Chapter 23, Problem 1Q

Interpretation Introduction

(a)

Interpretation:

The number of signals of $\text{2-pentanol}$, $\text{2,2-dimethylbutane}$, Isopropyl acetate and $\text{2-acetoxybutane}$ in decoupled ${}^{\text{13}}\text{C}$ NMR should be given.

Concept introduction:

Nuclear magnetic resonance spectroscopy is applied for the identification of the structure of molecules. The energy in the radiofrequency region is suitable for NMR. Nuclear magnetic resonance results from the spin of the nucleus of an atom. The value of I is obtained using the atomic number and the mass number of an atom. The non-zero magnetic moment of an isotopic nucleus is detectable by the NMR technique.

Any nucleus with both an even atomic number and the mass number has 0 nuclear spins. There are a total of $\left(2\text{I}+1\right)$ energy levels that are allowed for a nuclear spin (I). In the absence of a magnetic field, energy levels are equal in energy that is degenerate.

However, energy levels become non-degenerate in the presence of a magnetic field.

Deuterated chloroform $\left({\text{CDCl}}_{\text{3}}\right)$ is a common NMR solvent generally used because it dissolutes a broad range of organic compounds and is inexpensive.

The total signal intensity of each set of proton is given by the height of each set of steps. The integration value defines the relative number of each kind of proton in the molecule.

In NMR spectrum, the intensity of signals is plotted against the magnetic field or frequency. Nuclei that are non-equivalent show only one peak in the NMR spectrum. However, protons absorb at different frequencies that are non-equivalent.

An increase in the electron density that surrounds the nucleus shields it from the applied field. This results in a net decrease in the field experienced by the nucleus. The value of the observed chemical shift of the signal therefore decreases, and, on a typical NMR spectrum, the signal moves to the right, which is called an upfield shift because, at a constant frequency, a slightly higher applied magnetic field is required for resonance to occur. De-shielding is the effect of a decline in the electron density around a nucleus which leads to shifting in the peaks of a chemical shift towards left in the NMR spectrum that results in an increase in delta values, hence downshift.

Explanation of Solution

The decoupled ${}^{\text{13}}\text{C}$ NMR signals of a compound that is pure tell about the number of different kinds of carbon atoms present in the molecule. If the number of carbons in compound exceeds the number of signals, it indicates the presence of element of symmetry, which makes carbon atoms equivalent to each other.

  $\text{2-pentanol}$ has 5 different types of carbon atoms so 5 signals are obtained for it.

  $\text{2,2-dimethylbutane}$ has 4 different types of carbon atoms as 2 methyl substituents have equivalent chemical environment. So they show 4 signals.

Isopropyl acetate has 4 different types of carbon as here also 2 methyl groups in the isopropyl group that have equivalent chemical environment. So they show 4 signals.

  $\text{2-acetoxybutane}$ has 5 different types of carbon atoms so 5 signals are obtained for it.

Interpretation Introduction

(b)

Interpretation:

The number of signals of $para\text{-aminobenzoic acid}$, $\text{methyl 2-hydroxybenzoate}$, $\text{1-phenyl-2-methylpropane}$ and $\text{1,3-cyclopentadiene}$ in decoupled ${}^{\text{13}}\text{C}$ NMR should be given.

Concept introduction:

Nuclear magnetic resonance spectroscopy is applied for the identification of the structure of molecules. The energy in the radiofrequency region is suitable for NMR. Nuclear magnetic resonance results from the spin of the nucleus of an atom. The value of I is obtained using the atomic number and the mass number of an atom. The non-zero magnetic moment of an isotopic nucleus is detectable by the NMR technique.

Any nucleus with both an even atomic number and the mass number has 0 nuclear spins. There are a total of $\left(2\text{I}+1\right)$ energy levels that are allowed for a nuclear spin (I). In the absence of a magnetic field, energy levels are equal in energy that is degenerate.

However, energy levels become non-degenerate in the presence of a magnetic field.

Deuterated chloroform $\left({\text{CDCl}}_{\text{3}}\right)$ is a common NMR solvent generally used because it dissolutes a broad range of organic compounds and is inexpensive.

The total signal intensity of each set of proton is given by the height of each set of steps. The integration value defines the relative number of each kind of proton in the molecule.

In NMR spectrum, the intensity of signals is plotted against the magnetic field or frequency. Nuclei that are non-equivalent show only one peak in the NMR spectrum. However, protons absorb at different frequencies that are non-equivalent.

An increase in the electron density that surrounds the nucleus shields it from the applied field. This results in a net decrease in the field experienced by the nucleus. The value of the observed chemical shift of the signal therefore decreases, and, on a typical NMR spectrum, the signal moves to the right, which is called an upfield shift because, at a constant frequency, a slightly higher applied magnetic field is required for resonance to occur. De-shielding is the effect of a decline in the electron density around a nucleus which leads to shifting in the peaks of a chemical shift towards left in the NMR spectrum that results in an increase in delta values, hence downshift.

Explanation of Solution

The decoupled ${}^{\text{13}}\text{C}$ NMR signals of a compound that is pure tell about the number of different kinds of carbon atoms present in the molecule. If the number of carbons in compound exceeds the number of signals, it indicates the presence of element of symmetry, which makes carbon atoms equivalent to each other.

  $para\text{-aminobenzoic acid }$ has 5 different types of carbon atoms as it is symmetrical molecule with a plane of symmetry in it so 5 signals are obtained for it.

  $\text{methyl 2-hydroxybenzoate}$ has 8 different types of carbon atoms so they show 4 signals.

  $\text{1-phenyl-2-methylpropane}$ has 7 different types of carbon as here also 2 methyl groups in the propane have equivalent chemical environment. Also the ortho and meta carbons on phenyl group, relative to the substituent are in similar environment hence are equivalent. So they show 7 signals.

  $\text{1,3-cyclopentadiene}$ has 3 different types of carbon atoms because of the presence of plane of symmetry in it so 3 signals are obtained for it.

Interpretation Introduction

(c)

Interpretation:

The number of signals for cyclohexane, $trans\text{-1,4-dimethylcyclohexane}$ and $trans\text{-1,2- dimethylcyclohexane}$ in decoupled ${}^{\text{13}}\text{C}$ NMR should be given.

Concept introduction:

Nuclear magnetic resonance spectroscopy is applied for the identification of the structure of molecules. The energy in the radiofrequency region is suitable for NMR. Nuclear magnetic resonance results from the spin of the nucleus of an atom. The value of I is obtained using the atomic number and the mass number of an atom. The non-zero magnetic moment of an isotopic nucleus is detectable by the NMR technique.

Any nucleus with both an even atomic number and the mass number has 0 nuclear spins. There are a total of $\left(2\text{I}+1\right)$ energy levels that are allowed for a nuclear spin (I). In the absence of a magnetic field, energy levels are equal in energy that is degenerate.

However, energy levels become non-degenerate in the presence of a magnetic field.

Deuterated chloroform $\left({\text{CDCl}}_{\text{3}}\right)$ is a common NMR solvent generally used because it dissolutes a broad range of organic compounds and is inexpensive.

The total signal intensity of each set of proton is given by the height of each set of steps. The integration value defines the relative number of each kind of proton in the molecule.

In NMR spectrum, the intensity of signals is plotted against the magnetic field or frequency. Nuclei that are non-equivalent show only one peak in the NMR spectrum. However, protons absorb at different frequencies that are non-equivalent.

An increase in the electron density that surrounds the nucleus shields it from the applied field. This results in a net decrease in the field experienced by the nucleus. The value of the observed chemical shift of the signal therefore decreases, and, on a typical NMR spectrum, the signal moves to the right, which is called an upfield shift because, at a constant frequency, a slightly higher applied magnetic field is required for resonance to occur. De-shielding is the effect of a decline in the electron density around a nucleus which leads to shifting in the peaks of a chemical shift towards left in the NMR spectrum that results in an increase in delta values, hence downshift.

Explanation of Solution

The decoupled ${}^{\text{13}}\text{C}$ NMR signals of a compound that is pure tell about the number of different kinds of carbon atoms present in the molecule. If the number of carbons in the compound exceeds the number of signals, it indicates the presence of element of symmetry, which makes carbon atoms equivalent to each other.

Cyclohexane has 1 type of carbon atom as it is symmetrical molecule with 6 planes of symmetry in it so 1 signal is obtained for it.

  $trans\text{-1,4-dimethylcyclohexane}$ has 3 different types of carbon atoms as it is symmetrical molecule with a plane of symmetry in it so they show 3 signals.

  $trans\text{-1,2- dimethylcyclohexane}$ has 4 different types of carbon as here also plane of symmetry is present which makes carbon equivalent in chemical environment. So they show 4 signals.

Interpretation Introduction

(d)

Interpretation:

The number of signals in the following structures in decoupled ${}^{\text{13}}\text{C}$ NMR should be given.

Concept introduction:

Nuclear magnetic resonance spectroscopy is applied for the identification of the structure of molecules. The energy in the radiofrequency region is suitable for NMR. Nuclear magnetic resonance results from the spin of the nucleus of an atom. The value of I is obtained using the atomic number and the mass number of an atom. The non-zero magnetic moment of an isotopic nucleus is detectable by the NMR technique.

Any nucleus with both an even atomic number and the mass number has 0 nuclear spins. There are a total of $\left(2\text{I}+1\right)$ energy levels that are allowed for a nuclear spin (I). In the absence of a magnetic field, energy levels are equal in energy that is degenerate.

However, energy levels become non-degenerate in the presence of a magnetic field.

Deuterated chloroform $\left({\text{CDCl}}_{\text{3}}\right)$ is a common NMR solvent generally used because it dissolutes a broad range of organic compounds and is inexpensive.

The total signal intensity of each set of proton is given by the height of each set of steps. The integration value defines the relative number of each kind of proton in the molecule.

In NMR spectrum, the intensity of signals is plotted against the magnetic field or frequency. Nuclei that are non-equivalent show only one peak in the NMR spectrum. However, protons absorb at different frequencies that are non-equivalent.

An increase in the electron density that surrounds the nucleus shields it from the applied field. This results in a net decrease in the field experienced by the nucleus. The value of the observed chemical shift of the signal therefore decreases, and, on a typical NMR spectrum, the signal moves to the right, which is called an upfield shift because, at a constant frequency, a slightly higher applied magnetic field is required for resonance to occur. De-shielding is the effect of a decline in the electron density around a nucleus which leads to shifting in the peaks of a chemical shift towards left in the NMR spectrum that results in an increase in delta values, hence downshift.

Explanation of Solution

The decoupled ${}^{\text{13}}\text{C}$ NMR signals of a compound that is pure tell about the number of different kinds of carbon atoms present in the molecule. If the number of carbons in compound exceeds the number of signals, it indicates the presence of element of symmetry, which makes carbon atoms equivalent to each other.

The number of signals in the structures is indicated as follows: