Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

8. Elimination Reactions

E2 - Anti-Coplanar Requirement

Organic Chemistry

8. Elimination Reactions

E2 - Anti-Coplanar Requirement

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4:01 minutes

Problem 71

Textbook Question

One of the following dichloronorbornanes undergoes elimination much faster than the other. Determine which one reacts faster, and explain the large difference in rates.

Verified step by step guidance

Step 1: Analyze the structures of the cis- and trans-dichloronorbornanes. The cis isomer has both chlorine atoms on the same side of the bicyclic ring system, while the trans isomer has the chlorine atoms on opposite sides of the ring system.

Step 2: Consider the elimination reaction mechanism. The reaction involves the removal of a hydrogen atom and a chlorine atom to form a double bond. This is typically an E2 elimination mechanism, which requires the hydrogen and chlorine atoms to be anti-periplanar (i.e., opposite sides of the same plane).

Step 3: Evaluate the geometry of the cis isomer. In the cis isomer, the hydrogen atom and chlorine atom that are candidates for elimination are not anti-periplanar due to the rigid bicyclic structure. This makes elimination less favorable.

Step 4: Evaluate the geometry of the trans isomer. In the trans isomer, the hydrogen atom and chlorine atom are anti-periplanar, which aligns perfectly for the E2 elimination mechanism. This makes elimination much faster for the trans isomer.

Step 5: Conclude that the trans isomer undergoes elimination much faster than the cis isomer due to the anti-periplanar geometry required for the E2 mechanism, which is achievable in the trans isomer but not in the cis isomer.

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Key Concepts

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

Elimination Reactions

Elimination reactions involve the removal of a small molecule from a larger one, typically resulting in the formation of a double bond. In organic chemistry, these reactions can follow either an E1 or E2 mechanism, which differ in their steps and conditions. Understanding the mechanism is crucial for predicting the rate and outcome of the reaction.

Steric Hindrance

Steric hindrance refers to the repulsion between bulky groups in a molecule that can impede reactions. In the context of elimination reactions, steric hindrance can affect the accessibility of the leaving group and the formation of the double bond. The spatial arrangement of substituents, such as in cis and trans isomers, plays a significant role in determining the reaction rate.

Cis-Trans Isomerism

Cis-trans isomerism is a form of stereoisomerism where the relative positioning of substituents differs around a double bond or a ring structure. In the case of dichloronorbornanes, the cis isomer may allow for a more favorable transition state during elimination due to less steric hindrance compared to the trans isomer, leading to a faster reaction rate.

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Related Practice

Textbook Question

Predict the elimination products of the following reactions, and label the major products.

a. cis-1-bromo-2-methylcyclohexane + NaOCH₃ in CH₃OH

Textbook Question

When the following stereoisomer of 2-bromo-1,3-dimethylcyclohexane is treated with sodium methoxide, no E2 reaction is observed. Explain why this compound cannot undergo the E2 reaction in the chair conformation.

Textbook Question

What is the major elimination product obtained from an E2 reaction of each of the following alkyl halides with hydroxide ion?

Textbook Question

a. Design an alkyl halide that will give only 2,4-diphenylpent-2-ene upon treatment with potassium tert-butoxide (a bulky base that promotes E2 elimination).

b. What stereochemistry is required in your alkyl halide so that only the following stereoisomer of the product is formed?

Textbook Question

Give the expected product(s) of E2 elimination for each reaction. (Hint: Use models!)

(a)

Textbook Question

The cis-diastereomer undergoes E2 elimination 500 times faster than the trans form. Explain.

Textbook Question

Which reactant in each of the following pairs undergoes an elimination reaction more rapidly? Explain your choice.

Textbook Question

Explain why only a substitution product and no elimination product is obtained when the following compound reacts with sodium methoxide:

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