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Lesson37 - Substitution at Aromatic Carbon We previously...

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Substitution at Aromatic Carbon 1) Acid-Base - hydrogen swap 2 & 3) Oxidation & Reduction 4) Substitution - replace C’s 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 π bond -XY break π 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.
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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
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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 + +
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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 σ a π -type interaction The LUMO shows positions of electron deficiency MO pictures of the cationic intermediate
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Four Mechanistic Pathways for Aromatic Substitution: II. Nucleophilic Aromatic Susbstitution [Ad N ] [E β ] 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
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N N H H H H H N N H H H H H Nu H H H H H Nu Nu N 2 + +
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