Chapter 12, Problem 12.10P

Interpretation Introduction

(a)

Interpretation:

Whether the $\text{C}\equiv \text{C}$ stretch in the compound ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}\text{C}\equiv \text{CH}$ is IR active or IR inactive is to be stated.

Concept introduction:

The IR rays are able to stretch the bond in the compound as they carry the energy sufficient to perform this task. The compound is said to be IR active if the energy absorbed the bond leads to change in the dipole moment of the molecule.

Answer to Problem 12.10P

The $\text{C}\equiv \text{C}$ stretch in the compound ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}\text{C}\equiv \text{CH}$ is IR active.

Explanation of Solution

Change in the dipole moment of the molecule on IR absorption is the necessary condition to be IR active.

The $\text{C}\equiv \text{C}$ stretch in the compound ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}\text{C}\equiv \text{CH}$ leads to change in the dipole moment of the molecule as the molecule is unsymmetrical about the $\text{C}\equiv \text{C}$ bond.

Therefore, $\text{C}\equiv \text{C}$ stretch in the compound ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}\text{C}\equiv \text{CH}$ is IR active.

Conclusion

The $\text{C}\equiv \text{C}$ stretch in the compound ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}\text{C}\equiv \text{CH}$ is IR active due to the change in the dipole moment of the molecule.

Interpretation Introduction

(b)

Interpretation:

Whether the $\text{C}=\text{O}$ stretch in the compound ${\left({\text{CH}}_{\text{3}}\right)}_{\text{2}}\text{C}=\text{O}$ is IR active or IR inactive is to be stated.

Concept introduction:

The IR rays are able to stretch the bond in the compound as they carry the energy sufficient to perform this task. The compound is said to be IR active if the energy absorbed the bond leads to change in the dipole moment of the molecule.

Answer to Problem 12.10P

The $\text{C}=\text{O}$ stretch in the compound ${\left({\text{CH}}_{\text{3}}\right)}_{\text{2}}\text{C}=\text{O}$ is IR active.

Explanation of Solution

Change in the dipole moment of the molecule on IR absorption is the necessary condition to be IR active.

The $\text{C}=\text{O}$ stretch in the compound ${\left({\text{CH}}_{\text{3}}\right)}_{\text{2}}\text{C}=\text{O}$ leads to change in the dipole moment of the molecule. The $\text{C}=\text{O}$ bond is a polar bond already and if the bond is stretched then the distance of charge separation will change and thus the dipole moment as well.

Therefore, $\text{C}=\text{O}$ stretch in the compound ${\left({\text{CH}}_{\text{3}}\right)}_{\text{2}}\text{C}=\text{O}$ is IR active.

Conclusion

The $\text{C}=\text{O}$ stretch in the compound ${\left({\text{CH}}_{\text{3}}\right)}_{\text{2}}\text{C}=\text{O}$ is IR active due to the change in the dipole moment of the molecule.

Interpretation Introduction

(c)

Interpretation:

Whether the cyclohexane ring “breathing” or simultaneous stretching of the all $\text{C}-\text{C}$ bonds is IR active or IR inactive is to be stated.

Concept introduction:

The IR rays are able to stretch the bond in the compound as they carry the energy sufficient to perform this task. The compound is said to be IR active if the energy absorbed the bond leads to change in the dipole moment of the molecule.

Answer to Problem 12.10P

The simultaneous stretch of all $\text{C}-\text{C}$ bonds stretch in the cyclohexane is IR inactive.

Explanation of Solution

Change in the dipole moment of the molecule on IR absorption is the necessary condition to be IR active.

The simultaneous stretch of all $\text{C}-\text{C}$ bonds stretch in the cyclohexane does not lead to change in the dipole moment of the molecule. The simultaneous stretch of $\text{C}-\text{C}$ bonds in the cyclohexane leads to an increase in the size of the ring but the ring is still symmetric and does not have any polar bond and therefore, no change in the dipole moment.

Therefore, simultaneous stretch of all $\text{C}-\text{C}$ bonds stretch in the cyclohexane is IR inactive.

Conclusion

The simultaneous stretch of all $\text{C}-\text{C}$ bonds stretch in the cyclohexane is IR inactive due to the change in the dipole moment of the molecule.

Interpretation Introduction

(d)

Interpretation:

Whether the $\text{C}\equiv \text{C}$ stretch in the compound ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}\text{C}\equiv {\text{CCH}}_{\text{2}}{\text{CH}}_{\text{3}}$ is IR active or IR inactive is to be stated.

Concept introduction:

The IR rays are able to stretch the bond in the compound as they carry the energy sufficient to perform this task. The compound is said to be IR active if the energy absorbed the bond leads to change in the dipole moment of the molecule.

Answer to Problem 12.10P

The $\text{C}\equiv \text{C}$ stretch in the compound ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}\text{C}\equiv {\text{CCH}}_{\text{2}}{\text{CH}}_{\text{3}}$ is IR inactive.

Explanation of Solution

Change in the dipole moment of the molecule on IR absorption is the necessary condition to be IR active.

The $\text{C}\equiv \text{C}$ stretch in the compound ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}\text{C}\equiv {\text{CCH}}_{\text{2}}{\text{CH}}_{\text{3}}$ does not lead to change in the dipole moment of the molecule as the molecule is symmetrical about the $\text{C}\equiv \text{C}$ bond.

Therefore, $\text{C}\equiv \text{C}$ stretch in the compound ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}\text{C}\equiv {\text{CCH}}_{\text{2}}{\text{CH}}_{\text{3}}$ is IR inactive.

Conclusion

The $\text{C}\equiv \text{C}$ stretch in the compound ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}\text{C}\equiv {\text{CCH}}_{\text{2}}{\text{CH}}_{\text{3}}$ is IR inactive due to the no change in the dipole moment of the molecule.

Interpretation Introduction

(e)

Interpretation:

Whether the symmetrical stretching of the $\text{N}-\text{O}$ bond in nitromethane is IR active or IR inactive is to be stated.

Concept introduction:

The IR rays are able to stretch the bond in the compound as they carry the energy sufficient to perform this task. The compound is said to be IR active if the energy absorbed the bond leads to change in the dipole moment of the molecule.

Answer to Problem 12.10P

The symmetric stretch of $\text{N}-\text{O}$ bond in the nitromethane is IR active.

Explanation of Solution

Change in the dipole moment of the molecule on IR absorption is the necessary condition to be IR active.

The symmetric stretch of $\text{N}-\text{O}$ bond in the nitromethane leads to change in the dipole moment of the molecule. The $\text{N}-\text{O}$ bond is a polar bond and the symmetric stretch of $\text{N}-\text{O}$ bond will lead to change in the distance between the charge separation and therefore, in the dipole moment.

Therefore, symmetric stretch of $\text{N}-\text{O}$ bond in the nitromethane is IR active.

Conclusion

The symmetric stretch of $\text{N}-\text{O}$ bond in the nitromethane is IR active due to the change in the dipole moment of the molecule.

Interpretation Introduction

(f)

Interpretation:

Whether the $\text{C}-\text{Cl}$ bond stretch in ${\left({\text{CH}}_{\text{3}}\right)}_{\text{3}}\text{C}-\text{Cl}$ is IR active or IR inactive is to be stated.

Concept introduction:

The IR rays are able to stretch the bond in the compound as they carry the energy sufficient to perform this task. The compound is said to be IR active if the energy absorbed the bond leads to change in the dipole moment of the molecule.

Answer to Problem 12.10P

The stretch of $\text{C}-\text{Cl}$ bond in the ${\left({\text{CH}}_{\text{3}}\right)}_{\text{3}}\text{C}-\text{Cl}$ is IR active.

Explanation of Solution

Change in the dipole moment of the molecule on IR absorption is the necessary condition to be IR active.

The stretch of $\text{C}-\text{Cl}$ bond in the ${\left({\text{CH}}_{\text{3}}\right)}_{\text{3}}\text{C}-\text{Cl}$ leads to change in the dipole moment of the molecule. The $\text{C}-\text{Cl}$ bond is a polar bond and the stretch of $\text{C}-\text{Cl}$ bond will lead to change in the distance between the charge separation and therefore, in the dipole moment.

Therefore, stretch of $\text{C}-\text{Cl}$ bond in the ${\left({\text{CH}}_{\text{3}}\right)}_{\text{3}}\text{C}-\text{Cl}$ is IR active.

Conclusion

The stretch of $\text{C}-\text{Cl}$ bond in the ${\left({\text{CH}}_{\text{3}}\right)}_{\text{3}}\text{C}-\text{Cl}$ is IR active due to the change in the dipole moment of the molecule.

Interpretation Introduction

(g)

Interpretation:

Whether the $\text{C=C}$ stretch in the compound $trans\text{-3-hexene}$ is IR active or IR inactive is to be stated.

Concept introduction:

The IR rays are able to stretch the bond in the compound as they carry the energy sufficient to perform this task. The compound is said to be IR active if the energy absorbed the bond leads to change in the dipole moment of the molecule.

Answer to Problem 12.10P

The $\text{C=C}$ stretch in the compound $trans\text{-3-hexene}$ is IR inactive.

Explanation of Solution

Change in the dipole moment of the molecule on IR absorption is the necessary condition to be IR active.

The structure of $trans\text{-3-hexene}$ is shown below.

Figure 1

The $\text{C=C}$ stretch in the compound $trans\text{-3-hexene}$ does not lead to change in the dipole moment of the molecule as the molecule is symmetrical about the $\text{C=C}$ bond.

Therefore, $\text{C=C}$ stretch in the compound $trans\text{-3-hexene}$ is IR inactive.

Conclusion

The $\text{C=C}$ stretch in the compound $trans\text{-3-hexene}$ is IR inactive due to the no change in the dipole moment of the molecule.