Chapter 24, Problem 24.37P

Interpretation Introduction

(a)

Interpretation:

The curved arrows for a concerted mechanism for each given reaction are to be drawn.

Concept introduction:

Curved arrows are drawn cyclically to account for the movement of electrons.

Curved arrows in the concerted mechanism are drawn from the number of electrons move in the reaction, $\text{10}$ electrons movement shown by five curved arrows and $\text{8}$ electrons movement shown by four curved arrows.

Answer to Problem 24.37P

The curved arrows for a concerted mechanism for each given reaction are:

(i)

(ii)

(iii)

Explanation of Solution

(i)

The given reaction is:

In the above reaction, one reactant has $\text{6π}$ electrons and the second reactant has $\text{4π}$ electrons, i.e., total $\text{10π}$ electrons move in the reaction. Therefore five curved arrows required for the concerted mechanism as shown below:

(ii)

The given reaction is:

In the above reaction, one reactant has $\text{2π}$ electrons and the second reactant has $\text{6π}$ electrons, i.e., total $\text{8π}$ electrons move in the reaction. Therefore four curved arrows required for the concerted mechanism as shown below:

(iii)

The given reaction is:

In the above reaction, one reactant has $\text{6π}$ electrons and the second reactant has $\text{4π}$ electrons, i.e., total $\text{10π}$ electrons move in the reaction. Therefore five curved arrows required for the concerted mechanism as shown below:

Conclusion

The curved arrows for a concerted mechanism for the given reaction are drawn using the total number of $\text{π}$ electrons in the reaction.

Interpretation Introduction

(b)

Interpretation:

The transition state formed in each given reaction is to be drawn.

Concept introduction:

Cycloaddition reactions proceeds through the cyclic transition state which represents the breaking and making of bonds.

Answer to Problem 24.37P

The transition state formed in the each given reaction is:

(i)

(ii)

(iii)

Explanation of Solution

(i)

The given reaction is:

The above cycloaddition reaction proceeds through a cyclic transition state, represents the breaking and making of bond by dash bond.

The transition state had shown the total number of $\text{π}$ electrons move in the reaction in cyclic form.

The transition state formed in the above reaction is marked in the box shown below:

(ii)

The given reaction is:

The above cycloaddition reaction proceeds through a cyclic transition state that represents the breaking and making of the bond by dash bond.

The transition state had shown the total number of $\text{π}$ electrons move in the reaction in cyclic form.

The transition state formed in the above reaction is marked in the box shown below:

(iii)

The given reaction is:

The above cycloaddition reaction proceeds through a cyclic transition state that represents the breaking and making of the bond by dash bond.

The transition state had shown the total number of $\text{π}$ electrons move in the reaction in cyclic form.

The transition state formed in the above reaction is marked in the box show below:

Conclusion

The transition state formed in the each given reaction is drawn using the curved arrows direction with number of $\text{π}$ electron moving in the recation.

Interpretation Introduction

(c)

Interpretation:

The nature of transition state either aromatic or antiaromatic for each reaction is to be identified.

Concept introduction:

The transition state aromatic if the number of electrons in that cyclic $\text{π}$ system is either $\text{2, 6, 10, 14, 18,}$ and so on. (These are called Hückel numbers.) Antiaromatic if the number of electrons in that cyclic $\text{π}$ system is either $\text{4, 8, 12, 16, 20,}$ and so on. (These are called anti-Hückel numbers.)

Answer to Problem 24.37P

(i) The transition state is aromatic.

(ii) The transition state is antiaromatic

(iii) The transition state is aromatic.

Explanation of Solution

(i)

The concerted mechanism for the given reaction is:

In the above reaction, $\text{10π}$ electrons are moving around the ring; $\text{10}$ is the Huckels number thus the transition state in the above reaction is aromatic.

(ii)

The concerted mechanism for the given reaction is:

In the above reaction, $\text{8π}$ electrons are moving around the ring; $\text{8}$ is the Huckels number thus the transition state in the above reaction is antiaromatic.

(iii)

The concerted mechanism for the given reaction is:

In the above reaction, $\text{10π}$ electrons are moving around the ring; $\text{10}$ is the Huckels number thus the transition state in the above reaction is aromatic.

Conclusion

The nature of transition state either aromatic or antiaromatic for each reaction is identified from the number of $\text{π}$ electrons moving around the ring.

Interpretation Introduction

(d)

Interpretation:

Based on the answer to part (c), whether the reaction is allowed or forbidden under normal conditions is to be identified.

Concept introduction:

The reaction proceeds through aromatic transition state is allowed whereas if the transition state is antiaromatic the reaction is forbidden.

Answer to Problem 24.37P

(i) The given reaction is allowed under normal conditions.

(ii) The given reaction is forbidden under normal conditions.

(iii) The given reaction is allowed under normal conditions.

Explanation of Solution

(i)

The given reaction proceeds through the aromatic transition state, i.e., the number of moving $\text{π}$ electrons is equal to the Huckels number, thus the given reaction is allowed under normal conditions.

(ii)

The given reaction proceeds through the antiaromatic transition state, i.e., the number of moving $\text{π}$ electrons is not equal to the Huckels number, thus the given reaction is forbidden under normal conditions.

(iii)

The given reaction proceeds through the aromatic transition state, i.e., the number of moving $\text{π}$ electrons is equal to the Huckels number, thus the given reaction is allowed under normal conditions.

Conclusion

Whether the reaction is allowed or forbidden under normal conditions is identified from the nature of the transition state form in the respective reaction.