Drawing the Products of an E2 Reaction

As we have seen, predicting the products of an E2 reaction is often more complex than predicting the products of an S],j2 reaction. With an E2 reaction, two major issues must be considered before drawing the products regiochemistry (Section 8.4) and stereochemistry (Section 8.5). Both of these issues are illustrated in the following example. 

LEARN the skill Predict the product(s) of the following reaction  

We must consider both the regiochemistry and the stereochemistry of this reaction. We will start with the regiochemistry. First identify all of the 3 positions that possess protons  

There are two (3 positions. The base (ethoxide) is not sterically hindered, so we expect the Zaitsev product (the more substituted alkene) as the major product. The less substituted alkene will be the minor product. Next, we must identify the stereochemistry of formation of each of the products. Let s begin with the minor product (the less substituted alkene), because its double bond does not exhibit stereoisomerism  

This alkene is neither E nor Z, so we don t need to worry about stereoisomerism of the double bond in this product. But with the major product we must predict which stereoisomer will be obtained. To do this, draw a Newman projection  

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PROBLEMS For each of the compounds below, draw the Zaitsev and Hoffman products of an E2 reaction. 

It would be absolutely wrong to say that the product will always be the tram isomer. The product obtained depends on the configuration of the starting alkyl hahde. The only way to predict the configuration of the resulting alkene is to analyze the substtate carefully, draw a Newman projection, and then determine which stereoisomeric product is obtained. The E2 reaction is said to be stereospecific, because the stereoisomerism of the product is dependent on the stereoisomerism of the substrate. The stereospecificity of an E2 reaction is only relevant when the P position has only one proton ... 

Problem 8.18 Given that an E2 reaction proceeds with anti periplanar stereochemistry, draw the products of each elimination. The alkyl halides in (a) and (b) are diastereomers of each other. How are the products of these two reactions related ... 

PROBLEMS Draw the Zaitsev and Hofmann products that are expected when each of the following compounds is treated with a strong base to give an E2 reaction. For the following problems, don t worry about identifying which product is major and which is minor, since the identity of the base has not been indicated. Just draw both possible products ... 

The molecule below has four stereoisomeric forms exoO exoCH2Br, exoO endoCH2Br, and so on. Examine electrostatic potential maps of the four ions and identify the most nucleophilic (electron-rich) atom in each. Examine the electron-acceptor orbital (the lowest-unoccuped molecular orbital or LUMO) in each and identify electrophilic sites that are in close proximity to the nucleophilic. Which isomers can undergo an intramolecular E2 reaction Draw the expected SN2 and E2 products. Which isomers should not readily undergo intramolecular reactions Why are these inert ... 

PROBLEMS For each of the reactions below, predict whether the reaction will proceed via an E2 mechanism, an El mechanism, or neither. For now, do not worry about drawing the products. We need to cover the next section before we can do that. Right now, just focus on determining which mechanism, if any, is possible. 

Do not draw a concerted E2 reaction for the dehydration of an aldol product. 

For the following reaction, draw all products that result from an E2 reaction, and draw the transition state for removal of the hydrogen that leads to the major product. Ignore stereoisomers as being different products. 

Draw the major product obtained when each of the following alkyl halides undergoes an E2 reaction a. CH3CHCH2CH3 c. CH3CHCH2CH3 e. CH3CHCH2CH2CH3... 

For each of the following compounds, draw the product that will be formed in an E2 reaction and indicate its configuration a. (lS,25)-l-bromo-l,2-diphenylpropane b. (lS,2i )-l-bromo-l,2-diphenylpropane... 

The reaction involves a secondary substrate and a basic nucleophile elimination should predominate. Because cthoxide is a strong base, the E2 pathway is followed. The structure of the product is predictable because of the preference for anti elimination. Draw a chair conformation thtit places the leaving group (Cl) in an axial position. 

For each of the following reactions, (1) decide whether an E2 or an El will occur, and (2) draw the major elimination product ... 

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