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Chem. 343/345 Reich/Nelsen Oxidation State of Organic Molecules The most reduced form of carbon is CH4, the most oxidized is CO2. Thus the oxidation state of a one-carbon fragment is unambiguous and defined by the number of C-H bonds that have been replaced by C-X bonds, where X = any electronegative element (see periodic table on previous page). Replacing C-H bonds by C-Metal bonds is not a redox process. A C=O double bond is equivalent to two C-O single bonds (C(OH)2). Ox M = H, Li, MgBr, BH2 R M R X Red X = F, Cl, Br, I, OH, NH2 Reduction - Each step involves C-X → C-H Each step involves C-H → C-X, Oxidation Sum: 012 34 Reduction CH4 CH3-Cl CH2Cl2 CHCl3 CCl4 Oxidation CH3-MgBr CH3-OH CH2=O H-C(=O)OH CO2 CH3-NH2 CH2=NH H-C(=O)NH2 O=C=NH CH2(OH)2 H-C≡N O=CCl2 + C ≡O When there are C-C bonds involved, the situation is more complicated. Now, in addition to replacement of C-H by C-X, oxidations can also involve the removal of H2 by introduction of C-C and C-X double and triple bonds. To go left and right in the scheme below requires reduction and oxidation, respectively. Compounds in the same column are at the same oxidation state, and can be interconverted by nucleophilic substitutions (C-X by C-Y), electrophilic substitutions (C-H by C-M), additions or eliminations (of HX or MX fragments) and rearrangements. Reduction - Each step involves C-X → C-H or addition of H2 Each step involves C-H → C-X or loss of H2, Oxidation Sum: 0123456 CH3-CH3 CH2=CH2 HC≡CH HC≡COH HOC≡COH CH3-CH2-LiCH3-CH2Cl CH3-CHCl2 CH3-CCl3 ClCH2-CCl3 Cl2CH-CCl3 Cl3C-CCl3 O O O O O O O O CH3-CH2-BH2 CH3-CH2-OH CH3-C-H HOCH2-C-H H-C-C-H H-C-C-OH HO-C-C-OH OH O O O O O CH -C-OH HOCH -C-OH H-C-CCl Cl-C-C-Cl H2C=C 3 2 3 H O O O HOCH2-CH2OH CH2=C=OH2NCH2-C-OH Cl2CH-C-OH HO-C-C≡N O CH3-CH2-NH2 CH3-CH=NH CH3-C≡NNHO-CH2-C≡N H-C-C≡N ≡C-C≡N The oxidation level of any carbon fragment can then be defined as the sum of the number of C-X bonds and number of π bonds to carbon (C=C, C≡C, C=O, C=N, C≡N). Two molecules with the same number of carbons and the same sum are at the same oxidation state, and can, in principle, be interconverted without any redox chemistry, or by an equal number of reductions and oxidations. Redox Reagents M-X species are NOT redox reagents M = H, Li, MgBr, BH2 i X = F, Cl, Br, I, OH, NH2 H Br, H OH, H NH2, Na Br, Li N Pr, R3Si-Cl Addition or elimination of an M X species is not a redox reaction Conversion of M to X is Oxidation X X' Species are typically oxidizing agents Conversion of X to M is Reduction + - Ox Br Br, HO OH, R3N O, RS Cl, R2N Cl, RS SR, etc R M R X Red Addition of X X' to a double bond is an oxidation Elimination of X X' to form a double bond is a reduction Transition metals are more complicated - each Ox X X' one has a favorite redox resting place. For all + X X' metals the lower oxidation states are reducing Red agents. Some elements (among them Cr, Ru, Mn, Os) have accessible higher oxidation states M M' Species are typically reducing agents which behave as oxidizing agents. H H, R2B H, R3Sn H, (Li Li)n, R2B BR2, etc Cr2+ Cr3+ Cr6+ Addition of M M' to a double bond is a reduction Reductant Oxidant 2+ Elimination of M M' to form a double bond is an oxidation Pd° Pd Reductant Oxidant M M' Ox + M M' Red Reagents by Redox and Acid/Base Classes Oxidizing Agents: Free Radicals: Cl., Br., I. (Cl. and Br. both add and abstract H) Compounds with O-O bonds: PhC(=O)OOC(=O)Ph, tBuOOtBu (free radical initiators) O3, RCO3H, H2O2, O2 Halogen molecules and related species: Cl2, Br2, I2, Cl-OH, Br-OH (add to π bonds) Metals in high valence states: KMnO4, CrO3 (and other Cr(VI) reagents), OsO4, NalO4, MnO2, Fe(III), Cu(II), Ag(II) Reducing Agents: Hydrogenation: H2/catalyst (transition metals, esp. Pd, Pt, Ni) Metals: K°, Na°, Li°, Mg°, Zn° Hydride Donors: + t + BH3, LiAlH4, NaBH4, Li HAl(O Bu)3, Na BH3CN Inorganic Reducing Agents: Bisulfite (NaHSO3), Hypophosphorous acid (H3PO2), Thiosulfate (Na2S2O3) Nucleophiles/Bases: Anionic - Carbon Centered: RLi, RMgX, R2CuLi, N≡C: - + - + - Nitrogen Centered: iPr2N Li , H2N M , N3 - - - Oxygen Centered: RO , HO ,CH3C(=O)O Sulfur Centered: RS-, N≡C-S- Halogens: Cl-, Br-, I- Neutral R3N, H3N, ROH, H2O Acids: Mineral Acids: HCIO4, H2SO4, HNO3, HCl, HBr, Hl, H3PO4 Strong Organic Acids: RSO3H, CF3CO2H Carboxylic Acids: RCO2H Weaker Organic Acids: H-C≡N, ROH Basicity, nucleophilicity, and reducing power do tend to go together; electrons are given up in all three cases. Good nucleophiles like I- and RS- can give up one electron to an easily reduced compound as well as act as nucleophiles (making Y-C bonds). In the same vein, acidity and oxidizing power are related. Some strong acids like HCIO 4, HNO3, and H2SO4 can oxidize compounds as well as protonate them..
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