Chapter 27, Problem 27.1P

Interpretation Introduction

(a)

Interpretation: The given reaction is to be classified as an electrocyclic reaction, cycloaddition, or a sigmatropic rearrangement. The sigma bonds that are broken or formed in the given reaction are to be labeled.

Concept introduction: A chemical reaction that involves $\left[4+2\right]$ cycloaddition is called Diels-Alder reaction. The reactant molecules that give rise to the product are diene and dienophile. The rate of reaction is fast when the electronegative group is attached to dienophile. In the sigmatropic rearrangement, the migration of the sigma bond takes place. Electrocyclic reactions involve the conversion of $\text{σ}$ bonds into $\text{π}$ bonds or $\text{π}$ bonds into $\text{σ}$ bonds.

Answer to Problem 27.1P

The reaction is classified as an electrocyclic reaction. The sigma bond broken or formed in the given reaction is,

Explanation of Solution

The given reaction is shown below.

Figure 1

The sigma bond that is broken during the conversion of reactant into product is shown in red. The new bonds formed during the conversion are shown by curved arrows in the reactant. In the product, the ring is opened due to the rearrangement of $\text{π}$ bonds and dissociation of $\text{σ}$ bond. Therefore, the reaction is classified as an electrocyclic reaction.

Conclusion

The reaction is classified as an electrocyclic reaction. The sigma bond that is broken or formed in the given reaction is labeled in Figure 1.

Interpretation Introduction

(b)

Interpretation: The given reaction is to be classified as an electrocyclic reaction, cycloaddition, or a sigmatropic rearrangement. The sigma bonds that are broken or formed in the given reaction are to be labeled.

Concept introduction: A chemical reaction that involves $\left[4+2\right]$ cycloaddition is called Diels-Alder reaction. The reactant molecules that give rise to the product are diene and dienophile. The rate of reaction is fast when the electronegative group is attached to dienophile. In the sigmatropic rearrangement, the migration of the sigma bond takes place. Electrocyclic reactions involve the conversion of $\text{σ}$ bonds into $\text{π}$ bonds or $\text{π}$ bonds into $\text{σ}$ bonds.

Answer to Problem 27.1P

The reaction is classified as a cycloaddition reaction. The sigma bonds broken or formed in the given reaction are,

Explanation of Solution

The given reaction is shown below.

Figure 2

The sigma bonds that are formed during the conversion of reactant into a product are shown in red. In the reaction, $\text{π}$ bonds are broken and the formation of two sigma bonds takes place. Therefore, the reaction is classified as a cycloaddition reaction.

Conclusion

The reaction is classified as a cycloaddition reaction. The sigma bonds broken or formed in the given reaction are labeled in Figure 2.

Interpretation Introduction

(c)

Interpretation: The given reaction is to be classified as an electrocyclic reaction, cycloaddition, or a sigmatropic rearrangement. The sigma bonds that are broken or formed in the given reaction are to be labeled.

Concept introduction: A chemical reaction that involves $\left[4+2\right]$ cycloaddition is called Diels-Alder reaction. The reactant molecules that give rise to product are diene and dienophile. The rate of reaction is fast when the electronegative group is attached to dienophile. In the sigmatropic rearrangement, the migration of the sigma bond takes place. Electrocyclic reactions involve the conversion of $\text{σ}$ bonds into $\text{π}$ bonds or $\text{π}$ bonds into $\text{σ}$ bonds.

Answer to Problem 27.1P

The reaction is classified as an electrocyclic reaction. The sigma bond broken or formed in the given reaction is,

Explanation of Solution

The given reaction is shown below.

Figure 3

The sigma bond that is formed during the conversion of reactant into product is shown in red. The new bonds formed during the conversion are shown by curved arrows in the reactant. In the product, the ring gets closed due to the rearrangement of $\text{π}$ bonds and formation of $\text{σ}$ bond. Therefore, the reaction is classified as an electrocyclic reaction.

Conclusion

The reaction is classified as an electrocyclic reaction. The sigma bond broken or formed in the given reaction is labeled in Figure 3.

Interpretation Introduction

(d)

Interpretation: The given reaction is to be classified as an electrocyclic reaction, cycloaddition, or a sigmatropic rearrangement. The sigma bonds that are broken or formed in the given reaction are to be labeled.

Concept introduction: A chemical reaction that involves $\left[4+2\right]$ cycloaddition is called Diels-Alder reaction. The reactant molecules that give rise to product are diene and dienophile. The rate of reaction is fast when the electronegative group is attached to dienophile. In the sigmatropic rearrangement, the migration of the sigma bond takes place. Electrocyclic reactions involve the conversion of $\text{σ}$ bonds into $\text{π}$ bonds or $\text{π}$ bonds into $\text{σ}$ bonds.

Answer to Problem 27.1P

The reaction is classified as a sigmatropic reaction. The sigma bonds broken or formed in the given reaction are,

Explanation of Solution

The given reaction is shown below.

Figure 4

The sigma bond that is broken and formed during the conversion of reactant into product is shown in red. The product indicates that $\text{π}$ bond is rearranged and the shifting of methyl group from the first carbon to third carbon atom takes place. Hence, the old $\text{σ}$ bond is broken and a new $\text{σ}$ bond is formed. Therefore, the reaction is classified as a sigmatropic reaction.

Conclusion

The reaction is classified as a sigmatropic reaction. The sigma bonds broken or formed in the given reaction are labeled in Figure 4.