Organic Chemistry I & II
Organic Chemistry I & II

Organic Chemistry I & II

Lead Author(s): Steven Forsey

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Organic Chemistry I & II is designed for instructors who want an active, dynamic, and understandable approach to support their own efforts in the classroom. This ever-evolving textbook includes auto-graded questions, videos and approachable language in order to make difficult concepts easier to understand and implement.

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David R. Klein, “Organic Chemistry”, 3rd Edition

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Pricing

Average price of textbook across most common format

Top Hat

Steven Forsey, “Organic Chemistry”, Only one edition needed

Up to 40-60% more affordable

Lifetime access on any device

McGraw-Hill

Carey & Giuliano, “Organic Chemistry”, 10th Edition

$219

Hardcover print text only

Wiley

Solomons et al., “Organic Chemistry”, 12th Edition

$301

Hardcover print text only

Wiley

David R. Klein, “Organic Chemistry”, 3rd Edition

$301

Hardcover print text only

Always up-to-date content, constantly revised by community of professors

Constantly revised and updated by a community of professors with the latest content

Top Hat

Steven Forsey, “Organic Chemistry”, Only one edition needed

McGraw-Hill

Carey & Giuliano, “Organic Chemistry”, 10th Edition

Wiley

Solomons et al., “Organic Chemistry”, 12th Edition

Wiley

David R. Klein, “Organic Chemistry”, 3rd Edition

In-book Interactivity

Includes embedded multi-media files and integrated software to enhance visual presentation of concepts directly in textbook

Top Hat

Steven Forsey, “Organic Chemistry”, Only one edition needed

McGraw-Hill

Carey & Giuliano, “Organic Chemistry”, 10th Edition

Wiley

Solomons et al., “Organic Chemistry”, 12th Edition

Wiley

David R. Klein, “Organic Chemistry”, 3rd Edition

Customizable

Ability to revise, adjust and adapt content to meet needs of course and instructor

Top Hat

Steven Forsey, “Organic Chemistry”, Only one edition needed

McGraw-Hill

Carey & Giuliano, “Organic Chemistry”, 10th Edition

Wiley

Solomons et al., “Organic Chemistry”, 12th Edition

Wiley

David R. Klein, “Organic Chemistry”, 3rd Edition

All-in-one Platform

Access to additional questions, test banks, and slides available within one platform

Top Hat

Steven Forsey, “Organic Chemistry”, Only one edition needed

McGraw-Hill

Carey & Giuliano, “Organic Chemistry”, 10th Edition

Wiley

Solomons et al., “Organic Chemistry”, 12th Edition

Wiley

David R. Klein, “Organic Chemistry”, 3rd Edition

About this textbook

Lead Authors

Dr. Steven Forsey, Ph.D.University of Waterloo

Steven Forsey is currently a Professor at University of Waterloo, teaching a variety of organic chemistry courses to Chemistry, Science, Chemical Engineering, Nanotechnology and distance education students. He received his Ph.D. (2004) for Synthetic Organic Chemistry from University of Waterloo, Ontario. He is a recipient of the Excellence of Science Teaching Award and has acted as the Teaching Fellow for the Department of Chemistry since 2016.

Contributing Authors

Felix NgassaGrand Valley State University

Neil GargUCLA

Jennifer ChaytorSaginaw Valley State University

Greg DomskiAugustana College

Christian E. MaduCollin Community College

Christopher NicholsonUniversity of West Florida

Franklin OwEast Los Angeles College, UCLA

Robert S. PhillipsUniversity of Georgia

Grigoriy SeredaUniversity of South Dakota

Simon E. LopezUniversity of Florida

Brannon McCulloughNorthern Arizona University

Jason JonesKennesaw State University

José BoquinAugustana College

Stephanie BrouetSaginaw Valley State University

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Chapter 11: Substitution–Elimination: Discerning the Differences

Organic chemistry has much application in biotechnology. A group of bioengineers in Colorado have found a way to grow flowers that can change color throughout the day. [1]

Contents

Learning Objectives

  • Evaluate the attacking species as a strong or weak nucleophile and a strong or weak base.
  • Rank a series of attacking species in terms of their nucleophilicity and basicity.
  • Predict the major and minor products formed in substitution/elimination reactions by examining the attacking species, (nucleophilicity/basicity) the substrate (steric interaction around the electrophilic carbon), leaving group ability and solvent conditions.

11.1 Definitions

Note: Section 11.1 is the same as Section 9.1 and Section 10.1, review as needed.

Labeling carbons and hydrogens: The carbons bonded adjacent to a functional group are labeled outward, using Greek letters. The nomenclature can also apply to the hydrogens bonded to the carbons.

Figure 11.1. Greek letter labelling of carbons and hydrogens in hexan-3-one and 3-bromo-3-methylpentane​

In the example above, 3-bromo-3-methylpentane has 3 β carbons with 7 β hydrogens and 2 γ carbons with 6 γ hydrogens.

Elimination reaction: An elimination reaction of a neutral molecule involves the removal or elimination of substituents of the molecule to form an alkene in a one- or two-step mechanism. This type of elimination is often called beta-elimination because the elimination occurs between the alpha and beta carbons or adjacent atoms. When elimination happens in a one-step mechanism, it is called an E2 reaction while the two-step mechanism is referred to as an E1 reaction.

Figure 11.2. Generic elimination reaction​

Leaving group (LG): Leaving group must be able to leave as a relatively stable (i.e. non-reactive), weakly basic molecule or ion.

Unimolecular (E1): The rate of reaction is only dependent on the concentration of the substrate. The reaction rate is first order (unimolecular). The Rate of Reaction = k[substrate], where k is equal to the rate constant.

Bimolecular (E2): The rate of reaction is dependent on the concentration of the base and the substrate. The reaction rate is second order (bimolecular). Rate of Reaction = k[base][substrate], where k is equal to the rate constant.

Hofmann Rule This rule refers to a special β-elimination reaction in which the alkene having the smallest number of alkyl groups attached to the double bonded carbon atoms will be the predominant product (less stable alkene forms as the major product). The Hofmann rule is observed in elimination reactions with leaving groups like quaternary ammonium salts, tertiary sulfonium salts, and with bulky strong bases (NaOC(CH3)3) reacting with tertiary alkyl halides.

Figure 11.3. Elimination of 2-bromo-2-methylbutane with a bulky, strong base to produce major (Hofmann) product and minor (Zaitsev) product​

Regiochemistry: A reaction that can occur with different regions on a molecule to produce different products. A region is defined as a site on a molecule where a reaction can occur. Regiochemistry reflects the difference in the reactivity of the various sites.

Regioselective reaction: When there is more than one reactive site a regioselective reaction can occur. A regioselective reaction is one that produces one major product when many other products are possible. For example, the molecule 2-bromo-2-methylbutane has eight hydrogens on β carbons that can undergo elimination reaction in the presence of a base such as sodium ethoxide. Removal of a Ha hydrogen produces the minor product 2-methyl-but-1-ene while removal of a Hb hydrogen produces the major product.

Figure 11.4. Regioselective outcomes of elimination reaction of 2-bromo-2-methylbutane​.

Stereoselective reaction: A reaction in which a single reactant can produce two or more stereoisomeric products and one of these products is preferred over another. For example, the reaction of either enantiomer of 2-bromobutane will stereoselectively produce (E)-but-2-ene as the major product. Notice that it does not matter which enantiomer the starting material is; the product ratio is the same.

Figure 11.5. Stereoselectivity in elimination reactions of enantiomers of 2-bromobutane​

Stereospecific reaction: A reaction in which a single reactant can produce two or more stereoisomeric products and one of these products is exclusively formed over the other(s). For example; the elimination reaction of (1R,2R)-1-bromo-1,2-diphenylpropane with a strong base produces the (Z) stereoisomer whereas the elimination of the (1R,2S) diastereomer produces the (E) stereoisomer.

Figure 11.6. Stereospecificity in elimination reactions of diastereomers of 1-bromo-1,2-diphenylpropane​

Similarly, a SN2 reaction with 2-bromobutane and sodium methanethiolate (CH3SNa) will produce either (S)-sec-butyl(methyl)sulfane or (R)-sec-butyl(methyl)sulfane depending on which enantiomer you start with. This is another example of a stereospecific reaction.

​Figure 11.7. Stereospecificity in SN2 reactions of enantiomers of 2-bromobutane

Zaitsev’s Rule: An empirical rule that states; when two or more alkenes can be produced in an elimination reaction, the thermodynamically most stable alkene will predominate. The most thermodynamically stable alkene will be the alkene that has the most alkyl groups attached to the alkene carbons.

Figure 11.8. Elimination of 2-bromo-2-methylbutane with a strong base to produce major (Zaitsev) product and minor product​

11.2 Stability of Alkenes

Note: Section 11.2 is the same as Section 9.2 and Section 10.2, review as needed.

The relative stability of alkenes can be determined through values for heats of combustion. The amount of heat given off during combustion is related to the stability of the molecule. The larger the amount of heat given off the less stable the molecule is. A good analogy is the stress in the rubber of a balloon. A balloon that is inflated so that the rubber is stretched and is under a lot of stress will produce a much bigger bang when popped than a balloon whose rubber is soft and is not under a lot of stress. The liquid heats of combustion of six isomeric alkenes show how alkyl substitution affects the stability of alkenes. (Data obtained from: Handbook of Chemistry and Physics, 94th Edition, 2013–2014.)

Figure 11.9.
Determining the relative stability of alkenes as a function of substitution.

Trend: Double bond stability increases with increasing substitution (hyperconjugation) and the trans isomer is more stable than the cis isomer (steric interaction).

Hyperconjugation: Alkyl groups are electron donating and contribute electron density to the empty π* molecular orbital of the π bond. The simplified diagram below shows the favorable molecular orbital interaction that helps stabilize the double bond. Thus, it can be said that the electrons from the sigma bonds are delocalized to some extent into the double bond. This is similar to resonance although not as stabilizing. 

Figure 11.10. ​

Stability of alkenes increases with increasing substitution (the addition of alkyl groups) and decreasing steric interaction.

Alkenes are classified based on the number of carbon atoms bonded to the carbons that comprise the double bond. A monosubstituted alkene has one carbon atom bonded to the carbons of the double bond, a disubstituted alkene will have two carbon atoms, and so forth. The figure below shows the alkenes' stability trend.

Figure 11.11. Alkene stability with substitution from most to least stable


11.3 Substitution/Elimination: Review of Reactions

In the previous chapters you have gained knowledge of substitution and elimination reactions. It is now time to bring the reactions together and learn how reaction mechanism changes as different factors affecting the reaction change.

Now, to review the substitution and elimination reactions, relevant concepts are reviewed by way of asking self-test questions to make sure you know the material before going on.

11.3.1 Review of SN2 Reactions

Q11.1 - Level 1

Which of the following is a False statement about a SN_{N}2 reaction?

A

The reaction rate doubles if the concentration of the nucleophile doubles.

B

An intermediate is not formed.

C

It is a concerted reaction in which all bond-breaking and bond-forming occurs at the same time in the transition state.

D

If the attack of the nucleophile is on a chiral carbon inversion of the chiral center occurs.

E

The rate of the reaction = k\textit{k}[Nucleophile]


Q11.2 - Level 2

Predict the major product formed in the following reaction. The starting material is shown as a Fischer projection.

question description
A

1)

B

2)

C

3)

D

4)


Q11.3 - Level 2

Match the stereochemistry of the two substituted products formed.

question description
Premise
Response
1

1)

A

RR

2

2)

B

RR

C

SS

D

SS


Q11.4 - Level 1

Which is the electrophile in the following reaction?

question description
A

1)

B

2)

C

3)


Q11.5 - Level 1

Which halogen anion is the strongest nucleophile in a protic solvent such as water?

question description
A

1)

B

2)

C

3)

D

4)


Q11.6 - Level 1

For the following pairs click on the strongest nucleophile in a protic solvent. Click in the middle of the circle under the molecule.


Q11.7 - Level 1

For the following pairs click on the strongest nucleophile. Click in the middle of the circle under the molecule.


Q11.8 - Level 1

Sort the following compounds from fastest to slowest reaction rates with a nucleophile in an SN_N2 reaction.

question description
Premise
Response
1

Fastest reaction

A

Compound 2

2

Second fastest

B

Compound 3

3

Slowest reaction

C

Compound 1


Q11.9 - Level 1

Which of the following is a very poor leaving group?

question description
A

1)

B

2)

C

3)

D

4)


Q11.10 - Level 1

Would the reaction shown be faster in methanol or DMF?

question description
A

Ethanol

B

DMF


Q11.11 - Level 1

Which of the alkyl halides would undergo the slowest substitution reaction with the ethanethiolate ion?

question description
A

1)

B

2)

C

3)

D

4)


Q11.12 - Level 1

Sort the alkyl halides from most reactive to least reactive in a SN_N2 reaction.

question description
Premise
Response
1

Fastest reaction

A

3)

2

Second fastest

B

1)

3

Third fastest

C

2)

4

Slowest reaction

D

4)


11.3.2 Review: SN1 and SN1 vs. SN2

Q11.13 - Level 1

For the given reaction, how would the rate be affected if the concentration of tert\textit{tert}-butyl bromide was doubled?

question description
A

The rate would not change

B

The rate would triple

C

The rate would quadruple

D

The rate would be double


Q11.14 - Level 1

Would the following SN_N1 reaction produce a racemic mixture?

question description
A

Yes, a racemic mixture is produced

B

No, a racemic mixture is not produced


Q11.15 - Level 1

The given compounds were heated in methanol and the resultant SN_N1 products were determined. Match the products with the observed stereochemistry.

question description
Premise
Response
1

1)

A

Achiral product

2

2)

B

Enantiomers

3

3)

C

Achiral product

4

4)

D

Diastereomers


Q11.16 - Level 1

Which of the following alkyl halides would react the fastest when heated in methanol?

question description
A

1)

B

2)

C

3)

D

4)


Q11.17 - Level 1

Which of the following solvolysis reactions would you expect to have the slowest rate of reaction?

question description
A

1)

B

2)

C

3)


Q11.18 - Level 1

Sort the alkyl halides from fastest to slowest in an SN_N1 reaction.

question description
A

3)

B

2)

C

1)


Q11.19 - Level 1

Sort the following carbocations from most stable to least stable.

question description
A

1)

B

3)

C

2)

D

4)


Q11.20 - Level 2

What is/are the major substitution product(s) formed in the following reaction?

question description
A

1)

B

4)

C

An equal mixture of 3) and 4)

D

An equal mixture of 1) and 3)

E

All of the substitution products will be produced equally


Q11.21 - Level 1

Match the solvent to the type of reaction the solvent helps to promote.

Premise
Response
1

Dimethylformamide (DMF)

A

SN_N2

2

Hexamethylphosphoramide (HMPA)

B

SN_N1

3

Ethanol

C

SN_N1

4

Ammonia

D

SN_N2

E

SN_N1

F

SN_N2

G

SN_N1


Q11.22 - Level 2

What is/are the major product(s) formed in the following reaction?

question description
A

1)

B

2)

C

3)


Q11.23 - Level 2

What is/are the major product(s) formed in the following reaction?

question description
A

1)

B

2)

C

3)


11.3.3 Review: SN1/E1

Q11.24 - Level 2

For the given reaction, predict the major product(s).

question description
A

1)

B

2)

C

3)

D

4)

E

5)

F

Equal mixture of 4) and 5)


Q11.25 - Level 2

For the given reaction, predict the major product(s).

question description
A

1)

B

2)

C

3)

D

4)

E

5)

F

Equal mixture of 4) and 5)


11.3.4 Review: E2

Q11.26 - Level 2

When (1R,2R,4S)-2-bromo-4-isopropyl-1-methylcyclohexane is reacted with sodium ethoxide only one product forms. Which alkene is it? (Chair conformations provided as a guide)

question description
A

1)

B

2)