Organic Lecture Series BenzeneBenzene && AromaticityAromaticity Chapter 21 1 Concept of Aromaticity Organic Lecture Series The underlying criteria for aromaticity were recognized in the early 1930s by Erich Hückel, based on molecular orbital (MO) calculations To be aromatic, a compound must: 1. be cyclic 2. have one p orbital on each atom of the ring 3. be planar or nearly planar so that there is continuous or nearly continuous overlap of all p orbitals of the ring 4. have a closed loop of (4n + 2) π electrons in the cyclic arrangement of p orbitals 2 Organic Lecture Series Benzene - Resonance Model • The concepts of hybridization of atomic orbitals and the theory of resonance, developed in the 1930s, provided the first adequate description of benzene’s structure – the carbon skeleton is a regular hexagon – all C-C-C and H-C-C bond angles 120° H H 120° sp2 -sp2 120° C C σ H C 120° C σ H 109 pm CC sp2 -1s H H 139 pm 3 Organic Lecture Series Cyclohexane-Chair conformation Benzene- “flat” unsaturated system 4 Organic Lecture Series Benzene – Molecular Orbitals The π system of benzene: 1. the carbon framework with the six 2p orbitals 2. overlap of the parallel 2p orbitals forms one torus above the plane of the ring and another below it 3. this orbital represents the lowest-lying pi-bonding molecular orbital 5 Organic Lecture Series Benzene - Resonance Model • Benzene is represented as a hybrid of two equivalent Kekulé structures – each makes an equal contribution to the hybrid and thus the C-C bonds are neither double nor single, but something in between Benzene as a hybrid of two equivalent contributin g s tructures 6 Organic Lecture Series Benzene - Resonance Model • Resonance energy: the difference in energy between a resonance hybrid and the most stable of its hypothetical contributing structures in which electrons are localized on particular atoms and in particular bonds – one way to estimate the resonance energy of benzene is to compare the heats of hydrogenation of benzene and cyclohexene 7 Organic Lecture Series 3(-119.7 kJ/mol) 8 Organic Lecture Series Nomenclature-Monosubstituted • Monosubstituted alkylbenzenes are named as derivatives of benzene – many common names are retained TolueneEthylbenzene Cumene Styrene OH NH2 CHO COOH OCH3 Phenol AnilineBenzaldehyde Benzoic acid Anisole (commit to memory) 9 Organic Lecture Series Benzyl and phenyl groups: CH3 CH2 - Benzene Phenyl group, Ph- Toluene Benzyl group, Bn- OO H 3CO Ph 1-Phenyl-1pentanone 4-(3-Methoxyphenyl)- (Z)-2-Phenyl- 2-butanone 2-butene 10 Organic Lecture Series Disubstituted Benzenes • Locate two groups by numbers or by the locators ortho (1,2-), meta (1,3-), and para (1,4-) 11 Organic Lecture Series Disubstituted Benzenes – where one group imparts a special name, name the compound as a derivative of that molecule: CH 3 NH2 COOH CH 3 NO2 Cl CH Br 3 4-Bromotoluene 3-Chloroaniline 2-Nitrobenzoic acid m-Xylene (p-Bromotoluene) (m-Chloroaniline) (o-N itrob enzoic acid ) Toluene is Aniline is Benzoic Acid the parent the parent is the parent name name name 12 Organic Lecture Series Disubstituted Benzenes where neither group imparts a special name, locate the groups and list them in alphabetical order CH 2 CH 3 NO2 42Br 3 1 2 1 Cl 1-Chloro-4-ethylbenzene 1-Bromo-2-nitrobenzene (p-Chloroethylbenzene) (o-Bromonitrobenzene) 13 Organic Lecture Series Phenols • The functional group of a phenol is an -OH group bonded to a benzene ring OH OH OH OH OH CH3 OH Phenol 3-Methylphenol 1,2-Benzenediol 1,4-Benzenediol (m-Cresol) (Catechol) (Hydroquinone) 14 Organic Lecture Series Acidity of Phenols • Phenols are significantly more acidic than alcohols, compounds that also contain the OH group Phenoxide: + - + + OH H2 O O H3 O pKa = 9.95 + - + CH3 CH2 OH H2 O CH3 CH2 O + H3 O pKa = 15.9 Ethoxide 15 Organic Lecture Series Acidity of Phenols – the greater acidity of phenols compared with alcohols is due to the greater stability of the phenoxide ion relative to an alkoxide ion: O O O O O H H H These 2 Kekulé These three contributing structures structures are delocalize the negative charge equivalent onto carbon atoms of the ring 16 Organic Lecture Series Acidity of Phenols Alkyl and halogen substituents effect acidities by inductive effects: – alkyl groups are electron-releasing – halogens are electron-withdrawing OH OH OH OH OH CH3 Cl CH3 Cl Phen ol m-Cresol p-Cresol m-Chlorophenol p-Chororophenol pKa 9.95 pKa 10.01 pKa 10.17 pKa 8.85 pKa 9.18 17 Organic Lecture Series Acidity of Phenols – nitro groups increase the acidity of phenols by both an electron-withdrawing inductive effect and a resonance effect OH OH OH NO2 NO2 Phenol m-Nitrophenol p-Nitrophenol pK a 9. 95 pK a 8.28 pK a 7. 15 18 Organic Lecture Series Acidity of Phenols – part of the acid-strengthening effect of -NO2 is due to its electron-withdrawing inductive effect – in addition, -NO2 substituents in the ortho and para positions help to delocalize the negative charge O O delocalization of negative charge onto oxygen further increases the resonance stabilization of phenoxide ion + + – N ––N O O O O 19 Organic Lecture Series Acidity of Phenols • Phenols are weak acids and react with strong bases to form water-soluble salts – water-insoluble phenols dissolve in NaOH(aq) - + OH++ NaOH O Na H2 O Ph en ol Sodium Sodium Water p Ka 9.95 hydroxide phenoxide pK a 15.7 (stronger acid) (weaker acid) 20 Organic Lecture Series Acidity of Phenols – most phenols do not react with weak bases such as NaHCO3; they do not dissolve in aqueous NaHCO3 ++- + OH NaHCO3 O Na H2 CO3 Ph en ol Sodium Sodium Carbonic acid p Ka 9.95 bicarbonate phenoxide p Ka 6.36 (weaker acid) (stronger acid) 21 Organic Lecture Series Alkyl-Aryl Ethers • Alkyl-aryl ethers can be prepared by the Williamson ether synthesis – but only using phenoxide salts and haloalkanes – haloarenes are unreactive to SN2 reactions - + X + RO Na no reaction 22 Alkyl-Aryl Ethers Organic Lecture Series The following two examples illustrate 1. the use of a phase-transfer catalyst 2. the use of dimethyl sulfate as a methylating agent NaOH, H O, CH Cl OH+ CH =CHCH Cl 2 2 2 2 2 + - Bu4 N Br Phenol 3-Chloropropene (Allyl chloride) OCH2 CH= CH2 Phenyl 2-propenyl ether (Allyl phenyl ether) O NaOH, H2 O, CH2 Cl2 OH + CH3 OSOCH3 Bu N+ Br - O 4 Phenol Dimethyl sulfate OCH3 + Na2 SO4 Methyl phenyl ether 23 (Anisole) Benzylic Oxidation Organic Lecture Series • Benzene is unaffected by strong oxidizing agents such as H2CrO4 and KMnO4 – halogen and nitro substituents are also unaffected by these reagents – an alkyl group with at least one hydrogen on its benzylic carbon is oxidized to a carboxyl group CHR O C OH H2CrO4 or KMnO4 Arene Benzoic Acids Not responsible for mechanism 24 Organic Lecture Series Benzylic Oxidation CH3 COOH H2 CrO4 O2NCl O2NCl 2-Chloro-4-nitrotoluene 2-Chloro-4-nitrobenzoic acid – if there is more than one alkyl group on the benzene ring, each is oxidized to a -COOH group O O K2 Cr 2 O7 H3 C CH3 HOC COH H2 SO4 1,4-Dimethylbenzene 1,4-Benzenedicarboxylic acid (p-xylene) (terephthalic acid) Not responsible for mechanism 25 Organic Lecture Series Benzylic Chlorination • Chlorination (and bromination) occurs by a radical mechanism X CHR2 R2C X2 heat or h Arene Benzyl Halides CH Cl CH3 heat 2 or light + + Cl2 HCl Toluene Benzyl ch loride 26 Organic Lecture Series Benzylic Bromination • Bromination can be conducted with NBS: Br NBS (PhCO2) 2 , CCl4 Ethylbenzene 1-Bromo-1-phenylethane (racemic) 27 Organic Lecture Series Benzoyl Peroxide as a Radical initiator Benzoyl Peroxide O O O O h ν or Δ O O 2 C O O Phenyl Radical 28 Organic Lecture Series Benzylic Reactions • Benzylic radicals (and cations also) are easily formed because of the resonance stabilization of these intermediates – the benzyl radical is a hybrid of five contributing structures C C C C C 29 Organic Lecture Series Benzylic Halogenation – benzylic bromination is highly regioselective Br NBS (PhCO2 )2 , CCl4 Ethylbenzene 1-Bromo-1-phenylethane (the only product formed) – benzylic chlorination is less regioselective Cl heat Cl or light + Cl2 + Ethylbenzene 1-Chloro-1- 1-Chloro-2- phenylethane phenylethane (90%) (10%) 30 Hydrogenolysis Organic Lecture Series • Hydrogenolysis: cleavage of a single bond by H2 – among ethers, benzylic ethers are unique in that they are cleaved under conditions of catalytic hydrogenation H2 C H2 HO R O R CH3 Pt or Pd the desired the side product product this bond is cleaved Me O Pd/ C + H2 OH + Benzyl butyl ether1-Butanol Toluene 31 Organic Lecture Series Benzyl Ethers • The value of benzyl ethers is as protecting groups for the OH groups of alcohols and phenols – to carry out hydroboration/oxidation of this alkene, the phenolic -OH must first be protected; it is acidic enough to react with BH3 and destroy the reagent 2. BH •THF 1. ClCH2 Ph 3 OH Et 3N3.OPh H2 O2 /NaOH 2-(2-Propenyl)phenol (2-Allylphenol) OH H2 OH Pd/ C OPh OH 2-(3-Hydroxypropyl)phenol 32.