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Organic Chemistry, Fifth Edition

Janice Gorzynski SmithUniversity of Hawai’i

Chapter 7Modified by Dr. Juliet Hahn

Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.

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SN2 Reaction in Adrenaline Synthesis

End Exam III

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• Organic synthesis is the systematic preparation of acompound from a readily available starting material by one ormany steps.

• Nucleophilic substitution reactions, especially SN2, are usedto introduce a wide variety of functional groups into amolecule, depending on the nucleophile.

• Organic synthesis has produced many useful compounds(e.g., pharmaceuticals, pesticides, and polymers used ineveryday life).

• Chemists may rely on synthesis to prepare useful substancessuch as a natural product produced by organisms, but in onlyminute amounts (e.g., Taxol used in cancer treatment).

Organic Synthesis

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Organic Synthesis Using Alkyl Halides

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• To carry out the synthesis of a particular compound, we mustthink backwards, and ask ourselves the question:“What starting material and reagents are needed to make it?”

• If a nucleophilic substitution is being used, determine whatalkyl halide and what nucleophile can be used to form a specificproduct.

Thinking Backwards in Organic Synthesis

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• To determine the two components needed for synthesis,remember that the carbon atoms come from the organicstarting material, in this case, a 1° alkyl halide.

• The functional group comes from the nucleophile, HO¯ inthis case.

• With these two components, we can “fill in the boxes” tocomplete the synthesis.

Approaches Used in Organic Synthesis

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Organic Chemistry, Fifth Edition

Janice Gorzynski SmithUniversity of Hawai’i

Chapter 8

Modified by Dr. Juliet Hahn

Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.

8

• Elimination reactions involve the loss of elements fromthe starting material to form a new π bond in the product.

General Features of Elimination

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• In both example reactions a base removes the elementsof an acid, HX, from the organic starting material.

Elimination of HX

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• Removal of the elements HX is calleddehydrohalogenation.

• Dehydrohalogenation is an example of β elimination.

• The curved arrow formalism shown below illustrateshow four bonds are broken or formed in the process.

Dehydrohalogenation

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• The most common bases used in elimination reactionsare negatively charged oxygen compounds, such as HO¯and its alkyl derivatives, RO¯, called alkoxides.

Common Bases for Dehydrohalogenation

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• Find the α carbon.

• Identify all β carbons with H atoms.

• Remove the elements of H and X from the α and β carbonsand form a π bond.

Drawing Products of Dehydrohalogenation

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• Alkenes are hydrocarbons containing a carbon-carbondouble bond.

• Each carbon of the double bond is sp2 hybridized.

• The alkene carbons are trigonal planar.

• The bond angles are 120o.

Alkenes

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• The double bond of an alkene consists of a σ bond and aπ bond.

Alkene Structure

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• Alkenes are classified according to the number of carbonatoms bonded to the carbons of the double bond.

Figure 8.1

Classifying Alkenes

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• Recall that even though there is free rotation around singlebonds, rotation about double bonds is restricted.

Figure 8.2

Restricted Rotation About Double Bonds

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• Because of restricted rotation, two stereoisomers of 2-butene are possible.

• cis-2-Butene and trans-2-butene are diastereomers (i.e.,non-mirror image stereoisomers).

Stereoisomers of Alkenes

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• Whenever the two groups on each end of a carbon-carbon double bond are different from each other, cis-trans isomers are possible.

Alkene Diastereomers

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• In general, trans alkenes are more stable than cis alkenesbecause the groups bonded to the double bond carbonsare further apart, reducing steric interactions.

Stability of Trans Alkenes

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• The stability of an alkene increases as the number of Rgroups bonded to the double bond carbons increases.

Stability in Alkenes

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• The higher the percent s-character, the more readily anatom accepts electron density.

• Thus, sp2 carbons are more able to accept electrondensity and sp3 carbons are more able to donateelectron density.

• Increasing the number of electron donating groups on acarbon atom able to accept electron density makes thealkene more stable.

Stability in Alkenes

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• The 2-butenes (disubstituted) are more stable than1-butene (monosubstituted).

• trans-2-Butene is more stable than cis-2-butene (lesscrowding).

Relative Stability of Butenes

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• There are two mechanisms of elimination—E2 and E1,just as there are two mechanisms of substitution, SN2and SN1.

• The E2 mechanism is called bimolecular elimination.

• The E1 mechanism is called unimolecular elimination.

• The E2 and E1 mechanisms differ in the timing of bondcleavage and bond formation, analogous to the SN2 andSN1 mechanisms.

• E2 and SN2 reactions have some features in common, asdo E1 and SN1 reactions.

Elimination Mechanisms

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• The most common mechanism for dehydrohalogenationis the E2 mechanism.

• It exhibits second-order kinetics, and both the alkylhalide and the base appear in the rate equation.

• The reaction is concerted—all bonds are broken andformed in a single step.

E2 Mechanism

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E2 Mechanism

• There are close parallels between E2 and SN2 mechanismsin how the identity of the base, the leaving group, and thesolvent affect the rate.

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End class 10/20/17F