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© Prentice Hall 2001Chapter 101 On Line Course Evaluation for Chemistry 350/Section 26260 We are participating in the online course evaluation Please log.

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Presentation on theme: "© Prentice Hall 2001Chapter 101 On Line Course Evaluation for Chemistry 350/Section 26260 We are participating in the online course evaluation Please log."— Presentation transcript:

1 © Prentice Hall 2001Chapter 101 On Line Course Evaluation for Chemistry 350/Section We are participating in the online course evaluation Please log on Click on Spring 2003 Evaluation That will take you to Dinput.asp Dinput.asp Your password is your address

2 © Prentice Hall 2001Chapter 102 Stereochemistry of Elimination Reactions If the elimination reaction removes two substituents from the same side of the molecule it is syn elimination If the elimination reaction removes two substituents from opposite sides of the molecule it is anti elimination

3 © Prentice Hall 2001Chapter 103 The E2 Reaction: Stereochemistry In an E2 reaction, the bonds to the eliminated substituents must be in the same plane syn-periplanar anti-periplanar eclipsed conformation staggered conformation

4 © Prentice Hall 2001Chapter 104 The E2 Reaction: Stereochemistry The E2 reaction is regioselective (Zaitsev’s rule) The E2 Reaction is also stereoselective

5 © Prentice Hall 2001Chapter 105 The E2 Reaction: Stereochemistry The E isomer is preferred,as it has the bulkier groups on opposite sides of the double bond

6 © Prentice Hall 2001Chapter 106 The E2 Reaction: Stereochemistry Note that in the formation of 2-pentene there were two and only 2 hydrogens on the  carbon This is the requirement if a pair of E,Z isomers is to result from predominantly anti elimination

7 © Prentice Hall 2001Chapter 107 The E2 Reaction: Stereochemistry When only one hydrogen is on the  carbon predominantly anti elimination leads to high stereoselectivity (2S,3S)-2-bromo-3-phenylbutane (E)-2-phenyl-2-butene (2S,3R)-2-bromo-3-phenylbutane (Z)-2-phenyl-2-butene

8 © Prentice Hall 2001Chapter 108 The E1 Reaction: Stereochemistry Because both syn and anti elimination can occur, an E1 reaction forms both E and Z products regardless of whether the  -carbon is bonded to one or two hydrogens

9 © Prentice Hall 2001Chapter 109 Stereochemistry of Elimination Reactions When the  carbon has two hydrogens, products of the E2 and E1 reactions are stereoselective Both E and Z isomers are formed The major product will have the bulkier groups on opposite sides of the double bond

10 © Prentice Hall 2001Chapter 1010 Stereochemistry of Elimination Reactions When there is just one hydrogen on the  carbon, E2 reaction is gives predominantly the anti elimination product with a high degree of stereoselectivity E1 reaction is gives mostly the product with the bulkier groups on opposite sides of the double bond (usually E)

11 © Prentice Hall 2001Chapter 1011 E2 Reactions of Cyclic Compounds E2 reaction of cyclic compounds follows the same stereochemical rules as from open-chain compounds

12 © Prentice Hall 2001Chapter 1012 E2 Reactions of Cyclic Compounds The E2 reaction of menthyl chloride violates Zaitsev’s rule

13 © Prentice Hall 2001Chapter 1013 E1 Reactions of Cyclic Compounds When a cyclohexyl chloride undergoes an E1 reaction, there is no requirement that the two groups to be eliminated be diaxial

14 © Prentice Hall 2001Chapter 1014 E1 Reactions of Cyclic Compounds Carbocation rearrangements must be considered for E1 reactions

15 © Prentice Hall 2001Chapter 1015 Competition Between Substitution and Elimination The E2 reaction The S N 2 reaction

16 © Prentice Hall 2001Chapter 1016 Competition Between Substitution and Elimination Conditions that favor E2 also favor S N 2 Conditions that favor E1 also favor S N 1 No need to worry about S N 2/E1 or S N 1/E2 combinations First decide whether the reaction would favor S N 2/E2 or S N 1/E1 reactions If the halide is primary, only S N 2/E2 need be considered If the halide is secondary or tertiary, S N 2/E2 or S N 1/E1 depends on reaction condition

17 © Prentice Hall 2001Chapter 1017 Competition Between Substitution and Elimination S N 2/E2 reactions are favored by a high concentration of a good nucleophile/strong base S N 1/E1 reactions are favored by a poor nucleophile/weak base

18 © Prentice Hall 2001Chapter 1018 Competition Between S N 2 and E2 Primary halides generally undergo substitution, although if the halide or the base is bulky, elimination is possible Secondary halides are more difficult to predict The stronger and bulkier the base, the more elimination product is produced The higher the temperature, the more elimination product is produced Tertiary halides never undergo S N 2 reaction Elimination is the only possibility

19 © Prentice Hall 2001Chapter 1019 Competition Between S N 1 and E1 Because S N 1 and E1 reactions both proceed through a carbocation, they have the same rate-determining step Primary halides do not undergo either S N 1 and E1 reactions For secondary and tertiary halides, raising the temperature increases the elimination product

20 © Prentice Hall 2001Chapter 1020 Williamson Ether Synthesis The Williamson ether synthesis is a good way to prepare ethers

21 © Prentice Hall 2001Chapter 1021 Williamson Ether Synthesis This reaction is an S N 2 reaction and therefore benefits from a high concentration of alkoxide ion

22 © Prentice Hall 2001Chapter 1022 Williamson Ether Synthesis The alkoxide ion is prepared by using sodium metal or sodium hydride to remove a proton from an alcohol


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