Question

9.51 (.) Consider the Cope rearrangement, a reaction we will study in chapter 20. (a) Using the knowledge we have gained in this chapter, suggest a one-step, concerted mechanism which explains the formation of B from A. (b) Which side of the reaction would you expect to be favored? Justify your answer. (c) Which product, A or B, would you expect to be hydrogenated with the more exothermic heat of hydrogenation?

Answer 1

i) 3,3-sigmatropic rearrangement of 1,5-dienes under thermal conditions is called Cope rearrangement.

It a concerted reaction and occurs via a cyclic aromatic transition state

It is highly stereospecific reaction

The groups present on the 1,5-diene retains their stereochemistry in the product.

ii) In the above reaction, the equilibrium favors towards reactant side (A) because the 1,5-disubstituted alkene is thermodynamically more stable than 3,4-methyl-1,5-hexadiene.

iii) Since A is more thermodynamically more stable than B, hence B will have large exothermic heats of hydrogenation.

meobstitutet disabstituted alkene Thermodynamically more stable alkene- less table