Unveiling the Secrets of Organic Molecules
MSc Chemistry MSc III SEM

Unveiling the Secrets of Organic Molecules 2023 useful

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Unveiling the Secrets of Organic Molecules. The third semester MSc Organic Spectroscopy question paper aims to test the knowledge and understanding of students regarding advanced spectroscopic techniques and their applications in the structural elucidation and analysis of organic compounds.

The paper is designed to challenge students to think critically and creatively to solve problems related to spectroscopic data interpretation and analysis.

Unveiling the Secrets of Organic Molecules

Unveiling the Secrets of Organic Molecules
Unveiling the Secrets of Organic Molecules

why ethylene protons are de-shielded than acetylene protons?

The protons in ethylene (C2H4) are more de-shielded than the protons in acetylene (C2H2) due to the differences in the electron density surrounding the two molecules.

In ethylene, the two carbon atoms are bonded to each other through a double bond, which consists of one sigma bond and one pi bond. The pi bond consists of two electrons that are localized in the region above and below the plane of the molecule. This electron density creates a magnetic field that opposes the external magnetic field applied during NMR spectroscopy, causing the protons in the molecule to experience a higher chemical shift and appear more de-shielded.

Unveiling the Secrets of Organic Molecules

In acetylene, the two carbon atoms are bonded to each other through a triple bond, which consists of one sigma bond and two pi bonds. The pi bonds in acetylene are more diffuse than the pi bond in ethylene, which means that the electron density is spread out over a larger region. This results in a weaker magnetic field opposing the external magnetic field, causing the protons in acetylene to experience a lower chemical shift and appear less de-shielded than the protons in ethylene.

In summary, the electron density surrounding the pi bond in ethylene is more localized and concentrated, resulting in a stronger magnetic field opposing the external magnetic field and causing the protons to appear more de-shielded in NMR spectroscopy.

Unveiling the Secrets of Organic Molecules 2023 useful

define the terms chemical equivalence and magnetic equivalence of protons in NMR spectroscopy

Chemical equivalence and magnetic equivalence are important concepts in NMR spectroscopy that are used to describe the behavior of protons in a molecule.

Chemical equivalence refers to the situation where two or more protons in a molecule have the same chemical environment, meaning that they experience the same electronic environment and are chemically indistinguishable. This means that they will produce a single NMR signal that represents the average of the signals of the chemically equivalent protons. For example, in methane (CH4), all four hydrogen atoms are chemically equivalent and produce a single NMR signal.

Unveiling the Secrets of Organic Molecules

Magnetic equivalence refers to the situation where two or more protons in a molecule have the same magnetic environment, meaning that they experience the same magnetic field strength and are magnetically indistinguishable. This means that they will produce a single NMR signal that represents the average of the signals of the magnetically equivalent protons.

For example, in ethane (C2H6), the two methyl groups are magnetically equivalent because they have the same distance and orientation relative to the external magnetic field.

Unveiling the Secrets of Organic Molecules

It is important to note that chemical equivalence and magnetic equivalence are not always the same. Two protons may be chemically equivalent but not magnetically equivalent if they have different distances or orientations relative to the external magnetic field.

On the other hand, two protons may be magnetically equivalent but not chemically equivalent if they have different electronic environments, such as if they are in different functional groups or if they are affected by nearby electronegative atoms or groups.

Unveiling the Secrets of Organic Molecules

how will you distinguish t-butyl benzene and isobutyl benzene by mass spectrometry?

T-butyl benzene and isobutyl benzene can be distinguished by mass spectrometry based on the molecular ion peaks and the fragmentation patterns observed in their mass spectra.

T-butyl benzene has a tert-butyl group attached to the benzene ring, while isobutyl benzene has an isopropyl group attached to the benzene ring. In the mass spectrum of t-butyl benzene, the parent ion peak (M+) will be observed at a higher m/z value than in the mass spectrum of isobutyl benzene due to the larger molecular weight of t-butyl benzene.

Unveiling the Secrets of Organic Molecules 2023 useful

Additionally, the fragmentation patterns observed in the mass spectra of the two compounds can provide further information for differentiation. T-butyl benzene is expected to undergo fragmentation of the tert-butyl group, which would result in the loss of a 2-methyl propyl radical (C4H9), while isobutyl benzene is expected to undergo fragmentation of the isopropyl group, which would result in the loss of an isopropyl radical (C3H7).

Unveiling the Secrets of Organic Molecules

Therefore, the mass spectrum of t-butyl benzene would show a fragment peak at m/z 120, corresponding to the loss of the 2-methyl propyl radical, while the mass spectrum of isobutyl benzene would show a fragment peak at m/z 91, corresponding to the loss of the isopropyl radical.

In summary, mass spectrometry can be used to distinguish t-butyl benzene and isobutyl benzene based on the molecular ion peaks and the fragmentation patterns observed in their mass spectra.

Unveiling the Secrets of Organic Molecules

Unveiling the Secrets of Organic Molecules
Unveiling the Secrets of Organic Molecules

explain why the stretching frequency of the carbon-nitrogen triple bond is greater than that of a single bond.

The stretching frequency of a carbon-nitrogen triple bond is greater than that of a carbon-nitrogen single bond because the bond is shorter and stronger, and the atoms involved in the bond vibrate at a higher frequency.

In a carbon-nitrogen triple bond, there are three sets of bonding electrons shared between the carbon and nitrogen atoms, which creates a stronger bond compared to a single bond where there is only one set of bonding electrons. The triple bond also has a shorter bond length than a single bond, which means that the carbon and nitrogen atoms are held closer together and experience stronger attractive forces.

When a molecule containing a carbon-nitrogen triple bond is exposed to infrared radiation, the bond between the carbon and nitrogen atoms will absorb energy and vibrate at a characteristic frequency.

Unveiling the Secrets of Organic Molecules 2023 useful

This frequency is higher than the frequency of vibration of a single bond because the bond is shorter and stronger, and requires more energy to vibrate.

In contrast, a single bond between carbon and nitrogen atoms has a longer bond length and weaker attractive forces. When a molecule containing a carbon-nitrogen single bond is exposed to infrared radiation, the bond between the carbon and nitrogen atoms will absorb energy and vibrate at a lower frequency.

In summary, the stretching frequency of a carbon-nitrogen triple bond is greater than that of a single bond because the bond is shorter and stronger, and requires more energy to vibrate. This is due to the presence of three sets of bonding electrons and stronger attractive forces between the carbon and nitrogen atoms in the triple bond.

Unveiling the Secrets of Organic Molecules

mention the different types of electronic transitions in UV-vis spectroscopy. arrange them in the decreasing order of their energy.

UV-Vis spectroscopy is a technique used to study electronic transitions in molecules. There are several types of electronic transitions in UV-Vis spectroscopy, arranged here in decreasing order of energy:

  1. π → π* transitions: In this type of transition, an electron in a π bonding orbital is excited to a higher energy π* antibonding orbital. This transition occurs in molecules with conjugated double bonds, such as benzene, and has the highest energy.
  2. n → π* transitions: In this type of transition, an electron in a non-bonding (n) orbital is excited to a higher energy π* antibonding orbital. This transition occurs in molecules with lone pairs of electrons, such as carbonyl compounds, and has a slightly lower energy than π → π* transitions.
  3. σ → σ* transitions: In this type of transition, an electron in a σ bonding orbital is excited to a higher energy σ* antibonding orbital. This transition occurs in molecules with single bonds, such as alkanes, and has the lowest energy of the three types of transitions.

It is important to note that the exact energy of each type of transition can vary depending on the specific molecule and its electronic structure.

In summary, the different types of electronic transitions in UV-Vis spectroscopy are π → π* transitions, n → π* transitions, and σ → σ* transitions, arranged in decreasing order of energy.

Unveiling the Secrets of Organic Molecules

state the rule of multiplicity in 1H NMR spectroscopy. give an example.

The rule of multiplicity in 1H NMR spectroscopy states that the number of peaks observed in a signal is determined by the number of neighboring, nonequivalent hydrogen atoms. This rule is also known as the n+1 rule, where n is the number of neighboring nonequivalent hydrogen atoms.

For example, if a proton in a molecule is adjacent to three nonequivalent hydrogen atoms, then its signal in the 1H NMR spectrum will be split into a quartet. The quartet arises from the interaction between the spin of the observed proton and the spin of its three neighboring nonequivalent hydrogen atoms. According to the n+1 rule, the number of peaks in the signal is equal to n+1, which in this case is four.

Unveiling the Secrets of Organic Molecules

The splitting pattern resulting from the rule of multiplicity can provide important information about the chemical environment of the observed proton and its neighboring nonequivalent hydrogen atoms. By analyzing the splitting patterns, chemists can deduce the connectivity and stereochemistry of a molecule.

In summary, the rule of multiplicity in 1H NMR spectroscopy states that the number of peaks observed in a signal is determined by the number of neighboring, nonequivalent hydrogen atoms, and the n+1 rule can be used to predict the splitting pattern.

Unveiling the Secrets of Organic Molecules

name the different types of relaxation processes in NMR. Which process is responsible for the NMR phenomenon?

There are two types of relaxation processes in NMR spectroscopy:

Unveiling the Secrets of Organic Molecules
Unveiling the Secrets of Organic Molecules
  1. T1 relaxation (spin-lattice relaxation): In this process, the energy of the excited spin system is transferred to the surrounding lattice. The excited nucleus loses energy and returns to its equilibrium state aligned with the external magnetic field.
  2. T2 relaxation (spin-spin relaxation): In this process, the nuclear spins interact with each other and with their local magnetic environment. The magnetic field experienced by each nucleus fluctuates due to the motion of nearby nuclei, resulting in a loss of coherence between the spins.

The NMR phenomenon is due to the T1 and T2 relaxation processes. When a sample is placed in an external magnetic field and irradiated with radiofrequency energy, the nuclei in the sample absorb and re-emit the energy.

Unveiling the Secrets of Organic Molecules 2023 useful

The rate of absorption and re-emission is determined by the relaxation times T1 and T2, which in turn depend on the molecular environment and motion of the nuclei in the sample.

T1 relaxation is responsible for the recovery of magnetization after a radiofrequency pulse is applied, while T2 relaxation is responsible for the decay of transverse magnetization.

Unveiling the Secrets of Organic Molecules

By measuring the T1 and T2 relaxation times, chemists can obtain information about the molecular motion and environment of the nuclei in a sample, which can be used to determine the chemical structure and composition of the sample.

FOR BSC CHEMISTRY NOTES

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