Chemistry 125: Lecture 54 February 21, 2011 Acetylenes Allylic Intermediates & Dienes Linear and Cyclic Conjugation (4n+2) Aromaticity This For copyright notice see final page of this file Generalization to Acetylenes Addition of HBr Stepwise / Markovnikov Addition of H2O “Keto-Enol Tautomerism” Regioselection Addition of H2 Stepwise / Stereoselection Acidity and base-catalyzed isomerization e.g. J&F Sec. 10.6-10.11 pp. 444-455 Stepwise Addition of HBr to Alkyne FeBr3 1-Hexyne + HBr 15°C with “inhibitor” to trap radicals 2-Bromo-1-hexene isolated in 40% yield 100 to 1000x slower than comparable ionic addition to alkene, because vinyl cation is not so great. Gas Phase Ionization CH3-CH2-Cl CH2=CH-Cl 193 kcal/mole CH3-CH2+ + Cl- 225 kcal/mole CH2=CH+ + Cl- Stepwise Addition of HBr to Alkyne FeBr3 1-Hexyne + HBr 15°C with “inhibitor” to trap radicals 2-Bromo-1-hexene isolated in 40% yield 2,2-Dibromohexane HBr can add again to the bromoalkene (obviously more slowly) to give a second Markovnikov addition If the bromo substituent slows addition to an alkene, why is there Markovnikov orientation? Br is a “schizophrenic” substituent: both electron withdrawing (), and electron-donating (). Hydration and Hydrogenation of Alkynes HC CR Hg(OAc)2 H+ / H22O O HgOAc + HC CR -H H+ “Keto-Enol Tautomerism” HgOAc R H H C C+ + H O H Markovnikov Enol NaBH4 HKetoneR C C an easy allylic rearrangement H O H (favors ketone Cf. Lecture 37) Ketone "Enol" ve Bond Energies O O C H C C H H H H H H H H C C C H H H C=O 179 C-O 86 Bonds that change C-C 83 C=C 146 (the others should cancel in taking the difference) C-H 99 O-H 111 sum 361 sum 343 Can one sum bond energies to get -(3/4) 18 -13.5 Kcalc = 10 = 10 accurate"Heats of Atomization"? Kobs = 10-7 = 10-(3/4) 9.3 Ketone Why is Enol 9 kcal/mole "Too" Stable? C(sp2)-H stronger than C(sp3)-H (they shouldn’t actually cancel) "Enol" •• O O C H C C H H H H H H H H C C C H H H C=O 179 C-O 86 C-C 83 C=C 146 +O H "Resonance C-H 99 O-H 111 Stabilization” sum 361 sum C 343 from H H C 10-13.5C Intramolecular Kcalc = 10-(3/4) 18 = HOMO-LUMO -7 H -(3/4) Kobs = 10 = 10 H 9.3 H Mixing HC CR HgOAc Hg(OAc)2 + HC H + / H 2O CR -H+ H2O R H H C C+ H O Markovnikov Enol Hydration with Either Regiospecificity HC CR R’2B R’2B-H H e.g. “disiamylborane” BH3 + 2 (what is R’?) H C C R vinylborane Ketone HO HOOH HO- C H H C H R Anti-Markovnikov Enol (hindered R’2BH adds only once) Aldehyde R-C H2 C-R C Lindlar Catalyst ( Pd / CaCO3 / Pb ) deactivate Pd to stop at alkene Hydrogenation with Either Stereospecificity n-Pr-C C-n-Pr Na / NH3 H H C R R syn addition 97% for R = (CH2)3CO2CH3 n-Pr H C C H n-Pr anti addition 80-90% “dissolving metal reduction” Na NH3 e-(NH3)n + Na+ solvated electron First e R-C C-R NH2 First H+ H e- R-C C-R NH2 H R-C C-R R H C C R C C R Vinyl radicals are sp2 but they invert easily R Second Vinyl anions are sp2 and invert very slowly H R C C (remember XH3) R Second e- H+ H H R R H anti addition H NH2 R H C C C C R e NH2 (because of radical isomerism) C C R Vinyl radicals are sp2 but they invert easily R Alkyne Acidity and Isomerization e.g. J&F Sec. 12.4 pp. 516-518 Approximate “pKa” Values 50 CH3-CH2CH2CH2H ~ 52 pKa * 40 CH3-C C-CH2H CH3-CH=C=CHH (best E-match C-H) sp2 C_ (no overlap) : CH3-CH2CH=CHH ~ 44 _ sp3 C _ (allylic) C HOMO - overlap (better E-match N-H) ~ 34 H2NH ~ 38 30 CH3-CH2C CH : ~ 25 sp C_ (no overlap) 20 = 16 HOH (bad E-match O-H) 10 (e.g. J&F Acidity of 1-Alkynes Secs. 3.14 p. 129; 12.4 p. 516-518) CH3-CH=C=CH CH3-C C-CH2 50 H+(aq) + k 1013 10-38 /sec kcal/mol 40 30 20 t1/2 = 0.69/k 1025 sec = 1017 yrs 104 time since Big Bang Equilibrium & Rate CH3-CH2C C pKa 38 pKa 25 Ka 10-38 Ka 10-25 G 4/3 25 = 33 G 4/3 38 = 51 10 4.8 4.1 0 [0] CH3-C C-CH3 -10 CH3-CH=C=CH2 CH3-CH2C CH 0.1% 0.03% at equilibrium CH3-C C-CH2 50 H+(aq) + 40 + HOfavors dissn. by 21 kcal CH3-CH=C=CH Equilibrium & Rate CH3-CH2C C kcal/mol (4/3 16) 30 + H N 2 20 t1/2 30 yrs @ 300K favors dissn. by 45 kcal 2 min @ 150°C (4/3 34) 10 0 CH3-CH=C=CH2 CH3-C C-CH3 -10 0.0001% at equilibrium CH3-CH2C CH -7.2 Trick to obtain terminal acetylene: _ Equilibrate with RNH base (in RNH2 solvent at room temp) to form terminal anion. “Quench” by adding water which + donates H to terminal anion and _ _ to RNH , leaving OH , which is too weak to allow equilibration. Or add H +, _ so even [OH ] is very low. Conjugation & Aromaticity Conjugated Pi Systems C C C O Yoke Jungere Jugóm (to Join) e.g. J&F Ch. 12-13 The Localized Orbital Picture (Pairwise MOs and Isolated AOs) Is Our Intermediate between H-like AOs and Computer MOs When must we think more deeply? When does conjugation make a difference? Experimental Evidence Allylic Stabilization: Cation Anion R-Cl R+ + Cl(gas phase Cl Radical Bond Dissociation Energy (kcal/mol) pKa kcal/mol) 193 OH 16 H 101 H 89 4/3 6 = 8 kcal Cl Cl 172 171 as good as secondary OH O OH 10 5 Conjugation worth ~ 13 kcal ! Allylic Cation Intermediates: Addition of HX to Butadiene Reason for Kinetic Distribution? H + Br + HBr -78°C FeBr Br- 3 H Kinetic vs. Thermodynamic Control -78°C H 80% 15% Br + H H 20% 85% e.g. J&F 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 e.g. J&F Sec. 12.11a,b pp. 541-543 Allylic Transition States: SN1 krel for solvolysis in 1:1 EtOH/H2O at 45°C Cl Cl << 0.01 Cl [100] 0.01 Cl Cl Cl 0.05 43 6300 + Cl 0.07 + Cl 39 methylation is effective where charge is (C1,C3) e.g. J&F Sec. 12.11a,b pp. 541-543 Allylic Transition States: SN2 krel for Displacement by EtO- in EtOH at 45°C Cl [1] Cl 1.9 Cl Cl 37 97 560 Cl 33 Cl Allylic Anion Intermediates: RH Acidity H CH2 pKa ~52 4/3 x 9 ≈ 11 kcal/mole H allylic CH2 pKa 43 4/3 x 12 ≈ 16 kcal/mole benzylic H pKa 41 CH2 e.g. J&F Sec. 12.11d pp. 543-544 and Sec. 13.12 Allylic Free-Radical Intermediates: Allylic Bromination O Et2O 30 min. hn N Br O N-Bromosuccinimide (NBS) 58% yield K. Ziegler (1942) Cf. J&F Sec. 11.8 pp. 497-500, Sec. 12.11c p. 543 Ionic Preparation and Destruction of NBS O O N H NaOH N- O Br2 N Br + NaBr 0°C O O O pKa 9.5 undo with HBr “enol” to “ketone” OH N O + Br2 OH + N Br O + OH N Br O Br-Br Allylic Reactivity - Radical Addition 2 Rate [Br2]2 + Br2 Substitution H H Br+ CH2Cl2 25°C Dark Br Br + Br-Br-BrBr2 helps Br- leave from “Br+” in nonpolar solvent (like protonation of OH) initiator (hn, peroxide, etc.) Br• slow (selective) O Cl• also attacks this CH2 groupN Br H + HBr Br Br2 + Br H O Br N H or How to control O O Addn. vs. Subst.? Whenever a Br2 molecule is consumed, Keep dark one new Br2 molecule is created. or minimize [Br2] (tedious to impossible?) Automatically maintains minimal [Br2]. H Diene Stabilization: Hcombustion 768.9 ±0.3 761.6 ±0.2 Conjugation worth ~7 kcal Hformation 17.7 25.4 Conjugation worth ~8 kcal Hhydrogenation (kcal/mole) -30.2 -29.8 -30.0 -60.0 -60.4 -56.5 Conjugation worth ~ 4 kcal/mole End of Lecture 54 February 21, 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