Chapter 15 (Lecture 9)

Chapter 15 (Lecture 9) - Substituent Effects on the...

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Substituent Effects on the Reactivity and Orientation of Electrophilic Aromatic Substitution Substituent groups already on the benzene ring greatly influence both the reactivity of electrophilic attack, and the site (o,m,p) of attachment of the incoming electrophile. Classification of Substituents Substituents are classified as activating or deactivating relative to the reactivity of benzene. Substituents also are classified as ortho/para directing or meta directing . There is a fundamental theoretical connection between the reactivity influence and site direction of a substituent group.
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Activating Groups: Ortho-Para Directors The methyl and other alkyl groups are in this category. Example: The Nitration of Toluene Alkyl groups are also activating and ortho/para directing in other electrophilic aromatic substitutions such as halogenation, sulfonation, and Friedel-Crafts reactions. CH 3 toluene HNO 3 H 2 SO 4 more reactive than benzene CH 3 CH 3 CH 3 NO 2 NO 2 NO 2 + + o-nitrotoluene p-nitrotoluene m-nitrotoluene 59% 37% 4% The nitration of toluene proceeds faster than benzene and gives predominantly ortho and para nitrotoluene products.
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Other Substituent Groups that are Activating and Ortho/Para Directing OCH 3 NHCCH 3 O = OH NH 2 methoxy- acetamido- hydroxy- amino- Deactivating Groups: Meta Directors The nitro group, -NO 2 , is an example. The nitration of nitrobenzene proceeds at rate approximately 10 -8 times the rate of nitration of benzene, and the major product is meta-dintrobenzene .
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Other deactivating and meta-directing groups are: NO 2 nitrobenzene HNO 3 H 2 SO 4 much less reactive than benzene NO 2 NO 2 NO 2 NO 2 NO 2 NO 2 o-dinitrobenzene p-dinitrobenzene + + m-dinitrobenzene 6% 1% 93% COOH SO 3 H C-R CF 3 O = carboxyl- sulfo- acyl- trifluoromethyl-
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Halogen Substituents: Deactivating but Ortho/Para Directing The chloro and bromo substituents are unique in decreasing reactivity in electrophilic aromatic substitution, but producing mostly ortho and para products. Cl
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Chapter 15 (Lecture 9) - Substituent Effects on the...

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