Transcript ppt
Chemistry 125: Lecture 55 February 23, 2011 This Conjugated Dienes Theory of Linear and Cyclic Conjugation (4n+2) Aromaticity For copyright notice see final page of this file When does conjugation make a difference? Experimental Evidence Allylic Cation, Anion, Radical stabilized by ~13 kcal/mole. Allylic SN1/SN2 Transition States also stabilized. Diene Stabilization DHhydrogenation (kcal/mole) -30.2 -29.8 -30.0 -60.0 -60.4 -56.5 Conjugation worth only ~ 4 kcal/mole syn twist anti Central “single-bond” twist gives a 6 kcal barrier (vs. ~ 60 kcal for C=C twist) 6 Energy kcal/mole 4 2 ps orthogonal at 90° 0 0 45 90 135 Torsional Angle (°) 180 Why is conjugation worth more in allylic intermediates than in dienes? Because we can draw reasonable resonance structures? good bad Conjugation & Aromaticity Theory Simple Hückel MOs http://www.chem.ucalgary.ca/SHMO/index.html e.g. J&F Ch. 12-13 Two Ways to Think about Butadiene p System pp p pp Secondary mixing is minor : 4 Delocalized p s pp To maximize bonding-orbital overlap the central AOs are large in 1 and small in 2. p pp or Localized p/p* picture How different in overall stability? (because of poor E-match) Average same as localized : 4 p-orbitals (~3 kcal/mole max) Very Little Difference! Two Ways to Think about Butadiene p System pp p pp : 4 p-orbitals : 4 Delocalized p pp : : p pp Why ignore this mixing? Despite better E-match, it does not lower energy. Orthogonal (What is gained at two positions is lost at the others) Two Ways to Think about Butadiene p System nearer UV far UV (210 nm) (167 nm) : Although total energies are nearly the same with and without conjugation, there are substantial differences in HOMO & LUMO energies (Reactivity) and in HOMO-LUMO gap (Color). : : : 4 Delocalized p (~3 kcal/mole max) Localized p bond picture How different in overall stability? Very Little! Is There a Limit to 1 Energy for Long Chains? Chain length Normalized AO size Overlap per p bond Number Total of p overlap bonds stabilization (AO product) 2 1/2 1/2 1 1/2 4 1/4 1/4 3 3/4 8 1/8 1/8 7 7/8 1/N 1/N N.B. We are ignoring the smallish influence of overlap on normalization. Also we are using our own “overlap N stabilization” units, which are twice as large as the conventional “” units you Yes, the limit is 1, i.e. twice the will see in texts. N-1 (N-1)/N stabilization of the H2C=CH2 p bond. Similarly, the UMO destabilization limit is twice that of the H2C=CH2 p MO. MO Energy (units of 2) +1 Semicircle Mnemonic for p MO Energy in Conjugated Chains. Radius of circle = 2 stabilization of pH2C=CH2 [ limit of ±(N-1)/N ] Place points denoting length of chain evenly along circumference between upper and lower limit (+1 and -1). 0 -1 p 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 p p 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 p) allylic stabilization (vs. isolated LUMO decreases. in the radical, and of + (-) same 2-electron stabilization Color shiftintoward red. (anion). charge the cation for cation, radical, anion AROMATICITY Cyclic Conjugation worth ~30 kcal ! ~78 observed 49 ~22-24 predicted 26 Conjugation worth ~2 kcal 54 28 Heats of Hydrogenation (kcal/mole) Cf. J&F 13.5a pp. 580-581 Bringing the ends of a conjugated chain together to form a ring gives a lowest p MO with an additional bond. (much more effective than adjusting AO sizes) 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). +1 MO Energy (units of 2) unfavorable favorable 0 unfavorable End to End Interaction -1 favorable On bringing the ends of a chain together, odd-numbered p MOs (1, 3, 5, etc.) decrease in energy (favorable terminal overlap for 0,2,4… nodes), while even-numbered p MOs (2, 4, 6, etc.) increase in energy (unfavorable terminal overlap for 1,3,5… nodes). Thus having an odd number of occupied p MOs (more odd-numbered than even-numbered) insures overall p 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 p 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 p energies from semicircle mnemonic Circle Mnemonic for p 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 Transition State “Aromaticity” Cycloadditions & Electrocyclic Reactions e.g. J&F Sec. 13.6 pp. 582-595 Heteroaromatic Compounds N O H Pyridine H Furan H YY H H N H H Pyrrole Imidazole H H N.B. Single denotes contribution of 1 e to p system (redundant with double bond). (occurs in amino acid histidine) N H H H N H HH Relay for long-range proton transfer by enzymes X X- e.g. J&F Sec. 13.9 pp. 598-601 Furan 4 ABNs 2 ABNs 0 anti-bonding nodes (N.B. must click “Show Orbitals” to update energies after changing structure) SHMo2 (Simple Hückel Molecular Orbital Program) Crude p calculation shows heterocycle analogy. identical shape energy N lower energy N node highonNN density larger on N N lower energy Benzene Pyridine Cyclodecapentaene Naphthalene (SHMo2) same as ethylene same as butadiene same as cyclodecapentaene & butadiene Another Criterion of Aromaticity is the PMR Chemical Shift (coming soon, Chapter 15) Generalized Aromaticity H H H H H OH- H We’ll cover this frame on Friday. I’ve left it in because it is fair game for Monday’s exam. H pKa 15 vs. 16 for H2O H cyclo-C7H8 R H H H 6 p electrons (4n+2) cyclo-C7H7- pKa 39 (despite more resonance structures) e.g. J&F Sec. 13.6 p. 591 8 p electrons (4n, antiaromatic) R H Ph3C+ + R R unusually stable cation (triply benzylic) Same for cyclo-C7H8 (cycloheptatrienyl + + R Ph3CH R even more stable 2 p electrons (4n+2) e.g. J&F Sec. 13.6pp. 587, 592 cyclo-C7H7+ 6 p electrons (4n+2) “tropylium”) End of Lecture 55 February 23, 2011 Copyright © J. M. McBride 2011. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0). Use of this content constitutes your acceptance of the noted license and the terms and conditions of use. Materials from Wikimedia Commons are denoted by the symbol . Third party materials may be subject to additional intellectual property notices, information, or restrictions. The following attribution may be used when reusing material that is not identified as third-party content: J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0