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Ch. 7 - Structure and Synthesis of Alkenes; Elimination
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 7 - Structure and Synthesis of Alkenes; EliminationProblem 26
Chapter 7, Problem 26

Show that the (S,S) enantiomer of this (R,R) diastereomer of 1-bromo-1,2-diphenylpropane also undergoes E2 elimination to give the cis diastereomer of the product. (We do not expect these achiral reagents to distinguish between enantiomers.)

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Step 1: Understand the problem. The question asks us to demonstrate that the (S,S) enantiomer of the given (R,R) diastereomer undergoes E2 elimination to produce the cis diastereomer of the product. This involves analyzing the stereochemistry of the starting material and the elimination mechanism.
Step 2: Recall the E2 elimination mechanism. E2 elimination is a concerted reaction where a base abstracts a β-hydrogen, and the leaving group (in this case, bromine) departs simultaneously. The reaction requires an anti-periplanar geometry between the β-hydrogen and the leaving group for the elimination to occur.
Step 3: Analyze the stereochemistry of the (S,S) enantiomer. Use a molecular model or draw the Newman projection of the (S,S) enantiomer of 1-bromo-1,2-diphenylpropane. Identify the β-hydrogens and determine which one is anti-periplanar to the bromine atom.
Step 4: Predict the product of the elimination. When the β-hydrogen anti-periplanar to the bromine is removed, a double bond forms between the α and β carbons. The stereochemistry of the product depends on the spatial arrangement of the substituents on the double bond. For the (S,S) enantiomer, the elimination leads to the cis diastereomer of the product because the substituents on the double bond are on the same side.
Step 5: Confirm that the achiral reagents do not distinguish between enantiomers. Since the reagents used in the E2 elimination are achiral, they will not differentiate between the (R,R) and (S,S) enantiomers. Both enantiomers will undergo elimination to produce the same cis diastereomer of the product.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Stereochemistry

Stereochemistry is the study of the spatial arrangement of atoms in molecules and how this affects their chemical behavior. It is crucial for understanding enantiomers and diastereomers, which are types of stereoisomers. Enantiomers are non-superimposable mirror images, while diastereomers are not mirror images and can have different physical and chemical properties. This concept is essential for analyzing reactions involving chiral compounds.
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E2 Elimination Mechanism

The E2 elimination mechanism is a type of bimolecular elimination reaction where a base removes a proton from a β-carbon, leading to the formation of a double bond and the expulsion of a leaving group, such as a halide. This mechanism is stereospecific, meaning the spatial arrangement of substituents around the double bond affects the product formed. Understanding E2 is vital for predicting the outcome of reactions involving stereoisomers.
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Diastereomers and Their Reactivity

Diastereomers are stereoisomers that are not mirror images of each other and can exhibit different reactivity due to their distinct spatial arrangements. In the context of elimination reactions, the configuration of diastereomers can influence the pathway and products formed. Recognizing how diastereomers behave differently in reactions, such as E2 eliminations, is key to predicting the formation of specific products, like cis or trans isomers.
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Related Practice
Textbook Question

Make models of the following compounds, and predict the products formed when they react with the strong bases shown.

(b) meso-1,2-dibromo-1,2-diphenylethane + (CH3CH2)3N:

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Textbook Question

Make models of the following compounds, and predict the products formed when they react with the strong bases shown.

(c) (d,l)-1,2-dibromo-1,2-diphenylethane + (CH3CH2)3N:

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Textbook Question

For each reaction, decide whether substitution or elimination (or both) is possible, and predict the products you expect. Label the major products.

a. 1−bromo−1−methylcyclohexane + NaOH in acetone

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Textbook Question

Which of these reactions are likely to produce both elimination and substitution products?

a. 2-bromopentane + NaOCH3

b. 3-bromo-3-methylpentane + NaOMe. (Me = methyl, CH3)

c. 2-bromo-3-ethylpentane + NaOH

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Textbook Question

For each reaction, decide whether substitution or elimination (or both) is possible, and predict the products you expect. Label the major products.

c. chlorocyclohexane+NaOCH3 in CH3OH

d. chlorocyclohexane + NaOC(CH3)3 in (CH3)3COH

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Textbook Question

Make models of the following compounds, and predict the products formed when they react with the strong bases shown.

(a)

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