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Chemistry 125: Lecture 54 February 22, 2010 Linear and Cyclic Conjugation Allylic Intermediates (4n+2) Aromaticity This For copyright notice see final page of this file Is There a Limit to 1 Energy for Long Chains? Chain length Normalized AO size Overlap per bond Number Total of overlap bonds stabilization (AO product) N.B. Here we are using our own “overlap stabilization” units, which are twice as large as conventional “” units. 2 1/2 1/2 1 1/2 4 1/4 1/4 3 3/4 8 1/8 1/8 7 7/8 N 1/N 1/N N-1 (N-1)/N Yes, the limit is 1, i.e. twice the stabilization of the H2C=CH2 bond. Similarly, the LUMO destabilization limit is twice that of the H2C=CH2 MO.. MO Energy (units of 2) +1 0 Semicircle Mnemonic for MO Energy in Conjugated Chains. . . . p . : :: : : . . . . . Place points denoting length of chain evenly along circumference between upper and lower limit (+1 and -1). .. . N=2 an N=3 N=1 ethylene allyl isolated 2p AO N=4 1,3-butadiene etc. : All odd chains have a nonAs the conjugated chain lengthens, (difference is resonance stabilization bonding MO with nodes on more and more levels are crowded of butadiene vs. 2 isolated ethylenes) alternant carbons. It is the between -1 and +1, and the HOMOlocus of the “odd” electron p andgap ) allylic stabilization (vs. isolated LUMO decreases. in the radical, and of + (-) same 2 electron stabilization Color shiftintoward red. (anion). charge the cation . -1 Radius of circle = 2 stabilization of H2C=CH2 [ limit of ±(N-1)/N ] . for cation, radical, anion Allylic Intermediates: Allylic Free-Radical Bromination Sec. 11.8 pp. 497-500, Sec. 12.11c p. 543 NBS Allylic Intermediates: Addition of HX to Butadiene Sec. 12.9-12.10 pp. 534-541 Butadiene H+ Propenyl Cation -21.4 best potential best overlap LUMO+1 HOMO-1 HOMO LUMO +17.6 best overlap hyperconjugated C-H HOMO-4 LUMO+1 HOMO LUMO best product Propenyl Cation +132 best +152 potential +144 +99 best potential Surface Potential D Cl p. 1288 Cl- symmetrical (but for D) 3.1 : 1 -78° 25° 1.6 : 1 rapid ion-pair collapse competes with motion In a Very Viscous Solvent Can Short-Range Motion Constitute a Rate- (and Product-) Determining Step? CH3 H3C • CH3 (2) Shift D atom CH3 N N CD3 • H3C CD3 exothermic/easy/fast D CD2 CH3 CD3 CD3 If Step 1 (motion) is rate-limiting, H- and D-transfer products should form in equal amounts. CH3 (2) Shift H atom exothermic/easy/faster (because their motions should be equally fast) If Step 2 (atom shift) is rate-limiting, more H-transfer product should form. kH/kD > 1 (kinetic “isotope effect”) CD3 H3C CD3 CH2 H CD3 CD3 Kinetic vs. Thermodynamic Control Sec. 12.10 pp. 537-540 Allylic Intermediates, Transition States: SN1 and SN2 Sec. 12.11a,b pp. 541-543 Allylic Intermediates: RH Acidity Sec. 12.11d pp. 543-544 Cf. Benzylic Intermediates (sec. 13.12) e.g. Ph-CH2-H pKa = 41 AROMATICITY Ch. 13-14 predicted Conjugation worth ~30 kcal ! observed ! Cf. 13.5a pp. 580-581 Bringing the ends of a conjugated chain together to form a ring gives a lowest MO with one additional bonding interaction. Lowest MO will have energy = -N/N = -1 In a conjugated ring peripheral nodes must come in even numbers. e.g. cyclopropenyl E = -1 E = +1/2 0 nodes E = +1/2 2 nodes 2 nodes Energy Shifts on “Ring Formation” Shifts Alternate (because of node parity). . unfavorable . favorable : MO Energy (units of 2) +1 0 : -1 End to End Interaction : . : . unfavorable favorable On bringing the ends of a chain together, odd-numbered MOs (1, 3, 5, etc.) decrease in energy (favorable terminal overlap for 0,2,4… nodes), while even-numbered MOs (2, 4, 6, etc.) increase in energy (unfavorable terminal overlap for 1,3,5… nodes). Thus having an odd number of occupied orbitals (more odd-numbered than even-numbered) insures overall stabilization of ring (compared to chain). [though there may be strain in the bonds] an odd number of e-pairs Hückel’s Rule: 4n+2 electrons is unusually favorable in a conjugated ring. (where n in an integer) . 0 . . Same radius as for open chain . .. . Inscribe regular polygon with point down. Read MO energies on vertical scale. . . reactive SOMOs ! 3 cyclopropenyl 4 cyclobutadiene 6 benzene 4n “Antiaromatic”! Stabilized slightly destabilized Cation (vs. strongly stabilized hexatriene) : : : :: -1 : . . :: MO Energy (units of 2) +1 open-chain energies from semicircle mnemonic Circle Mnemonic for MO Energy in Conjugated Rings. . (vs. butadiene) + (vs. allyl ) • There is always an MO at Anion destabilized Radical less stabilized (vs.-1. allyl•) Generalization of Aromaticity: 4n+2 Stability Sec. 13.6 pp. 582-595 Transition State “Aromaticity” Heteroaromatic Compounds (Sec. 13.9 pp. 598-601) N . O H Pyridine H Furan H YY H H : N H H Pyrrole Imidazole H H H N.B. Single . denotes contribution of 1 e to system (redundant with double bond). (occurs in amino acid histidine) N . H H . N H HH Relay for long-range proton transfer by enzymes X X- End of Lecture 54 Feb. 22, 2010 Copyright © J. M. McBride 2010. Some rights reserved. 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