Organic Chemistry

334 CHAPTER 8 Reactions of Dienes • Ultraviolet and Visible Spectroscopy

44. a. Give the products of the following reaction:

b. How many stereoisomers of each product could be obtained?

45. As many as 18 different Diels–Alder products could be obtained by heating a mixture of 1,3-butadiene and 2-methyl-
    1,3-butadiene. Identify the products.


46. Many credit-card slips do not have carbon paper. Nevertheless, when you sign the slip, an imprint of your signature is made on the
    bottom copy. The carbonless paper contains tiny capsules that are filled with the colorless compound whose structure is shown here:

When you press on the paper, the capsules burst and the colorless compound comes into contact with the acid-treated paper, form-

ing a highly colored compound. What is the structure of the colored compound?

47. On the same graph, draw the reaction coordinate for the addition of one equivalent of HBr to 2-methyl-1,3-pentadiene and one
    equivalent of HBr to 2-methyl-1,4-pentadiene. Which reaction is faster?


48. While attempting to recrystallize maleic anhydride, Professor Nots O. Kareful dissolved it in freshly distilled cyclopentadiene
    rather than in freshly distilled cyclopentane. Was his recrystallization successful?


49. A solution of ethanol has been contaminated with benzene—a technique employed to make ethanol unfit to drink. Benzene has a
    molar absorptivity of 230 at 260 nm in ethanol, and ethanol shows no absorbance at 260 nm. How could the concentration of
    benzene in the solution be determined?


50. Reverse Diels–Alder reactions can occur at high temperatures. Why are high temperatures required?


51. The following equilibrium is driven to the right if the reaction is carried out in the presence of maleic anhydride:

What is the function of maleic anhydride?

52. In 1935, J. Bredt, a German chemist, proposed that a bicycloalkene cannot have a double bond at a bridgehead carbon unless one
    of the rings contains at least eight carbon atoms. This is known as Bredt’s rule. Explain why there cannot be a double bond at this
    position.

bridgehead

carbon

O

CH 3

CH 3

H 3 C

CH 3

+

∆

O

O O O

maleic anhydride

(CH 3 ) 2 N N(CH 3 ) 2

N(CH 3 ) 2

O

O

+ Br 2