Chapter 8, Problem 8.50P

Interpretation Introduction

(a)

Interpretation:

Whether an E2 elimination from the given alkyl halide precursor will exclusively produce $\text{(E)-anethol}$ or a mixture of stereoisomers is to be determined.

Concept introduction:

E2 elimination is a single step mechanism involving both the substrate molecule and the reagent molecule. It involves the elimination of a leaving group along with hydrogen from a carbon adjacent to the one with the leaving group. In E2 mechanism, a strong base extracts the proton with the $\text{C}-\text{H}$ bond pair moving to form a second bond between the two carbon atoms. Simultaneously, the leaving group departs with its pair of bond electrons. This requires that the leaving group and the extracted proton be anti to each other. This generally leads to the formation of only one isomer. However, if there are two hydrogen atoms on the carbon from which the proton is extracted, a mixture of stereoisomers is possible. The product distribution, in this case, depends on the stabilities of the conformers that lead to the two isomers.

Interpretation Introduction

(b)

Interpretation:

Whether an E1 elimination reaction, from the given precursor, will produce a pure stereoisomer or a mixture is to be determined.

Concept introduction:

E1 elimination is a two-step reaction. The leaving group breaks off in the first step taking its bond pair with it. A trigonal planar carbocation results from this step. Since the carbocation is planar, any one of the hydrogen atoms, on an adjacent carbon, can be removed as a proton. The associated $\text{C}-\text{H}$ bond pair can then move toward the carbocation to form the second (pi) bond between the two carbon atoms. Since there is no restriction on which hydrogen can be lost, a mixture of stereoisomers is formed.

Interpretation Introduction

(c)

Interpretation:

In each of the two reactions that produce a mixture, which isomer is produced in greater abundance is to be determined.

Concept introduction:

When a reaction produces a mixture of stereoisomers, the product distribution depends on two factors, the stability of the product and the stability of the conformers of the precursor in case of an E2 reaction. An E2 reaction requires that the proton and the leaving group be anti to each other. If there are two protons on the beta carbon, the relative stabilities of the two conformers will influence the product distribution.