Reactions of Aromatic Compounds

Transcription

1 2-1 Reactions of Aromatic Compounds lectrophilic Aromatic Substitution Reactions Aromatic hydrocarbons (= arenes) undergo a substitution reaction with electrophiles: + catalyst + xample: omination of benzene + e 3 Review: Alkenes undergo addition reactions: + CM 2312 Spring 2016 otes: C.J. ahrni

2 X 2, ex 3 X =, X + X 3 2 S S 3 S 3 2 S 4 R Al 3 R Al 3 R R General Mechanism for AS: Arenium Ions Step 1: The electrophile attacks the -system of benzene to form a non-aromatic cyclohexadienyl carbocation: + A + A Step 2: The proton is removed to reform the aromatic delocalized -system: + A + A CM 2312 Spring 2016 otes: C.J. ahrni

3 A + A + A alogenation of Benzene + e 3 25 C + e 3 heat Mechanism: CM 2312 Spring 2016 otes: C.J. ahrni

4 itration of Benzene 3, 2 S C Mechanism: Sulfonation of Benzene S 3, 2 S 4 S 3 Mechanism: CM 2312 Spring 2016 otes: C.J. ahrni

5 riedel-crafts Alkylation + R X Al 3 R Mechanism: 2-10 The carbocation intermediate can be also generated through protonation of an alkene, + 0 C or starting from an alcohol in the presence of a Lewis acid (B 3 ): + B 3 60 C Limitation: rearrangement reactions of carbocation intermediate: CM 2312 Spring 2016 otes: C.J. ahrni

6 riedel-crafts Acylation + Al 3 Mechanism: acetophenone riedel-crafts acylations can be also carried using carboxylic acid anydrides: + 1. Al acetic anhydride Mechanism: CM 2312 Spring 2016 otes: C.J. ahrni

7 Limitations of riedel-crafts Reactions 1. Rearrangement to more stable carbocation intermediate: Al % 64% 2. Vinyl or aryl halide do not form stable carbocations: + Al 3 + Al Aldehydes are not accessible through riedel-crafts reactions: Al 3 4. Poor yields with electronwithdrawing substituents: 2 (C 3 ) 3 R C 3 S 3 2 CM 2312 Spring 2016 otes: C.J. ahrni

8 Polyalkylation often occur: + Al emmensen Reduction Problem (slide 2-13): Al % 64% Solution: riedel-crafts acylation followed by emmensen reduction: + Al 3 Zn(g)/ reflux ote: only arylketones (or aldehydes) are reduced (e.g. carboxylic acids remain unaffected) CM 2312 Spring 2016 otes: C.J. ahrni

9 2-17 Problem (15.32): Starting from benzene, outline a synthesis for a) tert-butyl benzene: b) 1-phenyl-2-methylbutane: Problem: Propose a synthesis of -tetralone starting from benzene and succinic anhydride: CM 2312 Spring 2016 otes: C.J. ahrni

10 15.10, ffect of Substituents on Reactivity and rientation Substituents affect both the rate of reaction (kinetics) and the site of attack (regioselectivity) C 3 C activation deactivation 15.10, Activating Groups: rtho/para Directors Alkyl substituents: C 3 C 3 C 3 C 3 toluene 3 2 S % 4% 37% -donors (e.g. -, -R, 2, R 2 ): aniline >99% CM 2312 Spring 2016 otes: C.J. ahrni

11 15.10, The rate determining step in electrophilic aromatic substitution of substituted benzenes is the step that results in the formation of the arenium ion. Because this step is endothermic, there is a strong resemblance between the arenium cation and the transition state leading to it (ammond-lef er postulate) 15.10, Q + A Q + A ffect of electronwithdrawing (left) and electrondonating substituents (right) on the free energy pro les for the formation of the arenium ion: CM 2312 Spring 2016 otes: C.J. ahrni

12 15.10, C 3 C 3 C 3 C 3 C 3 C , C + A 3 C + A Alkyl groups: stabilization through inductive effect (hyperconjugation): C 3 C 3 C 3 para attack: C 3 C 3 C 3 ortho attack: C 3 C 3 C 3 meta attack: Stabilization of the arenium ion intermediate yields a lower activation barrier and thus the fastest reaction pathway for ortho and para substitution. CM 2312 Spring 2016 otes: C.J. ahrni

13 15.10, donors: resonance stabilization of arenium ion para attack: ortho attack: meta attack: lectron donating resonance stabilization applies with decreasing strength in the following order: R 2 2 > R > X 15.10, Deactivating Groups: Meta Directors nitrobenzene 3 2 S % 93% 1% lectronwithdrawing substituents (- 2, -C, -C, -C, -CR, -CR 2, -S 3 ) destabilize the arenium ion intermediate. CM 2312 Spring 2016 otes: C.J. ahrni

14 15.10, C 3 C 3 C 3 para attack: C 3 C 3 C 3 ortho attack: C 3 C 3 C 3 meta attack: Meta substitution yields the least destabilized intermediate and therefore the fastest reaction pathway , alo Substituents: Deactivating rtho/para Directors chlorobenzene 2 e % 6% 55% S chlorobenzene 30% 0% 70% CM 2312 Spring 2016 otes: C.J. ahrni

15 15.10, para attack: ortho attack: meta attack: alo substituents stabilize the arenium ion intermediate through resonance ( -donation): 15.10G, Table ffect of Substituents: verview rtho/para Directors Meta Directors Strongly activating: 2, R, R 2, Moderately activating: CC 3, CR C 3, R Weakly activating: C 3, C 2 5, R C 6 5 Strongly deactivating: 2 R 3+ C 3, C 3 Moderately deactivating: C S 3 C 2, C 2 R C, CR Weakly deactivating:,,, I CM 2312 Spring 2016 otes: C.J. ahrni

16 15.10, xamples: C 3 2 Al 3 C S 4 2 e 3 C 3 C Al 3 C 3 I Al B 2-32 Directing ffects on Disubstituted Rings C 3 C 3 C 3 C Al 3 C 3 C 3 C e 3 C 3 2 C 3 C(C 3 ) S 4 C 3 C(C 3 ) 3 CM 2312 Spring 2016 otes: C.J. ahrni

17 15.14B 2-33 C 3 C S C 4 3 C 3 C 3 C 3 2 Ac C 3 C 3 C C 3 2 S 4 C 3 C A S 4 Solution: 2 C 3 C pyridine CM 2312 Spring 2016 otes: C.J. ahrni

18 Reactions of the Side Chain of Alkylbenzenes 1. alogenation of the side chain: BS, light C 4 2. Addition to the double bond of alkenyl benzenes: peroxides (no peroxides) xidation of the side chain: C 3 1. KMn 4, a, heat Alkyl, alkynyl, and acyl benzenes are also degraded to benzoic acid: C 2 R C 3 1. KMn 4, a, heat CM 2312 Spring 2016 otes: C.J. ahrni

19 2-37 Problem (15.36): xplain why the following syntheses will fail: 1. 3 / 2 S 4 2. C 3 C/Al Zn(g)/ 1. BS, C 4, light 2. at, t, heat 3. 2, e Synthetic Strategies 1. Introducing substituent through electrophilic aromatic substitution: R R =,, I, S 3, 2, CR, alkyl 2. Modifying existing substituents: R 1 R 2 C 2 from alkyl C 2 R from CR C 2 from C 3 C 2 from C 2 CM 2312 Spring 2016 otes: C.J. ahrni

20 Using protective groups ighly activating substituents such as 2, R 2, and render the aromatic ring so reactive that undesirable reactions may take place. => Reduce reactivity through a protective group: 2 C 3 C 1. 2, 2 S 4 heat 2 base R 2. a R 4. rientation in disubstituted benzenes: => Consider directing effects of other substituents and effect of reagents/conditions on existing substituents 2-40 Problem (15.32j): Starting from benzene, outline a synthesis for p-bromonitrobenzene CM 2312 Spring 2016 otes: C.J. ahrni

21 2-41 Problem: Starting from benzene, outline a synthesis for m-chloroethylbenzene Problem: Starting from benzene, outline a synthesis for p-nitrobenzoic acid. CM 2312 Spring 2016 otes: C.J. ahrni

22 2-43 Problem (15.34e): Starting from toluene, outline a synthesis for 1-chloro-3-trichloromethylbenzene. 20.4B 2-44 Anilines: Synthesis and Reactivity Anilines can be prepared through reduction of nitro compounds: 3 2 reducing reagent 2 2 S 4 reducing reagents a) (1) e, ; (2) a b) (1) Zn, ; (2) a c) Sn 2 d) 2 /Pt (catalyst) e) 2 S, 3, t CM 2312 Spring 2016 otes: C.J. ahrni

23 Anilines react with nitrous acid to form diazonium salts, which can be converted to a wide range of derivatives: B 4, heat 2-45 Cu Cu 2 a 2 KI I CuC C 3 P 2 Cu 2, Cu Mechanism of diazotization: 2 a 2 CM 2312 Spring 2016 otes: C.J. ahrni

24 2-47 Problem: Starting from benzene, outline a synthesis for 3-chlorophenol Problem: Suggest a synthesis for 1,3,5-tribromobenzene. CM 2312 Spring 2016 otes: C.J. ahrni

25 20.8 Diazo coupling: reaction of diazonium salts with electron-rich arenes: 2-49 G G + G = R 2 or Azo compounds have found a wide range of applications: S 3 a Ca S Alizarine Yellow R (p Indicator) Methyl orange (p Indicator) 2 Prontosil (antibiotic ) Ct S 3 a S 3 a 2 S 3 a a 3 S range B (food dye) S 3 a range G (histological stain) riochrome Black T (complexometric Indicator) a 3 S 2 S 3 a a 3 S 2 S 3 a CM 2312 Spring 2016 Trypan Blue (vital stain to selectively color dead cells) The Trypan Blue exclusion assay identi es dead cells otes: C.J. ahrni

26 Phenols omenclature: phenol m-chlorophenol 1-naphthol 2-naphthol C 3 C 3 C 3 p-cresol m-cresol o-cresol hydroquinone resorcinol catechol Phenols: Acidity pk a = 9.89 phenol phenolate anion C pk a = CM 2312 Spring 2016 otes: C.J. ahrni

27 free energy transition state G, A + 2 G G = RT ln K a K a = [ 3 + ][A ] [A][ 2 ] A reaction coordinate lectron withdrawing substituents stabilize the negatively charged phenolate anion lectron donating substituents destabilize the negatively charged phenolate anion The degree of resonance stabilization depends on the substituent position: pk a = 7.15 pk a = 8.28 CM 2312 Spring 2016 otes: C.J. ahrni

28 2-55 Problem: rder the following compounds with increasing pk a : Phenols: Synthesis 1. ydrolysis of aryldiazonium salts (slide 2-45) 2 a 2 Cu 2, Cu Industrial syntheses: 2 a a + e 350 C high pressure 2 2 S 4 3 P C C C CM 2312 Spring 2016 otes: C.J. ahrni

29 Phenols: Reactions 1. Phenol oxygen as nucleophile: pyridine pyridine 1. a, 2 conc. 2. R Aromatic -system as nucleophile: excess S 4 conc. 2 S 4 CM 2312 Spring 2016 otes: C.J. ahrni

30 Kolbe Reaction: 1. a 2. C Mechanism: 2-60 Problem: Starting from phenol, suggest a synthesis for Aspirin Aspirin CM 2312 Spring 2016 otes: C.J. ahrni

31 2-61 Problem: Starting from phenol, suggest a synthesis for acetaminophen (Tylenol) Acetaminophen The aisen Rearrangement heat a CM 2312 Spring 2016 otes: C.J. ahrni

32 21.11B 2-63 Aryl alides: ucleophilic Aromatic Substitution ydrolysis reaction of halides: 2 r.t. 2 r.t. Aryl halides undergo substitution only under more drastic reaction conditions: a 350 C K B 2-64 Labeling experiment: 14 C K (l) 14 C C 2 The Benzyne limination-addition Mechanism: phenyl anion benzyne CM 2312 Spring 2016 otes: C.J. ahrni

33 21.11A 2-65 lectronwithdrawing substituents greatly facilitate the hydrolysis of aryl halides: 2 a C 2 a 100 C a C 2 2 ote: a meta-nitro group does not produce a similar activating effect => underlying mechanism is different from hydrolysis of unsubstituted halides ffect of alide: Reaction rate decreasing with >> 21.11A 2-66 Addition-limination Mechanism: + CM 2312 Spring 2016 otes: C.J. ahrni

34 2-67 Problem: Rank the following compounds in descending order of reactivity toward hydroxide ion: I II III IV V 2-68 Problem: Propose a mechanism for the following conversion: ac C S 2 3 C S 2 a CM 2312 Spring 2016 otes: C.J. ahrni

35 2-69 Problem: Predict the main product(s) of the following reactions: ac 3 C 3 Lit 2 C 3 t 2 Problem (21.19) The following scheme outlines the synthesis of the -blocker toliprolol. Give the structures of the intermediates and the nal product C C C toliprolol C 3 CM 2312 Spring 2016 otes: C.J. ahrni