4 - BENZENE: AROMATICITY, CONJUGATION AND ASSOCIATED REACTIVITY

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1 4 - BENZENE: AROMATICITY, CONJUGATION AND ASSOCIATED REACTIVITY During the early 1800's, a group of compounds of natural origin became collectively known as aromatic compounds. As several of these compounds were interconverted by chemical means, it became recognized that all were derived from benzene or related compounds. 1. Aromaticity (SF 14.1, 14.2, ; SFS 14.1,14.2, ) The term aromatic is presently applied not only to benzene and its derivatives, but to other certain compounds not related structurally to benzene yet having certain similar properties. These compounds were all characterized by a certain conjugated system (or systems) containing 2,6,10,14... electrons, i.e., (4n 2) electrons, where n = 0, 1, 2... in a single cycle. examples: CO 2 N N -all of the above systems contain 6 electron systems -certain ions can also be described as aromatic: cyclopentadienyl anion (6 ) cycloheptatrienyl cation (6 ) cyclopropenyl cation (2 ) CEM*3750 COURSE NOTES 1 of 13

2 TE FOLLOWING SECTION [a)] IS A READING SECTION AND SOULD BE ADDRESSED IN CONJUNCTION WIT OTER REFERENCE(S) a) uckel 4n 2 Rule and Molecular Orbitals Monocyclic planar systems of trigonally hybridized atoms having [4n 2] electrons (n = 1, 2, 3...) are aromatic (more stable than the corresponding open chain conjugated system). Those with 4n electrons (4, 8, 12...) are antiaromatic (less stable than their open chain counterparts). Once you have counted the (even) number of electrons in the molecule, then determine whether the value equates to a 4n system or a 4n2 system. Examination of the M.O. energy levels for monocyclic conjugated systems shows that cyclic systems having 2, 6, 10, (i.e., 4n 2) electrons will have a closed shell electron configuration. Those with 4, 8, (i.e., 4n) electrons are predicted to have two singly-occupied M.O.'s. This means there will be two unpaired electrons unless distortion of the molecule occurs. Calculations actually show that distortion should occur in such cases. E.g., cyclobutadiene distorts to a rectangle, cyclooctatetraene to a tub. Below are the molecular orbitals of benzene with a perspective from the top. They are aligned with their corresponding molecular orbital energy level. CEM*3750 COURSE NOTES 2 of 13

3 The energy level diagrams for several aromatic and anti-aromatic follow. Note that ions can also be aromatic or antiaromatic. cyclobutadiene cyclopentadienyl cyclopentadienyl benzene cation anion cycloheptatrienyl cycloheptatrienyl cyclooctatetraene cation anion As you can see, cyclic systems containing an even number of carbons can be uncharged polyenes. Radical cations and anions such as benzene radical anion (C ) contain an odd number of electrons and the uckel rule does not apply to such species. owever, it does apply to the 2 or -2 ion of the even cyclic systems. E.g., benzene with or - two electrons. Odd species must be anions, radicals or cations. Cyclopentadienyl is the most synthetically useful of the charged aromatic species. Cyclopentadiene itself has a low pk a of 16 - it is one of the most acidic hydrocarbons known. The reason of course is its ability to adopt the benzene-like electronic configuration. base indene fluorene Related compounds that are not as acidic include indene and fluorene. CEM*3750 COURSE NOTES 3 of 13

4 b) Aromatic Character Certain properties are associated with aromatic character, or "aromaticity". The most important of these are the following: i) The molar enthalpy of formation f, of aromatic compounds is more exothermic (or less endothermic) than that produced from average covalent bond enthalpies. ii) Aromatic species have a great tendency to be formed and preserved in a wide variety of chemical reactions. some examples: CO 2 P 2 O 5 Na 2 Cr 2 O 7 O heat 2 SO 4 CO 2 iii) The presence of a diamagnetic ring current induced by a magnetic field causing aromatic protons to appear at low field in the proton NMR spectrum. c) Structure of Benzene for comparison: resonance hybrid, all C-C bond lengths = 1.40 Å C 3 -C 3 (C sp 3-C sp 3) C 2 =C-C=C 2 (C sp 2-C sp 2) C 2 =C 2 (C sp 2=C sp 2) 1.54 Å 1.48 Å 1.33 Å avg. = ca Å is also a common designation, but is less useful for electron bookkeeping CEM*3750 COURSE NOTES 4 of 13

5 d) Resonance Energy ( ) 2 (g) = kcal/mol ( ) 2 (g) = kcal/mol Expected: 3 (-28.4) kcal/mol = kcal/mol Unexpected Extra Stability: ( ) kcal/mol = 35.9 kcal/mol = Empirical Resonance Energy Our estimate of empirical resonance energy (the unexpected lower enthalpy of benzene) depends on our choice of model compound. Other similar compounds can also be used. 2. NMR Spectra of Benzene Derivatives a) Chemical Shift-- the following are approximate chemical shift values, ppmdownfield from TMS: R-C ArC R 2 C Ar-C 2 -R 2.6 R 3 C 1.5 C 2 =C 2 5 ( ) Ar C C- 2-3 Substituents that are electron donating shield the protons and move them upfield while substituents that are electron withdrawing deshield the protons and move them downfield. CEM*3750 COURSE NOTES 5 of 13

6 b) Spin-spin coupling - aromatic protons single line, 7.27 ppm (6) R all 's have approximately the same chemical shift -usually a singlet where is electronegative such as for chlorobenzene and nitrobenzene o-, m- and p- protons have significantly different chemical shifts. o- and m- protons each coupled to 4 magnetically non-equivalent 's. The p- proton is coupled to 2 magnetically non-equivalent 's. Result: complex spectrum. singlet Y if and Y have significantly different electronegativities, spectrum resembles approximately a doublet of doublets J Y or and or -always complex when = R Y ortho coupling 6-8 z meta coupling 2-3 z para coupling 0-1 z CEM*3750 COURSE NOTES 6 of 13

7 3. Side Chain Chemistry of Benzene Derivatives a) Benzylic alogenation (SF 15.12; SFS 15.12) Benzene itself does not undergo free radical halogenation. The hydrogen abstraction step is endothermic and the chain reaction cannot be sustained. Instead, benzene eventually undergoes addition of Cl 2, under radical halogenation conditions (Cl 2, h ) to form a mixture of benzene hexachlorides C 6 6 Cl 6 (8 geometric isomers possible). Cl = 8 kcal/mol Cl owever, side chain reactions represent a different story: chlorinations of alkylbenzenes: Cl C 2 C 3 CC 3 Cl C 2 C 2 Cl 2 h Cl - not particularly selective or useful - relative reactivity: 56/2 : 44/3 = 1.9 : 1 But with bromination: Br C 2 C 3 CC 3 Br 2 h Br very selective >99% Related procedure: NBS in CCl 4, with thermal initiation using benzoyl peroxide or AIBN. CEM*3750 COURSE NOTES 7 of 13

8 Using either bromination method it is possible to replace 1, 2 or 3 (for Me group) benzylic hydrogens with bromine atoms. The selectivity is determined during the hydrogen abstraction step. (SS 10.6; SFS 10.6) Ar C benzylic hydrogen Bond Dissociation Energies: (kcal/mol) C C 2 =C-C 2 86 C 3 C 2 98 (1 o ) C 2 88 C 3 CC 3 95 (2 o ) (C 3 ) 3 C 93 (3 o ) 111 Cl Br 87.5 The benzyl radical is stabilized by resonance C 2 C 2 C 2 C 2 Why is bromination more selective? Initiation: 2 h or 2 Propagation: R R R 2 R This step determines selectivity. CEM*3750 COURSE NOTES 8 of 13

9 b) Nucleophilic Substitution at the Benzylic Carbon (SF/SFS 15.15) i) S N 2 Mechanism (bimolecular, direct displacement) Benzyl halides are ca. 100 times more reactive in S N 2 displacement than the corresponding ethyl halides. C 2 Cl I - acetone C 2 I Cl - relative rate 93 C 3 C 2 Cl I - acetone C 3 C 2 I Cl - 1 Benzyl halides will also react cleanly with Grignard reagents with few or no side reactions. C 2 Br C 2 R RMg ether or TF MgBr ii) S N 1 Mechanism (unimolecular, carbocation intermediate) S N 1 reactivity: 1º, ipropyl (2º) = allyl (1º/1º) < benzyl < tbutyl (3º) This reactivity corresponds to carbocation stability R-Cl C 3 CC 3 relative rate 3 Cl PhC 2 Cl 100 C 3 C 3 CCl C 3 PhCC 3 Cl PhCPh Cl In stabilizing a carbocation, one phenyl has approximately the same effect as two C 3 's. CEM*3750 COURSE NOTES 9 of 13

10 Stabilizing mode of phenyl group: C 2 C 2 C 2 C 2 Ionic salts of Ph 3 C (e.g., Ph 3 C BF 4 - ) can be isolated in crystalline form and are long-lived in polar aprotic solvents such as C 3 CN and C 3 NO 2. c) Side Chain Oxidation (SF/SFS C) O O benzylic alcohols: R MnO 2 hydrocarbon solvent R Common oxidizing agents: Na 2 Cr 2 O 7 2 SO 4 (aq). CrO 3, etc. MnO 2 is specific for allylic and benzylic alcohols. C 3 CO 2 oxidizing agents 1. Na 2 Cr 2 O 7 2 SO 4 (aq) 2. aq. KMnO 4 3. dilute NO 3 CR1R2 Other alkyl chains can be cleaved/oxidized to the simple acid, as long as there is at least one benzylic hydrogen. In these instances, there is less preparative use to the reaction. NO 2 and halogens on the ring do not interfere. CEM*3750 COURSE NOTES 10 of 13

11 d) Acidity compound C 3 C 2 - pka ca C 3 41 (N 3 : ca. 34) Can make carbanion using N 2 - in N 3 ( ). Ph-C 2 -Ph Ph 3 C C 3 n-butyllithium (n-buli) C 2 Li The benzylic anion is stabilized: C 2 C 2 C 2 C 2 e) Side Chain Reduction i) Catalytic reduction of the benzene ring by hydrogenolysis ( 2 (g) and Pt, Pd or Rh) is much more difficult than the reduction of alkenes, alkynes and most other unsaturated functional groups except for carboxylates. ii) Unlike alkyl systems, benzyl alcohols and benzyl ethers can be hydrogenated with Pd on C. ClO 4 is usually required. CEM*3750 COURSE NOTES 11 of 13

12 OR Pd/C, 2 ClO 4 R = hydrogen or hydrocarbon unit 4. Birch Reduction (The destruction of aromaticity) (S/SF 15.16) In general the reaction requires Na, Li or K in a mixture of liquid N 3 and alcohol Li (or Na or K) N 3 - EtO -further reduction much slower mechanism: e - etc. e - goes into the lowest molecular orbital benzene radical anion EtO EtO - (N 3 is not acidic enough to protonate the radical anion intermediate) CEM*3750 COURSE NOTES 12 of 13

13 e - EtO EtO - The reaction is compatible with the following substituents: -CO 2 /-CO 2 -, -R, -OC 3. It is not compatible with -NO 2, -CN, carbonyl, halogen. Some examples: CO 2 C3 C 3 CO 2 OC3 OC 3 aq O O Cl CEM*3750 COURSE NOTES 13 of 13

16. Chemistry of Benzene: Electrophilic Aromatic Substitution. Based on McMurry s Organic Chemistry, 7 th edition

16. Chemistry of Benzene: Electrophilic Aromatic Substitution Based on McMurry s Organic Chemistry, 7 th edition Substitution Reactions of Benzene and Its Derivatives Benzene is aromatic: a cyclic conjugated