Lesson37 - Substitution at Aromatic Carbon 1 Acid-Base hydrogen swap 2&amp 3 Oxidation&amp Reduction 4 Substitution replace Cs

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Unformatted text preview: Substitution at Aromatic Carbon 1) Acid-Base - hydrogen swap 2 & 3) Oxidation & Reduction 4) Substitution- replace Cs substituent (-X) with another (-Y), neither being -H X C Y C Y + X + neither X nor Y being hydrogen Y replaces X 5) Elimination- loss of XY elements with concomitant pi bond formation 6) Addition- gain of XY elements with concomitant loss of pi bond reduction X C H C oxidation H replaces X X replaces H change in the number of C-H bonds in relation to the number of C-X bonds X C Z Z Y C + X Y elimination addition form bond-XY break bond +XY base H X H Y Y + X + base acid acid 7) Rearrangement- isomerization process (no atoms lost or gained); results in new bonding connectivity (one of many examples shown as there is no generic representation). R CH 3 R R stands for a generic "residue" Z = C, N, O We previously studied substitution reactions at sp 2 carbonyl carbon and learned that the process goes by variations of nucleophilic acyl substitution. Here we examine substitution at sp 2 aromatic carbon . These substitution reactions often follow the electrophilic aromatic substitution pathway, although 3 other pathways are possible in certain cases. Aromatic Substitution: Four Examples + HNO 3 N + H 2 SO 4 H O N OH O O O H OH I. Electrophilic Aromatic Substitution II. Nucleophilic Aromatic Substitution Cl NO 2 NO 2 Na OH H 2 O, 100 C O NO 2 NO 2 H Aromatic Substitution: Four Examples IV. Substitution via benzyne intermediate III. Substitution via arenediazonium ion CH 3 Cl Na NH 2 NH 3 ( l ), -33 C CH 3 NH 2 CH 3 NH 2 Na Cl + + Four Mechanistic Pathways for Aromatic Substitution: I. Electrophilic Aromatic Susbstitution [A E ] H + E E + H E H E H E H E H [D E ] resonance stabilized cationic intermediate this filled orbital shows a a -type interaction The LUMO shows positions of electron deficiency MO pictures of the cationic intermediate Four Mechanistic Pathways for Aromatic Substitution: II. Nucleophilic Aromatic Susbstitution [Ad N ] [E ] resonance stabilized anionic intermediate LG Nu Nu EWG EWG LG LG Nu LG Nu LG Nu EWG EWG EWG + + Nu LG The HOMO shows positions of surplus electron density MO picture of the anionic intermediate N N H H H H H N N H H H H H Nu...
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This note was uploaded on 01/25/2012 for the course CHEM 232 taught by Professor Miller during the Spring '08 term at University of Illinois, Urbana Champaign.

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Lesson37 - Substitution at Aromatic Carbon 1 Acid-Base hydrogen swap 2&amp 3 Oxidation&amp Reduction 4 Substitution replace Cs

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