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

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).

For each observation, the chemical shift of H₁ proton of methyl group (− ch₃) attached with the benzene ring of all the compounds enlisted in independent variable, has been obtained from same secondary sources which are enlisted in Background Information.

TMS (Tetra methyl silane) has been used as the standard solvent in the NMR Spectroscopy for all the compounds. This is because the compound has all the H atoms in the same chemical environment. Moreover, the compound is inert in nature and does not react with the sample at all. This is also volatile in nature and can easily evaporate.

The aim of the investigation is to understand how the presence of different substituents like electron releasing substituents (-OH, CH₃) and electron withdrawing (-NO₂, CHO and COOH) and their position will affect the electronic environment around a methyl proton and thus their values of chemical shift in a 1-H NMR spectrum. To do this, three positional isomers – ortho, meta and para has been considered in each of the category. For example, in case of the substituent – nitro (NO₂), three isomers – 2-methyl nitro benzene (ortho nitro toluene), 3-methyl nitro benzene (meta nitro toluene) and 4-methyl nitro benzene (para nitro toluene) have been considered. The chemical shift of the H atom in the methyl group (CH₃) for these compound will be collected from three different sources – spectra base, pub-chem and chemical book. (Refer to Page-4 for more details). An arithmetic mean will be computed. Following this, bar graph will be plotted for all the positional isomers -ortho, meta and para of each of the category – hydroxy methyl benzene (CH₃ and OH) , dimethyl benzene (CH₃ and CH₃) , nitro methyl benzene (CH₃ and NO₂) , methyl benzaldehyde (CH₃ and CHO) and methyl benzoic acid (CH₃ and COOH). Following this, concepts of resonance and inductive effects will be used to explore how these groups impacts the electron density of the H atom in the methyl group and thus the values of their chemical shift.

Figure - 10 indicates that the chemical shift (δ in ppm) of the ¹H nucleus of the methyl group (CH₃) for o-xylene (1,2-dimethyl benzene), m-xylene (1,3-dimethyl benzene) and p-xylene (1,4-dimethyl benzene). The variation in the structure is related to the difference in the position of the alkyl groups. In, o-xylene, the two methyl groups are adjacent to each other and the signal is obtained at 2.244 ppm. In, m-xylene, the two-methyl group differ by one C atom and the signal is obtained at 2.278 ppm while in p-xylene, the difference of the two-methyl group is for two C atoms and the signal is obtained at 2.294 ppm. Thus, it is clear that as the distance between the two methyl groups increases, the magnitude of the signal increases. This means that as the two methyl groups are going away from each other, the peak gets shielded. This indicates that as the methyl groups are spaced more away from each other, the ¹H nucleus of methyl groups gets more shielded; has more electron density around itself.

This paragraph aims to provide a scientific justification for the data displayed in Figure - 10. For the sake of convenience and coherent communication, the discussion must be read in refer to the diagram above. Let us assume that the data is for the H marked in red circle in the diagram above. In, o-xylene, the methyl group (in green color) is at the immediate next C and thus the +I effect (electron releasing effect) of this group on the other methyl group is maximum.

And it is also known that the significance of +I effect reduces as the distance increases. Hence, if the three samples are compared – o-xylene, m-xylene and p-xylene; are compared the electron density at the C atom which is connected to the H atom (in red circle) whose chemical shift is referred to is maximum in case of o- xylene and least in case of p-xylene. Thus, the H atom (in red circle) is most shielded in case of o-xylene and least shielded in case of p-xylene. This is why, the separation between the two-spin state of the nucleus is minimum for the H atom in o-xylene and maximum in case of p-xylene. This explains why the signal is minimum for o-xylene and maximum for p-xylene.

Figure - 12 indicates that the chemical shift (δ in ppm) of the 1H nucleus of the methyl group (CH₃) for o-cresol (2-methyl phenol), m-cresol (3-methyl phenol) and p-cresol (4-methyl phenol). The variation in the structure is related to the difference in the position of the alkyl groups. In, o-cresol, the one methyl group and one alcohol group are adjacent to each other and the signal is obtained at 2.240 ppm. In, m-cresol, the one methyl group and one alcohol group differ by one C atom and the signal is obtained at 2.290 ppm while in p-cresol, the difference of one methyl group and one alcohol group is for two C atoms and the signal is obtained at 2.253 ppm.

This paragraph aims to provide a scientific justification for the data displayed in Figure - 13. For the sake of convenience and coherent communication, the discussion must be read in refer to the diagram above. Let us assume that the data is for the H marked in red circle in the diagram above. In, o-cresol and p-cresol, the +R effect (electron releasing effect) of hydroxy group creates discrete negative charge on the C atom of methyl group. As a result, the electron density around the group increases. Consequently, due to up shielding of 1H nucleus present in the methyl group, the chemical shift decreases.

However, the higher value of chemical shift in p-cresol than o-cresol can be explained using Inductive effect. As, hydroxy group shows -I effect (electron withdrawing effect), it will attract electron cloud towards itself. As -I effect is affected by the distance, the electron withdrawal in o-cresol is more than that of p-cresol, as methyl group is adjacent to the hydroxy group in case of o-cresol, the electron density around the methyl group will be less than that of p-cresol resulting in down shielding of nucleus. Thus, the chemical shift in o- cresol should be more than that of p-cresol. However, it is not observed in the graph. It is possible that a hydrogen bond is formed between the H-nucleus of methyl group with the oxygen atom of the hydroxy group which may result in up shielding of nucleus of methyl group. As a result, the chemical shift in o-cresol will be less.

Figure - 14 indicates that the chemical shift (δ in ppm) of the ¹H nucleus of the methyl group (CH₃) for o-toluic acid (2-methyl benzoic acid), m-toluic acid (3-methyl benzoic acid) and p-toluic acid (4-methyl benzoic acid). The variation in the structure is related to the difference in the position of the alkyl groups. In, o-toluic acid, the one methyl group and one carboxylic group is adjacent to each other and the signal is obtained at 2.681 ppm. In, m-toluic acid, the one methyl group and one carboxylic group differ by one C atom and the signal is obtained at 2.451 ppm while in p-toluic acid, the difference of one methyl group and one carboxylic group is for two C atoms and the signal is obtained at 2.681 ppm.

This paragraph aims to provide a scientific justification for the data displayed in Figure - 14. For the sake of convenience and coherent communication, the discussion must be read in refer to the diagram above. Let us assume that the data is for the H marked in red circle in the diagram above. In, o-toluic acid and p-toluic acid, the -R effect (electron withdrawing effect) of carboxylic group creates discrete positive charge on the C atom of the benzene ring at the ortho and para position. As a result, the electron density around the methyl group at ortho and para position decreases. Consequently, due to down shielding of ¹H nucleus present in the methyl group, the chemical shift increases and goes down field.

Figure - 16 indicates that the chemical shift (δ in ppm) of the 1H nucleus of the methyl group (CH₃) for o-chloro methyl benzene (2-chloro-1-methyl benzene), m-chloro methyl benzene acid (3-chloro-1-methyl benzene) and p-chloro methyl benzene acid (4-chloro-1-methyl benzene). The variation in the structure is related to the difference in the position of the alkyl groups. In, for o-chloro methyl benzene, the one methyl group and one chlorine group is adjacent to each other and the signal is obtained at 2.509 ppm.

In, for m-chloro methyl benzene, the one methyl group and one chlorine group differ by one C atom and the signal is obtained at 2.466 ppm while in for p-chloro methyl benzene, the difference of one methyl group and one chlorine group is for two C atoms and the signal is obtained at 2.321 ppm.

This paragraph aims to provide a scientific justification for the data displayed in Figure - 14. For the sake of convenience and coherent communication, the discussion must be read in refer to the diagram above. Let us assume that the data is for the H marked in red circle in the diagram above. Halogens can show both - I Effect and +R Effect. They can show – I Effect because they are more electronegative than the carbon atom and can, thus, withdraw electron of the C − X towards itself. They can show +R Effect due to the presence of lone pair. However, unlike other groups they inductive effect is given a priority over the Resonance Effect. This is because of the mismatch in size of the p orbitals of the carbon atom and the halogen atom in the C − X bond which interferes with the delocalization of the pi-electrons and limits the process of resonance. For example, in the C − Cl, the 2p orbital of carbon will have to overlaps with the 3p orbital of chlorine in case they form a pi bond during resonance. Due to the mismatch of size between 2p and 3p, this overlap is not favorable. Hence, the formation of a double bond between carbon and chlorine is not favored. This limits the transfer of lone pair from the chlorine atom to the carbon atom through the process of resonance. And hence, the inductive effect executed by the halogen atom gets a priority over the resonance effect.

As the chlorine group attracts the bonded electrons towards itself from the neighbouring atoms, the electron density on the C atom of the methyl group decreases. As a result, the nucleus of H attached with the methyl group is down shielded and chemical shift increases. Inductive effect decreases with an increase in distance. Thus, electron withdrawing effect in o-chloro methyl benzene will be maximum, comparatively less in m- chloro methyl benzene and least in p-chloro methyl benzene and so as the electron density of the methyl group.

Thus, the chemical shift in o-chloro methyl benzene will be maximum, m-chloro methyl benzene will be less than that of the previous, and p-chloro methyl benzene will be the least.

Figure - 18 indicates that the chemical shift (δ in ppm) of the ¹H nucleus of the methyl group (CH₃) for o-nitro methyl benzene (2-methyl-1-nitrobenzene), m-nitro methyl benzene (3-methyl-1-nitrobenzene) and p-nitro methyl benzene (4-methyl-1-nitrobenzene). The variation in the structure is related to the difference in the position of the alkyl groups. In, o-nitro methyl benzene, the one methyl group and one nitro group is adjacent to each other and the signal is obtained at 3.455 ppm. In, m-nitro methyl benzene, the one methyl group and one nitro group differ by one C atom and the signal is obtained at 3.332 ppm while in p-nitro methyl benzene, the difference of one methyl group and one nitro group is for two C atoms and the signal is obtained at 3.459 ppm.

This paragraph aims to provide a scientific justification for the data displayed in Figure - 19. For the sake of convenience and coherent communication, the discussion must be read in refer to the diagram above. Let us assume that the data is for the H marked in red circle in the diagram above. In, o-nitro methyl benzene and p- nitro methyl benzene, the -R effect (electron withdrawing effect) of nitro group creates discrete positive charge on the C atom of the benzene ring at ortho and para position. As a result, the electron density in the C atom of the methyl group at the ortho and para position decreases. Consequently, the  ¹H nucleus of methyl group at the ortho and para position gets deshielded which causes the chemical shift to move down field and increases in magnitude.

Hence, if the three samples are compared – o-nitro methyl benzene, m-nitro methyl benzene and p-nitro methyl benzene; are compared the electron density at the C atom which is connected to the H atom (in red circle) whose chemical shift is referred to is minimum in case of m-nitro methyl benzene and maximum in case of o- nitro methyl benzene and p-nitro methyl benzene.

How does the chemical shift of the methyl protons in ring substituted methyl benzene depends on the type (methyl, hydroxy, chloro, carboxylic acid and nitro) and position (ortho, meta and para) of the substituent in (1,2-dimethyl benzene, 1,3-dimethyl benzene and 1,4-dimethyl benzene), (o-cresol, m-cresol and p- cresol), (2-methyl chloro benzene, 3-methyl chloro benzene and 4-methyl chloro benzene), (2-methyl benzoic acid, 3-methyl benzoic acid and 4-methyl benzoic acid), (2-methyl nitro benzene, 3-methyl nitro benzene and 4-methyl nitro benzene)?

• In, o-xylene, the methyl group (in green color) is at the immediate next C and thus the +I effect (electron releasing effect) of this group on the other methyl group is maximum. And it is also known that the significance of +I effect reduces as the distance increases. Hence, if the three samples are compared – o-xylene, m-xylene and p-xylene; are compared the electron density at the C atom which is connected to the H atom (in red circle) whose chemical shift is referred to is maximum in case of o- xylene and least in case of p-xylene. Thus, the H atom (in red circle) is most shielded in case of o- xylene and least shielded in case of p-xylene. This is why, the separation between the two-spin state of the nucleus is minimum for the H atom in o-xylene and maximum in case of p-xylene. This explains why the signal is minimum for o-xylene and maximum for p-xylene.
• In o-cresol and p-cresol, the +R effect (electron releasing effect) of hydroxy group creates discrete negative charge on the C atom of methyl group. As a result, the electron density around the group increases. Consequently, due to up shielding of 1H nucleus present in the methyl group, the chemical shift decreases. However, the higher value of chemical shift in p-cresol than o-cresol can be explained using Inductive effect. As, hydroxy group shows -I effect (electron withdrawing effect), it will attract electron cloud towards itself. As -I effect is affected by the distance, the electron withdrawal in o- cresol is more than that of p-cresol, as methyl group is adjacent to the hydroxy group in case of o- cresol, the electron density around the methyl group will be less than that of p-cresol resulting in down shielding of nucleus. Thus, the chemical shift in o-cresol should be more than that of p-cresol. However, it is not observed in the graph. It is possible that a hydrogen bond is formed between the H- nucleus of methyl group with the oxygen atom of the hydroxy group which may result in up shielding of nucleus of methyl group. As a result, the chemical shift in o-cresol will be less.
• In o-toluic acid and p-toluic acid, the -R effect (electron withdrawing effect) of carboxylic group creates discrete positive charge on the C atom of the benzene ring at the ortho and para position. As a result, the electron density around the methyl group at ortho and para position decreases. Consequently, due to down shielding of 1H nucleus present in the methyl group, the chemical shift increases and goes down field.
• As the chlorine group attracts the bonded electrons towards itself from the neighbouring atoms, the electron density on the C atom of the methyl group decreases. As a result, the nucleus of H attached with the methyl group is down shielded and chemical shift increases. Inductive effect decreases with an increase in distance.
• Thus, electron withdrawing effect in o-chloro methyl benzene will be maximum, comparatively less in m-chloro methyl benzene and least in p-chloro methyl benzene and so as the electron density of the methyl group. Thus, the chemical shift in o-chloro methyl benzene will be maximum, m-chloro methyl benzene will be less than that of the previous, and p-chloro methyl benzene will be the least.
• In o-nitro methyl benzene and p-nitro methyl benzene, the -R effect (electron withdrawing effect) of nitro group creates discrete positive charge on the C atom of the benzene ring at ortho and para position. As a result, the electron density in the C atom of the methyl group at the ortho and para position decreases. Consequently, the 1H nucleus of methyl group at the ortho and para position gets deshielded which causes the chemical shift to move down field and increases in magnitude. Hence, if the three samples are compared – o-nitro methyl benzene, m-nitro methyl benzene and p-nitro methyl benzene; are compared the electron density at the C atom which is connected to the H atom (in red circle) whose chemical shift is referred to is minimum in case of m-nitro methyl benzene and maximum in case of o-nitro methyl benzene and p-nitro methyl benzene.

• The values of chemical shift (secondary data) of all fifteen compounds has been obtained from 3 different data sources and a mean value has been considered. As a result, even if a presence of discrepancy of data in either of the source, it will not affect the exploration in any means. As a result, there will not be any significant error in the exploration.
• The value of standard deviation of the chemical shift of each compound obtained in this exploration is too less to have any error or discrepancy in the collected data. Thus the result obtained can be considered to be accurate.
• The chemical shift of the compounds obtained from the online sources can be explained using the electronic effect (Inductive effect and Resonance effect). Moreover, predictions that are claimed using the electronic effect are aligned with the observed data.

The secondary data obtained from different sources are not educational websites. As the domains are commercial, it makes the authenticity of the data sources questionable. Hence, it limits the accuracy of the data.

IR spectroscopy is another method of identifying the type of bonds present in a molecule. Another exploration can be framed where the effect of electronegativity on the wave number obtained from the IR data of trihalo- alkane and trihalo-aldehyde can be determined. Hence, the research question can be framed as follows: “How does the wave number obtained from the IR spectroscopy of Set 1: trichloromethane, tribromomethane and triiodomethane and Set 2: trichloromethnal, tribromomethnal and triiodomethnal depends on the electronegativity of halogens?”

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