Chapter 15, Problem 15.36P

Interpretation Introduction

(a)

Interpretation:

How UV-vis spectroscopy could be used to determine whether the given reaction has actually taken place or not is to be suggested with reason.

Concept introduction:

${\lambda }_{\max }$ depends heavily on the extent of conjugation. The value of the longest-wavelength ${\lambda }_{\max }$ increases as the extent of conjugation increases.

Compounds with lone pairs tend to have longer-wavelength absorptions than analogous compounds without lone pairs. The longest ${\text{λ}}_{\text{max}}$ for a species that has an available $\text{n }\to {\text{π}}^{\text{*}}$ transition (due to the presence of a lone pair) will be significantly longer than a comparable species that does not have such a transition available.

Answer to Problem 15.36P

The given reaction will take place as suggested by change in the ${\text{λ}}_{\text{max}}$ to longer wavelength.

Explanation of Solution

The given reaction is:

In the above reaction, both the reactant and product have two double bonds; in the product, both double bonds are in conjugation, but in the reactant, they are not. ${\lambda }_{\max }$ increases as the extent of conjugation increases. Thus, the ${\lambda }_{\max }$ value is shifted to longer wavelength when the given reaction takes place (occurs).

Thus, we can say that the given reaction has actually occurred.

Conclusion

Whether the given reaction has actually taken place is suggested on the basis of conjugation in the product and its effect on ${\text{λ}}_{\text{max}}$.

Interpretation Introduction

(b)

Interpretation:

How UV-vis. Spectroscopy could be used to determine whether the given reaction has actually taken place or not is to be suggested with reason.

Concept introduction:

${\lambda }_{\max }$ depends heavily on the extent of conjugation. The value of the longest-wavelength ${\lambda }_{\max }$ increases as the extent of conjugation increases.

Compounds with lone pairs tend to have longer-wavelength absorptions than analogous compounds without lone pairs. The longest ${\text{λ}}_{\text{max}}$ for a species that has an available $\text{n }\to {\text{π}}^{\text{*}}$ transition (due to the presence of a lone pair) will be significantly longer than a comparable species that does not have such a transition available.

Answer to Problem 15.36P

The given reaction does not occur as given, since the ${\text{λ}}_{\text{max}}$ has shifted to shorter wavelength.

Explanation of Solution

The given reaction is:

In the above reaction, the reactant has two double bonds in conjugation, and the product has only one double bond. Thus, ${\lambda }_{\max }$ value in the reaction shifted to shorter wavelength when the given reaction takes place.

Thus, we can say that the given reaction does not take place.

Conclusion

Whether the given reaction has actually taken place is suggested on the basis of conjugation in the product and its effect on ${\text{λ}}_{\text{max}}$.

Interpretation Introduction

(c)

Interpretation:

How UV-vis. Spectroscopy could be used to determine whether the given reaction has actually taken place or not is to be suggested with reason.

Concept introduction:

${\lambda }_{\max }$ depends heavily on the extent of conjugation. The value of the longest-wavelength ${\lambda }_{\max }$ increases as the extent of conjugation increases.

Compounds with lone pairs tend to have longer-wavelength absorptions than analogous compounds without lone pairs. The longest ${\text{λ}}_{\text{max}}$ for a species that has an available $\text{n }\to {\text{π}}^{\text{*}}$ transition (due to the presence of a lone pair) will be significantly longer than a comparable species that does not have such a transition available.

Answer to Problem 15.36P

The given reaction not take place as given, since the ${\text{λ}}_{\text{max}}$ has shifted to shorter wavelength

Explanation of Solution

The given reaction is:

In the above reaction, the reactant has three double bonds in conjugation, and the product has two double bonds not in conjugation. The value of the longest-wavelength ${\lambda }_{\max }$ increases as the extent of conjugation increases. But in this reaction, conjugation decreases.

Thus, ${\lambda }_{\max }$ value should shift to shorter wavelength if the given reaction takes place. Therefore, we can say that the reaction given reaction has not taken place.

Conclusion

Whether the given reaction actually took is suggested on the basis of conjugation in the product and its effect on ${\text{λ}}_{\text{max}}$.

Interpretation Introduction

(d)

Interpretation:

How UV-vis. Spectroscopy could be used to determine whether the given reaction has actually taken place or not is to be suggested with reason.

Concept introduction:

${\lambda }_{\max }$ depends heavily on the extent of conjugation. The value of the longest-wavelength ${\lambda }_{\max }$ increases as the extent of conjugation increases.

Compounds with lone pairs tend to have longer-wavelength absorptions than analogous compounds without lone pairs. The longest ${\text{λ}}_{\text{max}}$ for a species that has an available $\text{n }\to {\text{π}}^{\text{*}}$ transition (due to the presence of a lone pair) will be significantly longer than a comparable species that does not have such a transition available.

Answer to Problem 15.36P

The given reaction will take place as suggested by change in the ${\text{λ}}_{\text{max}}$ to longer wavelength.

Explanation of Solution

The given reaction is:

In the above reaction, both reactant and product have three triple bonds in conjugation, but the reactant has a lone pair. So, the reactant’s longest-wavelength absorption corresponds to a $\text{π }\to {\text{π}}^{\text{*}}$ transition. The $\text{n }\to {\text{π}}^{\text{*}}$ transition requires lower energy or longer-wavelength light.

Therefore the ${\text{λ}}_{\text{max}}$ shifted to longer, we can say that the given reaction actually take place.

Conclusion

Whether the given reaction actually took is suggested on the basis of conjugation in the product and its effect on ${\text{λ}}_{\text{max}}$.

Interpretation Introduction

(e)

Interpretation:

How UV-vis. Spectroscopy could be used to determine whether the given reaction actually took is to be suggested with reason.

Concept introduction:

${\lambda }_{\max }$ depends heavily on the extent of conjugation. The value of the longest-wavelength ${\lambda }_{\max }$ increases as the extent of conjugation increases.

Compounds with lone pairs tend to have longer-wavelength absorptions than analogous compounds without lone pairs. The longest ${\text{λ}}_{\text{max}}$ for a species that has an available $\text{n }\to {\text{π}}^{\text{*}}$ transition (due to the presence of a lone pair) will be significantly longer than a comparable species that does not have such a transition available.

Answer to Problem 15.36P

The given reaction will take place as suggested by change in the ${\text{λ}}_{\text{max}}$ to longer wavelength.

Explanation of Solution

The given reaction is:

In the above reaction, the reactant has three conjugated bonds, and the product has four conjugated double bonds. ${\lambda }_{\max }$ increases as the extent of conjugation increases. Thus, ${\lambda }_{\max }$ value in the reaction shifted to longer when the given reaction took place.

Thus, we can say that the given reaction is actually took place.

Conclusion

Whether the given reaction actually took is suggested on the basis of conjugation in the product and its effect on ${\text{λ}}_{\text{max}}$.