1. 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

2. Is There a Limit to  1 Energy for Long Chains? 8 1/  8 1/8 77/8 4 1/  4 1/4 33/4 Chain length 2 Normalized AO size 1/  2 Overlap per  bond (AO product) 1/2 Number of  bonds 1 Total overlap stabilization 1/2 N 1/  N 1/N N-1(N-1)/N Yes, the limit is 1, i.e. twice the stabilization of the H 2 C=CH 2  bond. Similarly, the LUMO destabilization limit is twice that of the H 2 C=CH 2   MO.. N.B. Here we are using our own “overlap stabilization” units, which are twice as large as conventional “  ” units.

3. N=2 ethyleneN=3 allyl N=4 1,3-butadiene : . p :  N=1 an isolated 2p AO :. +1 0 Semicircle Mnemonic for  MO Energy in Conjugated Chains. Radius of circle = 2  stabilization of  H 2 C=CH 2 Place points denoting length of chain evenly along circumference between upper and lower limit (+1 and -1).......... : : :. etc.... All odd chains have a non- bonding MO with nodes on alternant carbons. It is the locus of the “odd” electron in the radical, and of + (-) charge in the cation (anion). As the conjugated chain lengthens, more and more levels are crowded between -1 and +1, and the HOMO- LUMO gap decreases. Color shift toward red. MO Energy (units of 2  ) allylic stabilization same 2 electron stabilization for cation, radical, anion (vs. isolated p and  ) (difference is resonance stabilization of butadiene vs. 2 isolated ethylenes) [ limit of ±(N-1)/N ]

4. Allylic Intermediates: Allylic Free-Radical Bromination Sec. 11.8 pp. 497-500, Sec. 12.11c p. 543

5. NBS

6. Allylic Intermediates: Addition of HX to Butadiene Sec. 12.9-12.10 pp. 534-541

7. +17.6 -21.4 HOMO-4HOMOLUMO+1 LUMO HOMO-1LUMO+1LUMOHOMO Butadiene Propenyl Cation H+H+ hyperconjugated  C-H best overlap best potential best product best overlap

8. Propenyl Cation +152 +144 +132 +99 Surface Potential best potential best  potential

9. symmetrical (but for D) p. 1288 3.1 : 1 -78° 1.6 : 1 25° Cl - D Cl rapid ion-pair collapse competes with motion

10. D CD 2 CD 3 CD 3 H CH 2 If Step 1 (motion) is rate-limiting, H- and D-transfer products should form in equal amounts. (because their motions should be equally fast) If Step 2 (atom shift) is rate-limiting, more H-transfer product should form. k H /k D > 1 (kinetic “isotope effect”) In a Very Viscous Solvent Can Short-Range Motion Constitute a Rate- (and Product-) Determining Step? CH 3 CH 3 H 3 C CD 3 CD 3 CD 3 CH 3 CH 3 H 3 C (2) Shift D atom exothermic/easy/fast N N (2) Shift H atom exothermic/easy/faster CD 3 CD 3 CD 3 CH 3 H 3 C N N N N

11. Kinetic vs. Thermodynamic Control Sec. 12.10 pp. 537-540

12. Allylic Intermediates, Transition States: S N 1 and S N 2 Sec. 12.11a,b pp. 541-543

13. Allylic Intermediates: RH Acidity Sec. 12.11d pp. 543-544 Cf. Benzylic Intermediates (sec. 13.12) e.g. Ph-CH 2 -H pK a = 41

14. predicted observed ! Conjugation worth ~30 kcal ! AROMATICITY Ch. 13-14 Cf. 13.5a pp. 580-581

15. Bringing the ends of a conjugated chain together to form a ring gives a lowest  MO with one additional bonding interaction. In a conjugated ring peripheral nodes must come in even numbers. e.g. cyclopropenyl 0 nodes2 nodes Lowest MO will have energy = -N/N = -1 2 nodes E = -1E = +1/2

16. Energy Shifts on “Ring Formation” +1 0.... : : MO Energy (units of 2  ) : : End to End Interaction favorable unfavorable Shifts Alternate (because of node parity).

17. Hückel’s Rule: 4n+2  electrons is unusually favorable in a conjugated ring. 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 (where n in an integer)

18. : : : :.. There is always an MO at -1. Circle Mnemonic for  MO Energy in Conjugated Rings. Inscribe regular polygon with point down. +1 0 MO Energy (units of 2  ) Same radius as for open chain Read MO energies on vertical scale. : 4 cyclobutadiene 6 benzene 3 cyclopropenyl........... reactive SOMOs ! Cation strongly stabilized (vs. allyl + ) : : : 4n “Antiaromatic”! slightly destabilized (vs. butadiene) Stabilized (vs. hexatriene) : : : Radical less stabilized (vs. allyl).. Anion de stabilized - open-chain  energies from semicircle mnemonic

19. Generalization of Aromaticity: 4n+2 Stability Sec. 13.6 pp. 582-595 Transition State “Aromaticity”

20. : H X-X- Y : H. H : N Pyridine HH H H O Furan H HH H H N Pyrrole H H H N N Imidazole Heteroaromatic Compounds (Sec. 13.9 pp. 598-601) : : : :. Y-Y- H X :. Relay for long-range proton transfer by enzymes N.B. Single. denotes contribution of 1 e to  system (redundant with double bond). (occurs in amino acid histidine)

21. End of Lecture 54 Feb. 22, 2010 Copyright © J. M. McBride 2010. 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).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