Chem 215-216 HH W11-Notes – Dr. Masato Koreeda - Page 1 of 13 Date: February 9, 2011 Chapter 13. Alcohols, Diols, and Ethers Overview: Chemistry and reactions of sp3 oxygen groups, particularly oxidation of an alcohol, ether formation, and reactions of oxirane (epoxide) groups. I. What are alcohols, phenols, and ethers? Alcohols (R-OH)Primary alcohols (1°-alcohols)CH3OHmethanol methyl alcoholCH3CH2OHethanolethyl alcoholCH3CH2CH2OH1-propanoln-propyl alcoholpKa ~16-17H3CCCH2OHH3CH2-methyl-1-propanolisobutyl alcoholH3CCCH2OHH3CCH32,2-dimethyl-1-propanol neopentyl alcoholH3CCOHH3CHSecondary alcohols(2°-alcohols)pKa ~17-182-propanolisopropyl alcoholH3C-CH2COHH3CH2-butanolsec-butyl alcoholH3CCOHH3CCH3Tertiary alcohols(3°-alcohols)pKa ~192-methyl-2-propanoltert-butyl alcoholPhenols (Ph-OH)pKa ~10-12Ethers (R-O-R')CH3CH2-O-CH2CH3diethyl etherPh-O-CH=CH2phenyl vinyl etherRO-: alkoxymethoxyethoxyArO-: aryloxy phenoxyCH3CH2OCH3OPhOIUPAC namesCommon names II. Oxidation Oxidation – historical use of the term: (1) oxide (oxyd/oxyde) – the ‘acid’ form of an element; e.g., S + air → oxide of S (acid of sulfur) (2) oxidation or oxidize – to make such an acid, to make the oxide (3) oxygen – Lavoisier: substance in the air that makes acids; “the bringer of acids” = “oxygen” (4) oxidation or oxidize – to increase the % oxygen in a substance (reduction: to reduce the % oxygen) More modern definition: oxidation or oxidize – loss of electrons (coupled with reduction as gain of electrons) Note: The loss of electrons (oxidation) by one atom or compound must be matched by the gain of electrons (reduction) by another.Chem 215-216 HH W11-Notes – Dr. Masato Koreeda - Page 2 of 13 Date: February 9, 2011 III. Oxidation state (or number) counting (see: pp 513-4 of the textbook) ePeNC H-1 +1O C-2 +2C C0 0"the contribution of an H to the oxidation number of the C is -1""the contribution of a C to the oxidation number of the H is +1"C HHHHThe oxidation number of this C is -4.C OHHHH-2HCHO0Therefore,HHOther examples of oxidation numbers:HHHHHHBrBrHHBrBr-1-10 0-1-1-2-2000 0+1+1+1+1"reducing agent" "oxidizing agent"1.2.Zn+BrZnBrBrBr000+2-1-1 An atom with a formal charge: incorporate its charge # to its oxidation number. Namely, if an atom has a +1 charge, add +1 to its oxidation number. NOFor N3 x (- 1) from 3 carbon atoms = -3+1 from oxygen atom = +1+1 from the charge = +1overall: -1For O -1 from nitrogen atom = -1-1 from the charge = -1overall: -2Example: ================================================================== Hydrocarbon oxidation-reduction spectrum: C HHHH C OHHHH-2HCHO0HHOC OHOHOC O-4+2+4+4methane methanol formaldehyde(methanal)formic acid(methanoic acid)carbonic acid"oxidation"(oxygen insertion)"oxidation"(also: dehydrogenation)"reduction"(hydrogenation)O C O+H2Oincreasing %O; decreasing %H"oxidation"decreasing %O; increasing %H "reduction"note: used in biochem.: oxidase = dehydrogenase enzymeChem 215-216 HH W11-Notes – Dr. Masato Koreeda - Page 3 of 13 Date: February 9, 2011 IV. Oxidation of alcohols R CHHOHR CR'HOHR CHOR COOHR CR'OR CR'R"OHketonealdehydecarboxylic acid1°-alcohol2°-alcohol3°-alcohol[ox] [ox]- 2H+[ox]- 2H+- H+not easily oxidized Oxidation methods: There are hundreds that differ in experimental conditions, but these follow basically the process shown below. R CR'HOHdifficult to remove as HLG X-H+R CR'HOLGB"E2 - type" reactionR CR'OH B+LG-"converting this H to a leaving group"LG: leaving group Historically, most common reagents involve high-valency metals. 1. Cr (VI)-based reagents - all Cr(VI) reagents have toxicity problems OCrOOchromium trioxide(chromic anhydride)CrO3+6RCH2OH + CrO3R CHO+Cr3+"to be oxidized" "to be reduced"oxidizing agent Balancing the oxidation-reduction reaction R - CH2OHRCHO+ 2 H + 2 e3 x("two-electron" oxidation)Cr+6 + 3eCr3+2 xOverall,3 R - CH2OH + 2 Cr+6RCHO+ 6 H+ + 2 Cr+33"stoichiometry"Chem 215-216 HH W11-Notes – Dr. Masato Koreeda - Page 4 of 13 Date: February 9, 2011 1a Chromic acid [ CrO3 + H2O → H2CrO4] (hydrous conditions) CrO OHO OH+6problems: 1. acidic conditions2. can't stop at the stage of an aldehyde because of the presence of water • Jones’ reagent: CrO3/H2SO4/H2O historically, one of the most commonly used chromium +6-based reagents for the oxidation of alcohols • Chromate: Na2CrO4 (sodium chromate)/H2SO4/H2O • Dichromate: Na2Cr2O7 (sodium dichromate)/H2SO4/H2O O Cr O Cr OOOOONaNaNa2Cr2O7 1b. Anhydrous Cr+6 • CrO3•pyridine • pyridinium chlorochromate (PCC): one of the most widely used oxidants! • pyridinium dichromate (PDC) [(pyH+)2•Cr2O7-2] NHCrO OCl O+6prepared from pyridine, HCl, and CrO3pyridinium chlorochromate(PCC) Oxidation reactions of alcohols using these reagents are carried out in anhydrous organic solvents such as dichloromethane and, thus, the oxidation of a primary alcohol stops at the stage of an aldehyde. Mechanism for the oxidation with PCC: CRR'HO-HCrOOOClmore electrophilic than the Cr in CrO3 because of ClCRR'OHCrOOOHClNHNHCRR'OHCrOHNHOO-ClClstill Cr+6B:extremely acidic!CRR'OCrOHNHOOClstill Cr+6often referred to as"Chromate" esteroxidation stepB:C ORR'CrOOO+Cr+4(chromium is reduced!)Cr+4 becomes Cr+3 through redox-disproportionation.Chem 215-216 HH W11-Notes – Dr. Masato Koreeda - Page 5 of 13 Date: February 9, 2011 The oxidation of primary alcohols with Jones’ reagent: R CHHOHR CHOR COOHaldehydecarboxylic acid1°-alcoholCrO3, H2SO4H2Oacetone(solvent)not isolablemore than 2 mol equiv. of the reagent neededfaster step Often, an ester R-C(=O)-OR is a by-product. Mechanism: OCrO+6HO OHH AOCrOHO OHH-ARCH2OHOCrOHHOOHOCRH HHAOCrOHHOOHOCRHHBCr+4OCRHCr+6Cr+3"chromate ester"+oxidation stepaldehyde:doesn't stop here!OCRHHOHH AOCRHHOH2HOCRHHOHAOCrOHOOHHOCRHHOHOCrOHOHOHBOCRHHOOCrOHOHOHAAoxidation step!OCRHOcarboxylic acidCr+4+Cr+6"chromate ester"-A2 Cr+4Cr+3+Cr+5, etc.Chem 215-216 HH W11-Notes – Dr. Masato Koreeda - Page 6 of 13 Date: February 9, 2011 2. Non chromium-based Oxidation Reactions: “greener” methods i) Swern oxidation: • usually using dichloromethane (CH2Cl2) as the solvent • anhydrous (i.e., no water present!), non-acidic conditions • 1°-alcohol: stops at an
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