Question

The Diels–Alder reaction, a powerful reaction discussed in Chapter 16, occurs when a 1,3- diene such as A reacts with an alkene such as B to form the six-membered ring in C.

a. Draw curved arrows to show how A and B react to form C.

b. What bonds are broken and formed in this reaction?

c. Would you expect this reaction to be endothermic or exothermic?

d. Does entropy favor the reactants or products?

e. Is the Diels–Alder reaction a substitution, elimination, or addition?

Short answer

Answer

a.

b.

c. The reaction is exothermic.

d. The entropy will favor the formation of reactants.

e. The reaction is an addition reaction.

Step-by-step solution

Step-by-Step SolutionStep 1: Diels–Alder reaction

It involves the reaction of a diene with the dienophile in the presence of heat or light. This leads to the formation of a six-membered cyclic product.

Explanation for a, b, and c

a. The reaction of buta-1,3-diene (diene) with ethyl acrylate (dienophile) results in the formation of ethyl cyclohex-3-enecarboxylate.

The mechanism is shown below:

Curved arrows in the reaction

b. The bonds broken and formed are mentioned below:

Bonds broken and bonds formed

c. The bond dissociation energy of the C=C bond and C-C bond is taken as 602 kJ/mol and 346 kJ/mol respectively.

The change in the enthalpy is calculated as follows:

$\begin{array}{rcl}\mathrm{Change}\mathrm{in}\mathrm{enthalpy}& =& \sum _{}^{}\mathrm{BE}\left(\mathrm{reactants}\right)-\sum _{}^{}\mathrm{BE}(\mathrm{products})\\ & & =2\left(\mathrm{C}=\mathrm{C}\right)-\left[2\left(\mathrm{C}-\mathrm{C}\right)+\mathrm{C}=\mathrm{C}\right]\\ & & =2\left(602\right)-\left[2\times 346+\right]\mathrm{kJ}/\mathrm{mol}\\ & =& -90\mathrm{kJ}/\mathrm{mol}\end{array}$

Thus, the value of change in enthalpy is negative and the reaction is exothermic.

Explanation for d and e

d. The cyclic product is formed which means the degree of freedom would be reduced. Thus, the entropy will favor the formation of the reactants.

e. The given reaction is an example of Diels alder reaction. It involves [4+2] addition. Thus, the given reaction is an addition reaction.