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Benzene: Aromaticity, Conjugation and Associated Reactivity

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Benzene: Aromaticity, Conjugation and Associated Reactivity 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, 14.4-14.7; SFS 14.1,14.2, 14.4-14.7) 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: N N H CO H 2 -all of the above systems contain 6 electron systems -certain ions can also be described as aromatic: H H H + _ H H H + H H cyclopentadienyl cycloheptatrienyl cyclopropenyl anion (6) cation (6) cation (2) CHEM*3750 COURSE NOTES| 1 of 13 THE FOLLOWING SECTION [a)] IS A READING SECTION AND SHOULD BE ADDRESSED IN CONJUNCTION WITH OTHER REFERENCE(S) a) Huckel 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 4n+2 system. Examination of the M.O. energy levels for monocyclic conjugated systems shows that cyclic systems having 2, 6, 10, 14... (i.e., 4n + 2) electrons will have a closed shell electron configuration. Those with 4, 8, 12... (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. CHEM*3750 COURSE NOTES| 2 of 13 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 - (C6H6 ) contain an odd number of electrons and the Huckel rule does not apply to such species. However, 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 pKa 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 H H H H H H H indene fluorene Related compounds that are not as acidic include indene and fluorene. CHEM*3750 COURSE NOTES| 3 of 13 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 Hf, 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: CO2H Na2Cr2O7 P2O5 H2SO4 heat CO2H O 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: CH3-CH3 (Csp3-Csp3) 1.54 Å resonance hybrid, all C-C bond lengths = 1.40 Å CH2=CH-CH=CH2 (Csp2-Csp2) 1.48 Å 1.33 Å CH2=CH2 (Csp2=Csp2) avg. = ca. 1.40 Å is also a common designation, but is less useful for electron bookkeeping CHEM*3750 COURSE NOTES| 4 of 13 d) Resonance Energy ( ) + H2 (g) H = -28.4 kcal/mol ( ) + H2 (g) H = -49.3 kcal/mol Expected: 3 (-28.4) kcal/mol = -85.2 kcal/mol Unexpected Extra Stability: (85.2-49.3) 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-CH3 0.9 ArCH3 2.35 R2CH2 1.3 Ar-CH2-R 2.6 R3CH 1.5 CH2=CH2 5 (4.7-5.3) Ar-H 7-8 -CC-H 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. CHEM*3750 COURSE NOTES| 5 of 13 b) Spin-spin coupling - aromatic protons single line, 7.27 ppm (6H) R all H's have approximately the same chemical shift -usually a singlet X where X 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 H's. Thep - proton is coupled to 2 magnetically non-equivalent H's. Result: complex spectrum. X singlet X X 1 2 if X and Y have significantly different electronegativities, spectrum resembles approximately a doublet of Y doublets J 12 X X X X X Y -always complex or and or when X = R X Y ortho coupling 6-8 Hz meta coupling 2-3 Hz para coupling 0-1 Hz CHEM*3750 COURSE NOTES| 6 of 13 3. Side Chain Chemistry of Benzene Derivatives a) Benzylic Halogenation (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 H undergoes addition of Cl2, • under radical halogenation + Cl• + HCl conditions (Cl2, h) to form a mixture of benzene H = +8 kcal/mol hexachlorides C6H6Cl6 (8 geometric isomers possible). However, side chain reactions represent a different story: chlorinations of alkylbenzenes: Cl Cl CH2CH3 CHCH3 CH2CH2 h + Cl2 + + HCl - not particularly selective or useful - relative reactivity: 56/2 : 44/3 = 1.9 : 1 But with bromination: Br CH2CH3 CHCH3 h + Br2 + HBr very selective >99% Related procedure: NBS in CCl4, with thermal initiation using benzoyl peroxide or AIBN. CHEM*3750 COURSE NOTES| 7 of 13 Using either bromination method it is benzylic possible to replace 1, 2 or 3 (for Me group) Ar C H benzylic hydrogens with bromine atoms. The hydrogen selectivity is determined during the hydrogen abstraction step. (SS 10.6; SFS 10.6) Bond Dissociation Energies: (kcal/mol) 105 CH3 H CH2=CH-CH2 H 86 98 (1o) CH3CH2 H CH2 H 88 o CH3CHCH3 95 (2 ) H 93 (3o) 111 (CH3)3C H H H Cl 103.2 H Br 87.5 The benzyl radical is stabilized by resonance • CH2 CH2 CH2 CH2 • • • Why is bromination more selective? h or Initiation: X2 2 X• Propagation: X• + RH R• + HX This step determines selectivity. R• + X2 RX + X• CHEM*3750 COURSE NOTES| 8 of 13 b) Nucleophilic Substitution at the Benzylic Carbon (SF/SFS 15.15) i) SN2 Mechanism (bimolecular, direct displacement) Benzyl halides are ca. 100 times more reactive in SN2 displacement than the corresponding ethyl halides. relative rate CH2Cl CH2I acetone - + I- + Cl 93 - acetone - 1 CH3CH2Cl + I CH3CH2I + Cl Benzyl halides will also react cleanly with Grignard reagents with few or no side reactions. CH2Br CH2R + RMgX ether or + MgXBr THF ii) SN1 Mechanism (unimolecular, carbocation intermediate) SN1 reactivity: 1º , ipropyl (2º) = allyl (1º/1º) < benzyl < tbutyl (3º) This reactivity corresponds to carbocation stability R-Cl relative rate CH3CHCH3 3 Cl PhCH2Cl 100 CH3 CH3CCl 1.8 X 104 In stabilizing a carbocation, one phenyl has approximately CH3 the same effect as two CH3's. PhCHCH3 3.6 X 104 Cl PhCHPh 107 Cl CHEM*3750 COURSE NOTES| 9 of 13 Stabilizing mode of phenyl group: + CH2 CH2 CH2 CH2 + + + + + + + - Ionic salts of Ph3C (e.g., Ph3C BF4 ) can be isolated in crystalline form and are long-lived in polar aprotic solvents such as CH3CN and CH3NO2. c) Side Chain Oxidation (SF/SFS 15.13 C) OH O MnO2 benzylic alcohols: R R H hydrocarbon solvent Common oxidizing agents: Na2Cr2O7 + H2SO4 (aq). CrO3, etc. MnO2 is specific for allylic and benzylic alcohols. oxidizing agents CH3 CO2H 1. Na2Cr2O7 + H2SO4 (aq) 2. aq. KMnO4 3. dilute HNO3 CHR1R2 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. NO2 and halogens on the ring do not interfere. CHEM*3750 COURSE NOTES| 10 of 13 d) Acidity compound pKa CH3CH2-H ca. 50 H 43 CH3 41 (NH3: ca. 34) Ph-CH2-Ph 34 Can make carbanion - using NH2 in NH3 ( ). Ph3C-H 31.5 CH3 CH2Li + n-butyllithium (n-BuLi) The benzylic anion is stabilized: _ CH2 CH2 CH2 CH2 _ _ _ e) Side Chain Reduction i) Catalytic reduction of the benzene ring by hydrogenolysis (H2 (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.
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