1. Organic Chemistry Second Edition Chapter 7 David Klein
Answers to Selected Questions in Lecture PowerPoints
David Klein
Chapter 7
Alkenes: Structure and Preparation via Elimination Reactions
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Klein, Organic Chemistry 3e

2. 7.4 Alkene Isomerism
In cyclic alkenes with less than 8 atoms in the ring, only cis alkenes are stable. WHY? Huge amount of angle strain precludes it
Draw the structure for trans-cyclooctene
When applied to bicycloakenes, this rule is
called Bredt’s rule
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

3. 7.7 Stereospecificity of E2
In the transition state, the C-H and C-Br bonds that are breaking must be rotated into the same plane as the pi bond that is forming
Draw the transition state structure illustrating the coplanar geometry δ- δ+
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

4. 7.7 Stereospecificity of E2
Draw all of the possible products if each of the molecules below were to undergo dehydrohalogenation
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

5. 7.7 Stereospecificity of E2
Draw all of the possible products if each of the molecules below were to undergo dehydrohalogenation
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

6. 7.8 Predicting Products for E2
Predict the products for the eliminations below, and draw complete mechanisms
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

7. 7.9 The E1 Mechanism
Given the rate law for E1, which step in the mechanism is the rate-determining slow step? Formation of the carbocation
If the second step were the slow step, how would you write the rate law? Rate = k [substrate][base]
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

8. Additional Practice Problems
Name the following molecules
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

9. Additional Practice Problems
Label the molecules below as either cis or trans and either E or Z where appropriate
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

10. Additional Practice Problems
For the substrate, give both the kinetically favored E2 product and the thermodynamically favored E2 product. Explain what conditions can be used to favor each.
The Br leaves as the antiperiplanar proton is removed by the base at the less hindered position. A large bulky base will select for that pathway.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

11. Additional Practice Problems
For the substrate, give both the kinetically favored E2 product and the thermodynamically favored E2 product. Explain what conditions can be used to favor each.
Molecule must rotate first to allow for proton on the right hand side of the molecule to be positioned antiperiplanar to the leaving group.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

12. Additional Practice Problems
For the substrate, give both the kinetically favored E2 product and the thermodynamically favored E2 product. Explain what conditions can be used to favor each.
Regioselectivity: A less hindered base selects for this product with time and heat, because the more substituted double bond is more stable than the less substituted alternative accumulate once equilibrium is reached.
Stereoselectivity: Because the anti-periplanar geometry gives a much more stable transition state, the (Z) product shown is formed rather than the (E) product .
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

13. Additional Practice Problems
Since tertiary substrates react more readily than secondary or primary in both E1 and E2 mechanisms, what factor(s) usually controls which mechanism will dominate and why? The strength of the base is generally the deciding factor. With a stronger base, it drives the reaction kinetically, and reacts in one step giving E2. With a weak base, the E2 mechanism goes through a very high Eact, so in that case competing E1 is significant, especially when a good leaving group is present.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

14. Additional Practice Problems
Consider both regioselestivity and stereoselectivity to predict the major product for the elimination below.
Ring flip must occur first to place the leaving group in an axial position with an antiperiplanar proton.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

15. Additional Practice Problems
Consider both regioselestivity and stereoselectivity to predict the major product for the elimination below.
There is only 1 antiperiplanar proton, so the elimination can only occur on one side of the leaving group.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

16. Additional Practice Problems
Predict the major product if the alcohol below were treated with concentrated sulfuric acid. Be aware of the possible rearrangements.
First the OH group is protonated and then leaves to form the secondary carbocation (E1)
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

17. Additional Practice Problems
Predict the major product if the alcohol below were treated with concentrated sulfuric acid. Be aware of the possible rearrangements.
A hydride shift can occur on either side giving two possible tertiary carbocations.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

18. Additional Practice Problems
Subsequent proton transfer prodcuts 3 stereoisomers
(3R,5R)-1,3,5-trimethylcyclohexene is formed from this pathway regardelss of which side of the carbocation proton transfer occurs.
(3S,5R)-1,3,5-trimethylcyclohexene and (3R,5S)-1,3,5-trimethylcyclohexene are a racemic mixture formed when proton transfer occurs on alternative sides of the second carbocation.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e

19. Additional Practice Problems
Predict the major product for the following reactions considering competing substitution and elimination pathways. Major product shown, but minor products would also form in each case.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 3e