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Published byDale Payne Modified over 9 years ago
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Elimination Reactions
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Dehydrohalogenation (-HX) and Dehydration (-H 2 O) are the main types of elimination reactions.
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Dehydrohalogenation (-HX)
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E1 mechanism
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E2 mechanism
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Orientation of elimination: Zaitsev’s Rule In reactions of removal of hydrogen halides from alkyl halides or the removal of water from alcohols, the hydrogen which is lost will come from the more highly-branched -carbon. A. N. Zaitsev -- 1875 Less branched More branched
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Product formed from previous slide
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Comparing Ordinary and Bulky Bases
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1-butene: watch out for competing reactions!
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Substitution or Elimination? Factors to Consider: 1.How Basic is the Nucleophile? 2. Steric Hindrance at Reacting Carbon 3. Steric Hindrance at Nucleophile
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Summary of Reactivity
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Reactions of Benzene
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KMnO 4 oxidationno reaction Br 2 /CCl 4 additionno reaction HIadditionno reaction H 2 /Nireductionno reaction Reagent Cyclohexene Benzene
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Heats of hydrogenation and combustion are far lower than they should be. cyclohexene + H 2, Ni cyclohexane + 28.6 Kcal/mole 1,3-cyclohexadiene + 2 H 2, Ni cyclohexane + 55.4 Kcal/mole (predicted value = 2 X 28.6 = 57.2 Kcal/mole) benzene + 3 H 2, Ni, heat, pressure cyclohexane + 49.8 Kcal/mole (predicted value = 3 X 28.6 = 85.8 Kcal/mole) Heat of hydrogenation for benzene is 36 Kcal/mole lower than predicted!
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Chapter 16 Unusual Stability
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Facts about benzene: a)Formula = C 6 H 6 b)Isomer number: one monosubstituted isomer C 6 H 5 Y known three disubstituted isomers C 6 H 4 Y 2 known c)Benzene resists addition reaction, undergoes substitution reactions. d)Heats of hydrogenation and combustion are far lower than they should be. e)From X-ray, all of the C—C bonds in benzene are the same length and intermediate in length between single and double bonds.
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Nomenclature for benzene: monosubstituted benzenes: Special names:
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Disubsituted benzenes: ortho- meta- para- 1,2- 1,3- 1,4-
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Electrophilic Aromatic Substitution (Aromatic compounds) Ar-H = aromatic compound 1. Nitration Ar-H + HNO 3, H 2 SO 4 Ar-NO 2 + H 2 O 2.Sulfonation Ar-H + H 2 SO 4, SO 3 Ar-SO 3 H + H 2 O 3.Halogenation Ar-H + X 2, Fe Ar-X + HX 4.Friedel-Crafts alkylation Ar-H + R-X, AlCl 3 Ar-R + HX
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+ HO-NO 2 + H 2 SO 4 H 2 O-NO 2 + HSO 4 - + H 2 O-NO 2 H 2 O + NO 2 H 2 SO 4 + H 2 O HSO 4 - + H 3 O + HNO 3 + 2 H 2 SO 4 H 3 O + + 2 HSO 4 - + NO 2 + nitration
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resonance
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Mechanism for nitration:
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Mechanism for sulfonation:
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Mechanism for halogenation:
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Mechanism for Friedel-Crafts alkylation:
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Substituent groups on a benzene ring affect electrophilic aromatic substitution reactions in two ways: 1)reactivity activate (faster than benzene) or deactivate (slower than benzene) 2)orientation ortho- + para- direction or meta- direction Effect of Substituent
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-CH 3 activates the benzene ring towards EAS directs substitution to the ortho- & para- positions -NO 2 deactivates the benzene ring towards EAS directs substitution to the meta- position
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Common substituent groups and their effect on EAS: -NH 2, -NHR, -NR 2 -OH -OR -NHCOCH 3 -C 6 H 5 -R -H -X -CHO, -COR -SO 3 H -COOH, -COOR -CN -NR 3 + -NO 2 increasing reactivity ortho/para directors meta directors
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