Chapter 27, Problem 27.8P

Interpretation Introduction

(a)

Interpretation: The product formed when the given compound in Problem 27.6 undergoes photochemical electrocyclic ring opening or ring closure is to be predicted. The given process is to be labeled as conrotatory or disrotatory. The stereochemistry around tetrahedral stereogenic centers and double bonds is to be indicated.

Concept introduction: Electrocyclic reactions involve the conversion of $\text{σ}$ bonds into $\text{π}$ bonds or $\text{π}$ bonds into $\text{σ}$ bonds. These reactions can take place in photochemical and as well as thermal conditions. The photochemical and thermal reactions take place in the presence of light and heat respectively.

Curved arrows aid in determining the movement and flow of electrons in the reaction. The electrons that take part in the chemical reactions are shown by the curved arrows.

Answer to Problem 27.8P

The product formed in the given photochemical electrocyclic ring closure reaction is $trans-$$4,5-$ dimethylcyclohexa $-2,6-$ diene-$1,2$-dibenzene. The formation of the product takes place by conrotatory process. The stereochemistry around tetrahedral stereogenic centers is,

Explanation of Solution

The thermal electrocyclic ring closure reaction is shown below.

Figure 1

Electrocyclic reactions involve the conversion of $\text{σ}$ bonds into $\text{π}$ bonds or $\text{π}$ bonds into $\text{σ}$ bonds. In the product, the ring gets closed due to the rearrangement of $\text{π}$ bonds and the formation of new $\text{σ}$ bond. Therefore, the product formed in the given thermal electrocyclic ring closure reaction is $cis-$$4,5-$ dimethylcyclohexa $-2,6-$ diene-$1,2$-dibenzene.

Under photochemical condition, conrotatory process takes place in the given reaction. The orientation of substituents and stereochemistry around tetrahedral stereogenic centers and double bonds in the given reaction are shown below.

Figure 2

The conrotatory process involves the rotation of two orbitals in the same direction. The reaction in above figure implies that orbitals move in the same direction in the reactant molecule and the formation of trans product occurs. One methyl group is present above the plane and one methyl group is present below the plane in the product. Therefore, the product formed in the given photochemical electrocyclic ring closure reaction is $trans-$$4,5-$ dimethylcyclohexa $-2,6-$ diene-$1,2$-dibenzene.

Conclusion

The product formed in the given photochemical electrocyclic ring closure reaction is $trans-$$4,5-$ dimethylcyclohexa $-2,6-$ diene-$1,2$-dibenzene. The formation of the product takes place by conrotatory process. The stereochemistry around tetrahedral stereogenic centers is indicated in Figure 2.

Interpretation Introduction

(b)

Interpretation: The product formed when the given compound in Problem 27.6 undergoes photochemical electrocyclic ring opening or ring closure is to be predicted. The given process is to be labeled as conrotatory or disrotatory. The stereochemistry around tetrahedral stereogenic centers and double bonds is to be indicated.

Concept introduction: Electrocyclic reactions involve the conversion of $\text{σ}$ bonds into $\text{π}$ bonds or $\text{π}$ bonds into $\text{σ}$ bonds. These reactions can take place in photochemical and as well as thermal conditions. The photochemical and thermal reactions take place in the presence of light and heat respectively.

Curved arrows aid in determining the movement and flow of electrons in the reaction. The electrons that take part in the chemical reactions are shown by the curved arrows.

Answer to Problem 27.8P

The product formed in the given photochemical electrocyclic ring opening reaction is $trans-$$3,4-$ dimethylhexa $-2,4-$ diene. The formation of the product takes place by disrotatory process. The stereochemistry around tetrahedral stereogenic centers is,

Explanation of Solution

The thermal electrocyclic ring opening reaction is shown below.

Figure 3

Electrocyclic reactions involve the conversion of $\text{σ}$ bonds into $\text{π}$ bonds or $\text{π}$ bonds into $\text{σ}$ bonds. In the product, the ring is opened due to the rearrangement of $\text{π}$ bond and the breakdown of old $\text{σ}$ bond. Therefore, the product formed in the given photochemical electrocyclic ring opening reaction is $3,4-$ dimethylhexa $-2,4-$ diene.

Under photochemical condition, disrotatory process takes place in the given reaction. The movement of substituents and stereochemistry around tetrahedral stereogenic centers and double bonds in the given reaction are shown below.

Figure 4

The disrotatory process involves the rotation of two orbitals in the opposite direction. The reaction in above figure implies that orbitals move in the opposite direction in the reactant molecule and the formation of trans product occurs. One methyl group is present above the plane and one methyl group is present below the plane in the product shows the stereochemistry. Therefore, the product formed in the given photochemical electrocyclic ring opening reaction is $trans-$$3,4-$ dimethylhexa $-2,4-$ diene. The formation of the product takes place by disrotatory process. The stereochemistry around tetrahedral stereogenic centers is indicated in Figure 4.

Conclusion

The product formed in the given photochemical electrocyclic ring opening reaction is $trans-$$3,4-$ dimethylhexa $-2,4-$ diene. The formation of the product takes place by disrotatory process. The stereochemistry around tetrahedral stereogenic centers is indicated in Figure 4.