In our overview of chemical reactions we outlined three basic reaction types; substitution, elimination, and addition. Having considered nucleophilic aliphatic substitution reactions is some detail, we will now consider several examples of elimination reactions. Our first two examples are reactions which both display second order kinetics. They are labeled as E2 reactions; bimolecular eliminations.
When meso-1,2-dibromo-1,2-diphenylethane is treated with KOH the only alkene formed is E-2-bromo-1,2-diphenylethene. None of the Z steroisomer is formed:
Exercise 2 Draw the structure of Z-1-bromo-1,2-diphenylethene. Which isomer would you expect to be more stable? E Z
Exercise 3 Is the formation of E-1-bromo-1,2-diphenylethene kinetically or thermodynamically controlled?
Exercise 4 If the meso-1,2-dibromo-1,2-diphenylethane were in conformation B, shown in Exercise 1, when the elimination occured, which steroisomer would you expect to be formed? Z E
Exercise 5 In conformation B the dihedral angle between the H on the front carbon and the Br on the back carbon is 60 degrees. The dihedral angle between the H on the back carbon and the Br on the front carbon is also 60 degrees. What is the dihedral angle between the H on the front carbon and the Br on the back carbon in conformation C? degrees. What is the dihedral angle between the H on the back carbon and the Br on the front carbon in conformation C? degrees
Exercise 6 Given your answers to Exerises 4 and 5, and assuming that meso-1,2-dibromo-1,2-diphenylethane is formed from conformation C, what is the most reasonable dihedral angle between the H and the Br when these atoms are eliminated? degrees
Exercise 7 Draw the structure of the product of the following reaction:
Three points about the results shown in Figure 1 are noteworthy. First, menthyl chloride does not yield any 1-menthene. Second, 1-menthene is thermodynamically more stable than 2-menthene. Third, neomenthyl chloride reacts approximately 40 times faster than menthyl chloride.
Exercise 9 What is the H-C1-C2-Cl dihedral angle in menthyl chloride? What is the Hax-C3-C2-Cl dihedral angle in menthyl chloride? What is the Heq-C3-C2-Cl dihedral angle in menthyl chloride? Are there any H-C-C-Cl dihedral angles of 180 degrees in this conformation of menthyl chloride? Yes No
Exercise 10 Draw the other chair conformation of menthyl chloride. How many 1,3-diaxial interactions are there in it? Is this conformation more stable or less stable than the one shown in Figure 1? Are there any H-C-C-Cl dihedral angles of 180 degrees in this conformation of menthyl chloride? Yes No
Exercise 11 What is the H-C1-C2-Cl dihedral angle in neomenthyl chloride? What is the Hax-C3-C2-Cl dihedral angle in neomenthyl chloride? What is the Heq-C3-C2-Cl dihedral angle in neomenthyl chloride? How many H-C-C-Cl dihedral angles of 180 degrees are there in this conformation of neomenthyl chloride?
Exercise 12 Draw the other chair conformation of neomenthyl chloride. How many 1,3-diaxial interactions are there in it? Is this conformation more stable or less stable than the one shown in Figure 1? How many H-C-C-Cl dihedral angles of 180 degrees are there in this conformation of neomenthyl chloride? How many H-C-C-Cl dihedral angles of 180 degrees are there in the conformation of neomenthyl chloride shown in Figure 1?
Exercise 13 Offer a rationalization for the facts that 1. menthyl chloride does not yield any 1-menthene. 2. neomenthyl chloride reacts approximately 40 times faster than menthyl chloride.
