Jumat, 09 Oktober 2009

The E2 Mechanism

Introduction

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.

The E2 Mechanism

Stereochemistry

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 1 Select the most stable conformation of meso-1,2-dibromo-1,2-diphenylethane.

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:


The outcome of reaction 1 suggests that the E2 mechanism has certain stereochemical requirements. Apparently elimination of the H and the Br is not equally likely from all possible conformations of the starting material. While it appears that a conformation in which the dihedral angle of 180 degress between the H and the Br is preferred, free rotation about the central C-C bond makes it difficult to be certain about that conclusion. Figure 1 compares the results of two elimination reactions in molecules that were selected to reduce the uncertainty that attends conformational freedom.

Figure 1

Chair-Chair Conformations Revisited

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 8 How many 1,3-diaxial interactions are there in menthyl chloride? How many 1,3-diaxial interactions are there in neomenthyl 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.


Figure 2 rationalizes the results described above. Kinetic studies indicate clearly that both the base and the substrate are involved in the transition state of the reaction. The stereochemical results suggest strongly that the preferred conformation for the reaction is one in which the dihedral angle between the C-X bond and the adjacent C-H bond is 180 degrees. Presumably this arrangement allows for maximum orbital overlap as the various bonds are made and broken.

Figure 2

From Fact to Fancy


Exercise 14 Select the reaction that you think would occur faster.

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