby atom to distribute the electron cloud amongst its neighbouring atoms to decrease the electron density on itself. Hence, resulting in gaining stability of the compound. There are two types of Resonance or Mesomeric Effect. They are: +R Effect and – R Effect.

+R Effect: The phenomenon in which any group of an organic compound, distributes its pi electrons (electron releasing effect) amongst the nearby atoms is known as + R Effect.

Alcohol group (−OH) shows + R Effect.

-R Effect: The phenomenon in which any group of an organic compound, attracts pi electrons (electron releasing effect) from the nearby atoms towards itself is known as - R Effect.

Alkyl group denoted by - R shows - R Effect. Other than that, carboxylic acid (− COOH), nitro group (−N O₂) shows - R Effect.

Resonance Effect on ring substituted methyl benzene by the following groups - methyl, hydroxy, chloro, carboxylic acid and nitro are shown below using Resonating structures:

From the resonating structures of phenol, it is noted that there is generation of discrete negative polarity on ortho and para position of the benzene ring.

From the resonating structures of nitro-benzene, it is noted that there is generation of discrete positive polarity on ortho and para position of the benzene ring.

From the resonating structures of benzoic acid, it is noted that there is generation of discrete positive polarity on ortho and para position of the benzene ring.

To determine effect of electron density on the group where the 1H nucleus is attached, chemical shift of halomethane is obtained and analyzed. The following data comprise chemical shift in ppm of chloro methane, bromo methane and iodo methane in tabular form.

Here, it can be said that with an increase in electronegativity of halogen attached with the methyl group, the electron density on the methyl group decreases. Moreover, the decrease in electron density on the methyl group and resulting in 1H nucleus can be explained by the – I effect of halogens. The electron withdrawing capacity depends on the electronegativity of an atom. As the electronegativity decreases from chlorine to iodine (down the group of Gr 17 of Modern Periodic Table), the – I effect also decreases and hence, the electron density on the methyl group increases.

Here, with an increase in electron density on methyl group, the chemical shift decreases.

The values of chemical shift were taken from three different sources:

Source 1: Spectrabase is one of the largest open source repository of spectral data from John Wiley and Sons Inc. It includes spectral data obtained using various spectral techniques. Such as: Nuclear Magnetic Resonance (NMR) - CNMR, HNMR, XNMR, IR Spectroscopy, Mass Spectroscopy etc. Despite being a commercial website, it is one of the best open sources of reliable data on spectral analysis and techniques. The URL of the website is mentioned below:

http://www.spectrabase.com

Source 2: PubChem is one of the largest open source repository of spectral data after SpectraBase from National Library of Medicine operated by National Institutes of Health (NIH). It includes spectral data obtained using various spectral techniques. Such as: Nuclear Magnetic Resonance (NMR) - CNMR, HNMR, XNMR, IR Spectroscopy, Mass Spectroscopy etc. It is a website with a government domain and hence there is no question on reliability of data found in the website. The URL of the website is mentioned below:

http://www.pubchem.com

Source 3: ChemicalBook is commercial website especially used to find data or characteristics of different chemical compounds. It is similar to an educational website however, apart from the above mentioned websites, Chemical Book is not only concise with spectral data. It involves a vast data base of various compounds and their properties with application. The URL of this website is mentioned below:

http://www.chemicalbook.com

Exploration on a particular parameter of five different compounds, such as: 1. Xylene, 2. Cresol, 3. Methyl chloro benzene, 4. Toluic acid, and 5. Methyl nitro benzene has been carried in this secondary data analysis. For each compound, three positional isomers have been considered. They are – ortho, meta, and para. This isomerism (positional) is the principle independent variable in this exploration. The purpose is to understand how the positional isomerism would affect the values of chemical shift in a molecule.

The chemical shift of ¹H proton of methyl group (− CH₃) attached with the benzene ring is the dependent variable of the exploration. The chemical shift is measured in parts per million (ppm).