Chapter 15 (Lecture #10)

Chapter 15 (Lecture #10) - Oxidation of the Side Chain of...

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Oxidation of the Side Chain of Arenes Strong oxidizing agents oxidize alkyl and alkenyl side chains to aryl carboxylic acids . A standard reagent is hot alkaline potassium permanganate . CH 3 CH 3 p-xylene (i) KMnO 4 , KOH, heat (ii) H 3 O + COOH COOH terephthalic acid CH 2 CH 2 CH 2 CH 3 CH 3 1-p-tolylbutane (i) KMnO 4 , KOH, heat (ii) H 3 O + COOH COOH
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Other Vigorous Side-Chain Oxidations C 6 H 5 CH=CHCH 3 C 6 H 5 C CCH 3 C 6 H 5 CCH 2 CH 3 O = (i) KMnO 4 , KOH, heat (ii) H 3 O + alkenyl alkynyl acyl COOH benzoic acid
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Synthetic Applications and Strategies The influence of substituents on electrophilic aromatic substitution reactions requires special attention to the order in which they are introduced into a benzene ring in the syntheses of substituted benzenes. Example: The nitration and bromination of benzene. HNO 3 H 2 SO 4 NO 2 Br 2 Fe Br NO 2 Br Br 2 Fe HNO 3 H 2 SO 4 Br + NO 2 Br NO 2 m-bromonitrobenzene ortho para 38% 62%
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Example: The oxidation and nitration of toluene. CH 3 toluene (i) KMnO 4 , HO - , heat (ii) H 3 O + COOH HNO 3 H 2 SO 4 CH 3 CH 3 + NO 2 NO 2 HNO 3 H 2 SO 4 COOH NO 2 m-nitrobenzoic acid (i) KMnO 4 , HO - heat (ii) H 3 O + (i) KMnO 4 , HO - heat (ii) H 3 O + COOH COOH + NO 2 NO 2 ortho para
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Modification of Amino and Hydroxyl Groups by Acetylation These groups are highly activating in electrophilic aromatic substitution reactions: NH 2 aniline Br 2 aqueous solution NH 2 Br Br Br 2,,4,6-tribromoaniline It is difficult to limit this reaction to monobromination.
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Controlled Monobromination by way of Acetanilide NH 2 aniline (highly reactive) CH 3 COCCH 3 O = O = acetylation NHCCH 3 O = acetanilide (moderate reactivity) Br 2 acetic acid NHCCH 3 O = NHCCH 3 O = + Br Br (i) H 3 O + heat (ii) HO - (i) H 3 O + heat (ii) HO - hydrolysis NH 2 NH 2 Br Br o-bromoaniline p-bromoaniline Because of the interaction between the nonbonding electron pair on N and the carbonyl group, the acetamido group is less activating. -NHCCH 3 :O: = : -NH=CCH 3 :O: : - +
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Moderating Reactivity through Synthetic Modification Acetylation of Phenols and Anilines Both phenols (Ar- OH ) and anilines (Ar- NH 2 ) are extremely reactive in most electrophilic aromatic substitution reactions. Sometimes it is necessary to reduce this reactivity by synthetic modification of the substituent groups. :OH : phenol (more reactive) :NH 2 aniline (more reactive) CH 3 COCCH 3 O = O = acetic anhydride acetylation CH 3 COONa + CH 3 COCCH 3 O = O = acetic anhydride + acetylation :OCCH 3 : O = phenyl acetate (less reactive) :NHCCH 3 O = acetanilide (less reactive)
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An Explanation of the Decreased Reactivity The decrease in reactivity of phenol and aniline upon acetylation
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