Department CMIC Lecture 6 FR6 Free-Radicals: Chemistry and Biology
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1 Department CMIC Lecture 6 FR6 Free-Radicals: Chemistry and Biology Prof. Dipartimento CMIC Giulio Natta
2 Content 1. Introduction Current Status of Radicals Chemistry What is a Radical Free Radicals and Life 2. istorical Aspects 3. Electronic Structure and Bonding 4. Active Oxygen Specie, O 2, O 2, O 2, 1 O 2, 2 O 2, O Chemistry 2 O 2 and peroxides 5. Radical Reactions Atom transfer Addition to multiple bonds omolytic Aromatic Substitution Electron Transfer (oxidation-reduction) 6. Thermodynamics 7. Free Radical Kinetics First-order Reaction Second-order Reaction Steady-State Chain-reactions Redox chain reactions Inhibition 8. Radiation Chemistry Tools Specie: e (aq),, O, 2 O 2, 2, O 2 Pulse Radiolysis/Flash Photolysis 9. Lipid Peroxidation Chemistry Measurement Effects 10. Antioxidants Preventive Chain-breaking Small molecule (Vit. C/E, CoQ, Urate). Enzymes Chelates 11. Metals and Free Radical Chemistry Reactions Chelates 12. DNA and Protein (As radical targets) 13. Photo reactions Photochemistry Photosensitization 14. Detection of Radicals TBARS Fluorescence Cyt. C /NBT Strategies 1. SOD, CAT 15. EPR Detection of Radicals Direct Detection Spin Trapping Transition metal 16. Nitric Oxide/NOS 17. Oxygen radicals/ros
3 Thermodynamics Prof. Dipartimento CMIC Giulio Natta
4 Thermodynamic Values Useful in Free Radicals Chemistry a) Ionization Potential (IP) R R + + e b) Electron Affinity (AE) R + e - R c) Redox Potential (E ox ) R (sol) + e a R (sol) (E red ) R (sol) a R + (sol) + e d) Bond Energy (BDE) R-X R + X (+ q) e) Formation Enthalpy ( f ) Elements R (+ q) f) Radical Stabilization ( r ) ( f (R ) - f (R rif)) (BDE) in homolog series g) Acid strength (pk a ) R a R R + a R + + h) Coordination Constants (pk c ) R + A + a R-A + R + B - a R-B -
5 Ionization Potential of Some Radicals Radicals centered to electronegative elements show high IP Radicals alfa-substituted with electron-withdrawing groups show high IP Radicals substituted by electron-donor groups show low IP (easily oxidized species, involved in redox reactions). 25 N I.P. (ev) NCC 2 C 3 Cl CF 3 O Br OO S (C 3 ) 2 C PhC 2 PhO CCl 3 (C 3 ) 3 C OC 2 MeS Me 2 NC 2 0
6 Electron Affinity of Some Radicals Radicals centered to electronegative elements show high AE Carbon centered radicals alfa-substituted with electronwithdrawing groups show high AE Oxygen centered radicals show an AE in the order: phenoxy > alkoxy > peroxy. 4,5 4 Cl A.E. (ev) 3,5 3 2,5 2 F (NC) 2 C NCC 2 Br I PhO RO 1,5 1 C 3 MeC 2 MeSC 2 ROO 0,5 MeOC 2 0
7 Bond Dissociation Energy of Selected Molecules (kcal mol -1 ) Specie BDE* Specie BDE* Specie BDE* O Me--Me 88 Me C 3 CO Et--Me 85 Et-- 98 C 3 OO Pr--Me 84 n Pr-- 96 O 2 NO Bu t --Me 80 i Pr-- 95 O Bu t --Bu t 68 t Bu-- 91 PhO-- 85 PhC 2 --Me 72 PhC ONO-- 79 C 2 =CC 2 --Me 72 C 2 =CC 2-85 ClO-- 78 MeCO--Me 82 Ph 2 C-- 75 F MeCO--COMe 70 C Cl OC 2 --Me 83 C=C Br-- 88 C CPh 3 15 Ph I-- 71 Ph--Ph 100 C 2 =C N Ph--Me 93 C 3 CO-- 88 S-- 90 Me--F 106 OCO-- 90 Cl--Cl 58 Me--Cl 84 OC Br--Br 46 Me--I 56 F 3 C I--I 36 Me--O 92 Cl 3 C-- 96 F--F 38 Me--N 2 79 MeS SMe 73
8 Bond Dissociation Energies (kcal mol -1 ) sp 3 Carbons CF C C 3 C (C 3 ) 2 C (C 3 ) 3 C C 3 OC N CC C 3 COC C 6 5 C C 2 =CC (C 2 =C) 2 C sp 2 Carbons C 2 =C C sp Carbons C C π Bonds C C C=C C=O C N C=N C-X Bonds C-CI 78.9 C-Br 65.7 C-l 52.6 alogens F CI Br I O- Bonds O C 3 O OO C 6 5 O (t-bu) 2 NO S- Bonds C 3 S M- Bonds (C 3 ) 3 Si (C) 3 Sn Perkins, J.M., Radical Chemistry: The Fundamentals, Oxford University Press, Inc., New York, March, J.; Smith, M.B., March's Advanced Organic Chemistry, John Wiley & Sons, Inc., New York 2001.
9 Alkane f values (kcal mol -1 ) Carbon number n Branching Octanes: Octane ,2-dimethylhexane methylheptane ,3-dimethylhexane methylheptane ethyl-3-methylpentane methylheptane ethylhexane ,3,4-trimethylpentane ,5-dimethylhexane ,2,3-trimethylpentane ,4-dimethylhexane ,2,4-trimethylpentane ,3-dimethylhexane ,3,3-trimethylpentane ,4-dimethylhexane ethyl-2-methylpentane ,2,3,3-tetramethylbutane
10 Bond Dissociation Energies (BDEs) ,0 26,7 25,5 9,3 45,5 35, ,5-54,4-50,9-45,1-32,8 27,1 33, ,0-65,3-22,1-8,7 6,3-57,8-11,0-17, ,0-65,3-22,1-8,7 6,3-57,8-11,0-17,9 C ,9-56,0-20,0-8,2 3,4-48,0-5,5-20, ,9-56,0-20,0-8,2 3,4-48,0-5,5-20,0 C 3 C ,0-26,8-15,2-1,7-56,2-13,0-25, ,0-26,8-15,2-1,7-56,2-13,0-25,0 (C 3 ) 2 C ,0-70,1-34,7-22,9-9,5-65,2-20,0-32, ,0-70,1-34,7-22,9-9,5-65,2-20,0-32,1 (C 3 ) 3 C ,1-43,0-31,6-17,2-74,7-28,8-40, ,1-43,0-31,6-17,2-74,7-28,8-40,1 2 C=CC ,9-1,3 11,4 23,8-29,6-0, ,9-1,3 11,4 23,8-29,6-0,2 PhC ,0-30,2 4,5 20,0 30,4-22,6 21,0 7, ,0-30,2 4,5 20,0 30,4-22,6 21,0 7,1 Value in bold: group contribution; kj mol -1
11 eats of Formation of Some Radicals F Cl Br I O N 2 C 3 52,1 19,0 29,0 26,7 25,5 9,3 45,5 35,1 34,7-59,5-54,4-50,9-45,1-32,8 27,1 33,3 52,1 104,2 136,4 103,2 87,5 71,3 119,2 108,6 105,1 365,7 400,4 371,5 333,4 323,5 314,3 390,7 403,8 416,9 C 3 35,1 105,1 110,1 84,1 70,0 57,2 92,4 86,1 90,2 262,0 314,6 258,5 227,6 219,3 213,5 277,2 294,6 315,3 C 3 C 2 28,4 100,5 84,2 70,3 55,6 93,9 86,9 88,5 215,6 270,3 188,0 179,9 172,2 239,0 255,7 273,9 (C 3 ) 2 C 22,0 99,1 111,1 85,7 71,6 57,0 96,5 87,5 89,2 192,0 251,2 202,1 171,8 163,5 155,9 223,9 238,6 256,9 (C 3 ) 3 C 11,0 95,2 83,0 69,3 53,7 95,0 85,3 86,2 165,5 232,3 154,1 146,2 137,6 207,4 221,4 238,9 2 C=CC 2 40,9 88,1 71,2 56,2 42,6 79,8 76,2 229,5 259,3 176,4 167,2 160,6 226,3 263,0 PhC 2 49,5 89,6 61,2 74,0 56,2 44,6 81,4 74,0 77,5 216,5 239,2-17,3 157,6 145,6 141,0 206,3 222,6 242,7 Value in bold: group contribution; kj mol -1
12 Challenge: C 4 C 3 O (314.6) C 98.1 (254.3) C 92.4 (277.2) O (382) 90.2 (259.4) C O 2 f = kcal mol -1 le = ev (290.8 kcal mol -1 ) PA = kcal mol -1 2 f = kcal mol -1 le = ev (250.0 kcal mol -1 ) PA = kcal mol -1 2 f = kcal mol -1 le = ev (250.9 kcal mol -1 ) PA = kcal mol -1 Reagents? Radicals Acids Bases (66.6) 117 (180) C O 2 f = kcal mol -1
13 Radical Stabilization ( r, Kcal mol -1 ) Best evaluated comparing Bond Dissociation Energies in homologous series. X O I C 3 C 2 5 i C 3 7 t C 4 9 C 6 5 C 3 -X Benzyl-X Allyl-X (Me/Benzyl) (Me/Allyl) X C 3 C 2 O COOR CN COR Ph Vinyl (Ph) 2 (Ph) 3 OR SR r Resonance saturation effect
14 Correlations between Thermodynamic Quantities Acidity, Bond Energies and Redox Potentials E red(r) E ox(r) R ˉ R R + pk a (R) pk a (R.+ ) + + BDE - e - - e - R - R R+ E red(r ) E ox(r ) The following typical inequalities hold: a) E ox/red (R) E ox/red (R ) b) BDE (R) > BDE (R.+ o R.- ) c) pk a (R + ) >> pk a (R)
15 Correlations between Thermodynamic Quantities Elect. group loss, Bond Energies and Redox Potentials E red(rx) E ox(rx) RX - RX RX + X X - X X - X BDE pk(rx.- ) pk(rx) - e - - e - R - R R + E red(r ) E ox(r ) The following typical inequalities hold: a) E ox/red (RX) E ox/red (R ) b) BDE (RX) > BDE (RX + o RX - ) c) pk(rx - ) >> pk(rx)
16 BDE in Uncharged Radicals, Cation Radicals and Anion Radicals Uncharged Radicals Radical BDE kcal mol -1 C 2 -Me 96 Cation radicals R-. + σ o π R + + R + + (cation radical acidity) C 2 C 2-39 C 2 C 2 -Me 26 R+ PhC 3 Ph 2 C 2 Ph(C 3 ) 2 Me 3 N Et 3 N OC 2 -Me 12 OCMe 2 -Me 7 O=C -Me 11 (A) (B) C 3 C Anion radicals R-X. - σ o π R - + X R + X (anion radicals basicity) 3 C N C 3 C 3 3 C N X C C 3 + X RX C 3 -Cl CCl 4 C 3 I C 6 5 Cl Et 3 N (A) (B)
17 Acid-Base Properties of Radicals Uncharged Radicals Cation Radicals R-X- + S RX - + S + + R-X- + S RX + S + Radical pk a Molecule pk a C(O-) OC C 2 C 2 O C 3 C 2 O- 16 C(O-)CN ~ 4 -OC 2 CN ~ 9 C 2 COO- 4.3 C 3 COO- 4.5 C(N + 3 )COO 6.6 C 2 (N + 3 )COO 2.3 CR 2 -NCO CR 2 -NCO PhC 3 + PhO + N PhC PhO N 3 (N 4 + ) 18 (9.2)
18 Redox Properties of Radicals (vs. S.C.E.) Radical E 1/2 ox (V) E 1/2 red (V) Radical E 1/2 red (V) - C(COO) SO C 2 Ph O 2.63 CPh CPh C 2 O C(C 3 )O C(C 3 ) 2 O C 2 -N(C 3 ) CPhN(Bz) CN 2.56 Br 2.20 SCN 1.42 N I 1.09 NO (a) Wayner, D. D. M.; McPhee, D. J.; Griller, D. J. Am. Chem. Soc. 1988, 110, 132. (b) Sim, B. A.; Milne, P..; Griller, D.; Wayner, D. D. M. J. Am. Chem. Soc. 1990, 112, 6635.
19 Captodative Effect The synergic stabilizing effect on a radical center due to the simultaneous presence on the center of both electron-withdrawing and electron-donating groups. The phenomenon is explained by a succession of orbital interactions; the acceptor stabilizes the unpaired electron, which for this reason interacts more strongly with the donor than in the absence of the acceptor. BDE (R-) C(CO)2 99 C(NO2)2 99 R C 2 C X Y RC 2 X - Electron Withdrawing Group Y - Electron Donating Group C X Y C(t-Bu)2 98 LUMO C(OC3)2 91 CC3(OC3) 91 C(N2)CO 73 OMO SOMO Radical Stabilised C(N2)CO2 76
20 The Pecking Order of Redox Potentials Note that the donor antioxidants are found in the middle of the pecking order. Buettner GR. (1993) The pecking order of free radicals and antioxidants: Lipid peroxidation, α-tocopherol, and ascorbate. Arch Biochem Biophys. 300: [PDF] Redox Couple E '/mv (one-electron reductions) O, + / 2 O RO, + /RO (aliphatic alkoxyl radical) ROO, + /ROO (alkyl peroxyl radical) GS /GS (glutathione) PUFA, + /PUFA- (bis-allylic-) TO, + /TO (tocopherol) O 2, + / 2 O, O Asc, + /Asc - (Ascorbate) CoQ -, 2 + /CoQ Fe(III) EDTA/Fe(II) EDTA CoQ/CoQ O 2 /O Paraquat/Paraquat Fe(III)DFO/Fe(II)DFO RSSR/RSSR (GS) O/e aq
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