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Alcohols from Carbonyl Compounds Oxidation-Reduction & Organometallic Compounds

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Alcohols from Carbonyl Compounds Oxidation-Reduction & Organometallic Compounds Chapter 12 Alcohols from Carbonyl Compounds Oxidation-Reduction & Organometallic Compounds Ch. 12 - 1 1. Structure of the Carbonyl Group O Carbonyl compounds O O R H R R' Aldehyde Ketone O O O R' R OH R OR' R N Carboxylic acid Ester Amide R" Ch. 12 - 2 Structure o ~ 120 O ~ 120o C ~ 120o ● Carbonyl carbon: sp2 hybridized ● Planar structure Ch. 12 - 3 Polarization and resonance structure δ− O O + Cδ C Ch. 12 - 4 1A. Reactions of Carbonyl Compounds with Nucleophiles One of the most important reactions of carbonyl compounds is nucleophilic addition to the carbonyl group − Oδ O + nucleophilic Nu C δ C addition Nu Ch. 12 - 5 Two important nucleophiles: ● Hydride ions (from NaBH4 and LiAlH4) ● Carbanions (from RLi and RMgX) Another important reactions: O OH oxidation C R R H H reduction H 1o alcohol aldehyde Ch. 12 - 6 2. Oxidation-Reduction Reactions in Organic Chemistry Reduction of an organic molecule usually corresponds to increasing its hydrogen content or decreasing its oxygen content oxygen content hydrogen content decreases decreases O [H] O O [H] OH R OH reduction R H R H reduction R H H carboxylic aldehyde acid Ch. 12 - 7 The opposite reaction of reduction is oxidation. Increasing the oxygen content of on organic molecule or decreasing its hydrogen content is oxidation [O] OH [O] O [O] O RCH3 [H] R H [H] R H [H] R OH H lowest highest oxidation oxidation state state Ch. 12 - 8 Oxidation of an organic compound may be more broadly defined as a reaction that increases its content of any element more electronegative than carbon [O] [O] [O] Ar CH3 Ar CH2Cl Ar CHCl2 Ar CCl3 [H] [H] [H] Ch. 12 - 9 2A. Oxidation States in Organic Chemistry Rules ● For each C–H (or C–M) bond -1 ● For each C–C bond 0 ● For each C–Z bond +1 (where M = electropositive element and is equivalent to H, e.g. Li, K, etc.; Z = electronegative heteroatom, e.g. OR, SR, PR2, halogen, etc.) Calculate the oxidation state of each carbon based on the number of bonds it is forming to atoms more (or less) electronegative than carbon Ch. 12 - 10 Examples H Bonds to C: (1) H C H 4 to H = (- 1) x 4 = - 4 H Total = - 4 Oxidation state of C = - 4 Ch. 12 - 11 Examples H Bonds to C: (2) H C OH 3 to H = - 3 H 1 to O = +1 Total = - 2 Oxidation state of C = - 2 Ch. 12 - 12 Examples O Bonds to C: (3) C 2 to H = - 2 H H 2 to O = +2 Total = 0 Oxidation state of C = 0 Ch. 12 - 13 Examples O Bonds to C: (4) C 1 to H = - 1 H OH 3 to O = +3 Total = +2 Oxidation state of C = +2 Ch. 12 - 14 Overall order H H O O O H C H < H C OH < C < C < C H H H OH H H O oxidation - 4 - 2 0 +2 +4 state lowest highest oxidation oxidation state of state of carbon carbon Ch. 12 - 15 3. Alcohols by Reduction of Carbonyl Compounds H [H] R O OH [H] H H (1o alcohol) R O R OH OR' [H] R O O [H] HO H R R' R R' Ch. 12 - 16 3A. Lithium Aluminum Hydride LiAlH4 (LAH) ● Not only nucleophilic, but also very basic ● React violently with H2O or acidic protons (e.g. ROH) ● Usually reactions run in ethereal solvents (e.g. Et2O, THF) ● Reduces all carbonyl groups Ch. 12 - 17 Examples O OH 1. LiAlH4, Et2O (1) R OH 2. H+, H O R H 2 H O OH 1. LiAlH4, Et2O (2) + HOR' R OR' 2. H+, H O R H 2 H O OH 1. LiAlH4, Et2O (3) R H 2. H+, H O R H 2 H Ch. 12 - 18 Mechanism O H O + H Al H R OR' R OR' H H O R'O + R H H H Al H OH O O H H H R H R H H H Esters are reduced to 1o alcohols Ch. 12 - 19 3B. Sodium Borohydride NaBH4 ● less reactive and less basic than LiAlH4 ● can use protic solvent (e.g. ROH) ● reduces only more reactive carbonyl groups (i.e. aldehydes and ketones) but not reactive towards esters or carboxylic acids Ch. 12 - 20 Examples O NaBH4 OH (1) H R H H2O R H O NaBH4 OH (2) R' R R' H2O R H Ch. 12 - 21 Mechanism δ− O H O + H B H R R' R δ+ R' H H O OH H H R R' H Aldehydes are reduced to 1° alcohols & ketones are reduced to 2° alcohols Ch. 12 - 22 3C. Overall Summary of LiAlH4 and NaBH4 Reactivity reduced by LiAlH4 reduced by NaBH4 O O O O < < < R O R OR' R R' R H ease of reduction Ch. 12 - 23 4. Oxidation of Alcohols 4A. Oxidation of Primary Alcohols to Aldehydes O O [O] [O] R OH R H R OH 1o alcohol aldehyde carboxylic acid The oxidation of aldehydes to carboxylic acids in aqueous solutions is easier than oxidation of 1o alcohols to aldehydes It is, therefore, difficult to stop the oxidation of a 1o alcohol to the aldehyde stage unless specialized reagents are used Ch. 12 - 24 PCC oxidation ● Reagent PCC = [CrO3Cl] N H (Pyridinium chlorochromate) CrO3 + HCl + N N H [CrO3Cl] Pyridine Pyridinium (C5H5N) chlorochromate (PCC) Ch. 12 - 25 PCC oxidation PCC O R OH CH2Cl2 R H OH PCC O R R' CH2Cl2 R R' OH PCC R' No Reaction R CH2Cl2 R Ch. 12 - 26 4B. Oxidation of Primary Alcohols to Carboxylic Acids - O + KMnO4, OH H3O H2O, heat R O K O R OH H2CrO4 R OH (chromic acid) Chromic acid (H2CrO4) usually prepared by [CrO3 or Na2Cr2O7] + aqueous H2SO4 Jones reagent Ch. 12 - 27 Jones oxidation ● Reagent: CrO3 + H2SO4 ● A Cr(VI) oxidant O CrO3 + Cr(III) R OH H SO 2 4 R OH (orange solution) (green) OH O CrO3 + Cr(III) H SO R R' 2 4 R R' (orange solution) (green) OH CrO3 No Reaction H SO R R" 2 4 R' Ch. 12 - 28 4D. Mechanism of Chromate Oxidations Formation of the Chromate Ester H H O O H H O H3C O C + HO Cr O H3C O Cr O H C H 3 O C O O H C H H O H 3 H H H O H H H H O O O H3C O Cr O H H3C O Cr O C C O O H + OH H C H H3C H 3 H H Ch. 12 - 29 The oxidation step O O H3C H3C O Cr O C O + Cr O C OH H3C OH H3C H + + H O H H H O H Ch. 12 - 30 4E. A Chemical Test for Primary and Secondary Alcohols O CrO3 + Cr(III) R OH H SO 2 4 R OH (orange solution) (green) OH O CrO3 + Cr(III) H SO R R' 2 4 R R' (orange solution) (green) OH CrO3 No Reaction H SO R R" 2 4 R' Ch. 12 - 31 4F. Spectroscopic Evidence for Alcohols Alcohols give rise to broad O-H stretching absorptions from 3200 to 3600 cm-1 in IR spectra The alcohol hydroxyl hydrogen typically produces a broad 1H NMR signal of variable chemical shift which can be eliminated by exchange with deuterium from D2O Hydrogen atoms on the carbon of a 1o or 2o alcohol produce a signal in the 1H NMR spectrum between δ 3.3 and δ 4.0 ppm that integrates for 2 and 1 hydrogens, respectively The 13C NMR spectrum of an alcohol shows a signal between δ 50 and δ 90 ppm for the alcohol carbon Ch. 12 - 32 5. Organometallic Compounds Compounds that contain carbon-metal bonds are called organometallic compounds δ− δ+ C M C : M C M primarily ionic primarily covalent (M = Na or K) (M = Mg or Li) (M = Pb, Sn, Hg or Tl) Ch. 12 - 33 6. Preparation of Organolithium & Organomagnesium Compounds 6A. Organolithium Compounds Preparation of organolithium compounds Et2O R X + 2 Li RLi + LiX (or THF) Order of reactivity of RX ● RI > RBr > RCl Ch. 12 - 34 Example (80% - 90%) Et2O Br -10oC Li + + 2 Li LiBr Ch. 12 - 35 6B. Grignard Reagents Preparation of organomagnesium compounds (Grignard reagents) Et2O R X + Mg RMgX Et2O Ar X + Mg ArMgX Order of reactivity of RX ● RI > RBr > RCl Ch. 12 - 36 Example Br MgBr THF + Mg Ch. 12 - 37 7. Reactions of Organolithium and Organomagnesium Compounds 7A. Reactions with Compounds Con- taining Acidic Hydrogen Atoms δ− δ+ δ− δ+ RMgX ~ R:MgX RLi ~ R:Li Grignard reagents and organolithium compounds are very strong bases δ− δ+ δ+ δ− R MgX + H Y R H + Y + Mg2+ + X (or RLi) (Y = O, N or S) Ch. 12 - 38 Examples ● As base − (1) CH3MgBr + H2O H3C H + OH + Mg2+ + Br− MgBr − (2) + CH3OH + CH3O 2+ − + Mg + Br Ch. 12 - 39 Examples ● As base (3) H + H3C MgBr MgBr + H CH3 A good method for the preparation of alkynylmagnesium halides Ch. 12 - 40 7B. Reactions of Grignard Reagents with Epoxides (Oxiranes) Grignard reagents react as nucleophiles with epoxides (oxiranes), providing convenient synthesis of alcohols then H2O RMgBr O OH + R Ch. 12 - 41 Via SN2 reaction O O R R + H , H2O OH R o (1 alcohol) Ch. 12 - 42 Also work for substituted epoxides R OH O then H2O RMgBr + H H R' R' (2o alcohol) R OH O then H2O RMgBr + R" R" R' R' (3o alcohol) Ch.
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