Chapter 12

Chapter 12

CHAPTER 12 Substituted Benzene 12.1 Alkylbenzenes (Ar-R) Bezylic carbons: CH3 1 CH2CH3 CH(CH3)2 Reactions of Alkylbenzens 1. Free Radical Halogenation CH2CH3 2 Br2 hv Br CHCH3 Oxidation- 2 CH3 KMnO4 hot

CH2CH3 MnO4hot R CO2H KMnO4 hot CO2H CO2H CH2CH3 CHCH3 3 MnO4hot CO2H :Notice CH3 CCH3 CH3

MnO4hot No Reaction Benzylic carbon has no hydrogen CH3 CH3 MnO4hot MnO4hot 4 CO2H HO2C CO2H CO2H Phenols (ArOH) A phenol has a hydroxyl group directly attached to the benzene ring SP2- hybridized carbon OH C---O bond is not easily broken R-OH + HBr

5 No substitution or elimination reaction RBr + H2O Ar-OH + HBr No reaction SN1 or SN2 Acidity of Phenols + NaOH CH3CH2OH CH3CH2O- Na+ + pKa= 16 Not favored pKa=15.7 OH + NaOH O- Na+

+ H2O favored pKa= 15.7 pKa= 10 O- + R X 6 H2O a phenoxide ion SN2 OR + X- Esterfication of phenols O OH + CH3C

O O OCCH3 OCCH3 + CH3CO2H 7 The Kolbe Reaction OH 1) OH2) CO2 OH 3) H+ CO2H Mechanism OH OH - O-

+ O C O O H C O O- OH H+ OH 8 Salicylic acid CO2H CO2- Importance of Kolbe Reaction To Synthesize aspirin OH + CO2H O O CH3COCCH3 O

OCCH3 CO2H Acetylsalicylic acid (Aspirin) 9 The Reimer-Tiemann Reaction The reaction of phenol with chloroform in the .presence of base OH 10 OH- , 1) CHCl3, 2) H2O, H+ 70 C OH CH

O Salicylaldehyde :Mechanism OH - H CCl3 + - H2O H O- CCl2 + ..CCl CCl3 Cl- CCl2 dichlorocarbene 2

CHCl2 O OOH- O O H 11 H OH H H+ O- O CHCl O- Oxidation of phenols: phenol resists oxidation Oxidation of hydroquinones (1,4-dimethoxy benzenes) O

OH O + 2 H+ + 2e H O OH hydroquinone 1.4-benzoquinone OH O (quinone) O + OH

H O 1.2-benzoquinone 12 2 H+ + 2e - Preparation of Aniline HNO3 H2SO4 NO2 Fe, HCl NO2 + N H3Cl

H2, Pd/C or Pt 13 - OH- NH2 Benzenediazonium Salts NH2 + N ClN NaNO2, HCl, 0O C + H2O benzenediazonium chloride Excellent leaving abilty of N2 gas very reactive + N N 14 _ N 2

+ Nu - Nu Reactions of benzenediazinium Salts Br Cl CuCl CuBr 1) HNO3, H2SO4 2) H2, Pt NH2 NaNO 2, HCl, O 0C H3PO2 CuCN, KCN + N N Cl-

H2O, H OH KI 1) H+ 2) 15 CN I HBF4 heat F Coupling reactions + N + N N

N + N N + OH CO2H N N OH Azo compound NH2 NaNO2, HCl, 0O C CO2H 16 + N N + NHCH3 N N NHCH3 Synthesis involving diazonium salts

? CH3 ? ? ? CH3 HNO3, H2SO4 CH3 CuCN, KCN NO2 1) C N Fe, HCl 2) OH N2+ Cl- CH3 NaNO2 , O

CH3 17 NH2 HCl, O C Nucleophilic Aromatic Substitution X But + X Nu + - No SN1 or SN2 reaction Nu - Nucleophilic Aromatic Substitution

18 Nu + X- Examples 10% Cl NaOH OH o 350 300 atm increasing O2N reactivity toward Nu - Cl 10% NaOH o 160

O2N P-nitrophenol NO2 NO2 O2N Cl NO2 19 OH H2O warm O2N OH NO2 Other nucleophiles NO2 O2N Cl NO2 O2N

20 Cl NO2 NH3 heat CH3CH2Sheat O2N NH2 NO2 O2N SCH2CH3 Carbanion Mechanism: If the ring is activated toward nucleophilc substitution by an electron withdrawing group (I,e NO2) Two steps 1) addition of Nu: to form carbanion Loss of the halide ion) 2 OHO2N Cl slow

OH O2N - Cl Carbanion intermediate 21 fast O2N OH Carbanion intermediate is stabilized by resonance OO- O2 N OH Cl O+ N O- Cl

OH Cl O+ N O- 22 OH N+ OH Cl :Benzyne Mechanism If there is no electron withdrawing substituent on the .ring Cl NH2 , NH3 NH2 Mechanism Cl H - HCl NH2-

NH3 Benzyne NH2- 23 NH2 NH2- + NH2 Benzyne H H H 24 H Syntheses of substituted benzene compounds Cl How would you synthesize

NO2 HNO3 Cl2 H2SO4 NO2 FeCl3 NO2 But IF YOU START WITH CHLORINATION Cl Cl2 FeCl3 HNO3 H2SO4 o,p-director 25 Cl Cl

Cl + NO2 NO2 Syntheses ? ? Br ? NH2 HNO3 H2SO4 1) Fe, HCl NO2 ) OH 2 Br2, FeBr3 Br NO2 26

Note: Nitro group can be converted to arylamines which in turn can also be converted to aryldiazonium salts,=> making a diversity of substituted products 1) NO2 Fe, HCl NaNO2, HCl 2) OH- m-director NH2 0o o,p-director Nu N2+ Nu easily displaced Notice: Amino group does not undergo Friedel-Crafts reaction because an amino group reacts with Lewis acid since it is basic to

form strongly deactivating grouo NH2 27 + AlCl3 Al-Cl3 N+H2 strongly deactivating Nitrobenzene does not undergo Friedel-Crafts reactions NO2 O RX or RCX AlX3 No aromatic substitution Notice: An amino group is o,p-director, but an ammonium .group ( -NR3+) is an m-director and deactivating NH2 o,p-director 28

+ HNO3 N+H3 NO3m-director Sulfonic acid group (-SO3H) The sulfonic acid group is easily removed and can be displaced by a .variety of reagents SO3H SO3H 29 H2O, H+ 1) Fused NaOH + ) H O, H 2 2 + H2SO4

OH ? ? ? Br OH Br2, FeBr3 H2O, H+ Br HNO3 H2SO4 Br NO2 30 1) Fe, HCl 2) OH-

Br NH2 NaNO2,HCl 0o Br N2+ Br ? ? ? Br HNO3 H2SO4 CuBr NO2 Br2, FeBr3 Br NO2 31 1) Fe, HCl

2) OH- Br Br NH2 NaNO2,HCl 0 N2+ ? ? ? ? CO2H NH2 CH3CH2Cl, AlCl3 KMnO4 heat CH2CH3 HNO3, H2SO4 CH2CH3 NO2

1) Fe, HCl 2) OH CH2CH3 NH2 32 ? ? ? ? OCH3 CH3I HNO3, H2SO4 O- NO2 NaOH H2,Pd/C NH2 33 1)

NaNO2,HCl 2) H2O, H+ OH Cl ? ? ? ? CH3 CO2H KMno4, heat Cl2, FeCl3 CO2H 34 Cl ? ? ? ? CH3 CH3 HNO3, H2SO4 H3PO2

O 2N CH3 H2,Pd/C - H2N 1) CH3 2) 35 Cl2, FeCl3 NaNO2, HCl Cl N2 + Cl CH3

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