Question

5. This reaction uses water as a solvent to efficiently prepare the Diels-Alder product, even though the reactants and products are not soluble in water. Explain what is meant by "hydrophobic effect" and how it is used in the context of this reaction. You may use the back side of this worksheet to answer the question. (4 pts) IC

Answer 1

To assess the importance of the hydrophobicity of different parts of diene and dienophile on the aqueous acceleration of Diels-Alder reactions, second-order rate constants have been determined for the reactions of cyclopentadiene (1), 2,3-dimethyl-1,3-butadiene (4), and 1,3-cyclohexadiene (6) with N-methyl-, N-ethyl-, N-propyl-, and N-butylmaleimide (2a-d) in different solvents. All these reactions are accelerated in water relative to organic solvents as a result of enhanced hydrogen bonding and enforced hydrophobic interactions during the activation process. The beneficial influence of water as compared to 1-propanol on the rate of the Diels-Alder reaction of 4 with 2a-d increases linearly with the length of the alkyl chain of 2. In contrast, for the reaction of both 1 and 6 with 2a-d, no such effect was observed. This difference can be explained by a hydrophobic interaction between the methyl groups of 4 and the N-alkyl group of 2 during the activation process. In the reactions of 1 and 6, lacking the methyl substituents, this interaction is not possible and elongation of the alkyl chain from ethyl onward does not result in an additional acceleration by water. The enhanced hydrophobicity near the reaction center of dienes 4 and 6 compared to 1 results in an increased aqueous acceleration of the Diels-Alder reactions of the former dienes with 2a. These data indicate that an increase in the hydrophobicity close to the reaction center in the diene has a much more pronounced effect on the rate acceleration in water than a comparable increase in hydrophobicity in the dienophile further away from the reaction center. his destabilization becomes more pronounced when the nonpolar character of diene (close to the reaction center) or dienophile (distant from the reaction center) is increased. Likewise, an increase in the nonpolar character of 2 results in a destabilization of the activated complex. In contrast, addition of methyl or methylene units to the diene is not accompanied by a significant destabilization of the activated complex in water as compared to 1-propanol. We conclude that hydrophobic groups near the reaction center seem to lose their hydrophobic character completely in the activated complex of the Diels-Alder reaction, whereas more distant groups retain their nonpolar character throughout the reaction.