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Chem 215-216 HH W12-Notes – Dr. Masato Koreeda - Page 1 of 13 Date: February 13, 2012 Chem 215-216 HH W12-Notes – Dr. Masato Koreeda - Page 1 of 13 Date: February 13, 2012 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? IUPAC names Common names Alcohols (R-OH) Primary alcohols CH3OH methanol methyl alcohol (1°-alcohols) CH3CH2OH ethanol ethyl alcohol pKa ~16-17 CH3CH2CH2OH 1-propanol n-propyl alcohol H3C C CH2OH 2-methyl-1-propanol isobutyl alcohol H3C H H3C C CH2OH 2,2-dimethyl-1-propanol neopentyl alcohol H3C CH3 H C Secondary alcohols 3 C OH 2-propanol isopropyl alcohol (2°-alcohols) H3C H pKa ~17-18 H C-CH 3 2 C OH 2-butanol sec-butyl alcohol H3C H Tertiary alcohols H3C C OH 2-methyl-2-propanol tert-butyl alcohol (3°-alcohols) H3C CH pKa ~19 3 Phenols (Ph-OH) pKa ~10-12 diethyl ether Ethers (R-O-R') CH3CH2-O-CH2CH3 Ph-O-CH=CH2 phenyl vinyl ether RO-: alkoxy CH3O methoxy CH3CH2O ethoxy ArO-: aryloxy PhO phenoxy 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 W12-Notes – Dr. Masato Koreeda - Page 2 of 13 Date: February 13, 2012 III. Oxidation state (or number) counting (see: pp 513-4 of the textbook) Therefore, eN eP C H O C C C -1 +1 -2 +2 0 0 The oxidation number of this C is -4. H H C H -2 0 "the contribution of an H to the oxidation number of the C is -1" H "the contribution of a C to the oxidation number of the H is +1" H H H C OH C O H H Other examples of oxidation numbers: 1. -1 -1 +1H H +1 "reducing agent" "oxidizing agent" Br Br 0 0 H H 0 0 +1 H H +1 -1 -1 H H -2-2 H H Br Br 0 0 2. 0 0 0 +2 Zn + Br Br Zn Br Br -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. Example: For N 3 x (- 1) from 3 carbon atoms = -3 O +1 from oxygen atom = +1 overall: -1 +1 from the charge = +1 N For O -1 from nitrogen atom = -1 overall: -2 -1 from the charge = -1 ================================================================== Hydrocarbon oxidation-reduction spectrum: "reduction" (hydrogenation) +4 -4 -2 0 +2 +4 H O C O H H H HO H C H H C OH C O C O C O + H H H HO HO H2O methane methanol formaldehyde formic acid carbonic acid (methanal) (methanoic acid) "oxidation" "oxidation" (oxygen insertion) (also: dehydrogenation) increasing %O; decreasing %H "oxidation" decreasing %O; increasing %H "reduction" note: used in biochem.: oxidase = dehydrogenase enzyme Chem 215-216 HH W12-Notes – Dr. Masato Koreeda - Page 3 of 13 Date: February 13, 2012 IV. Oxidation of alcohols H H [ox] O [ox] O 1°-alcohol R C O R C R C H + H + O - 2H aldehyde - H H carboxylic acid H H [ox] O 2°-alcohol R C O R C R' - 2H+ R' ketone R" H 3°-alcohol R C O not easily oxidized R' Oxidation methods: There are hundreds that differ in experimental conditions, but these follow basically the process shown below. difficult to remove as H B "E2 - type" reaction H H H O LG X LG R C O R C O R C + H B R' -H+ R' - LG R' "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 +6 O O CrO3 Cr "to be oxidized" "to be reduced" O O chromium trioxide RCH OH + CrO + 3+ (chromic anhydride) 2 3 R C Cr H oxidizing agent Balancing the oxidation-reduction reaction O 3 x R - CH2OH R C + 2 H + 2 e ("two-electron" oxidation) H 2 x Cr+6 + 3e Cr3+ Overall, O +6 + +3 3 R - CH2OH + 2 Cr 3 R C + 6 H + 2 Cr H "stoichiometry" Chem 215-216 HH W12-Notes – Dr. Masato Koreeda - Page 4 of 13 Date: February 13, 2012 1a Chromic acid [ CrO3 + H2O → H2CrO4] (hydrous conditions) O O Cr +6 problems: 1. acidic conditions HO OH 2. 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 O Na2Cr2O7 Na O Cr O Cr O Na O O 1b. Anhydrous Cr+6 • CrO3•pyridine • pyridinium chlorochromate (PCC): one of the most widely used oxidants! + -2 • pyridinium dichromate (PDC) [(pyH )2•Cr2O7 ] O O Cr +6 N Cl O H pyridinium chlorochromate (PCC) prepared from pyridine, HCl, and CrO3 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: more electrophilic than the H Cr in CrO3 because of Cl O R O-H R O Cr O C O O C N R' R' H Cr O Cl H Cl H O N -Cl H often referred to as extremely acidic! "Chromate" ester B: H O R oxidation O R O C O R O Cr O step Cr O C N R' + O C R' R' O H O Cr O H Cl O H +4 still Cr+6 Cr B: still Cr+6 (chromium is reduced!) N Cl Cr+4 becomes Cr+3 through redox-disproportionation. H Chem 215-216 HH W12-Notes – Dr. Masato Koreeda - Page 5 of 13 Date: February 13, 2012 The oxidation of primary alcohols with Jones’ reagent: aldehyde H CrO , H SO H 3 2 4 H O O O R C O 2 R C R C acetone faster step H H O (solvent) H 1°-alcohol carboxylic acid not isolable more than 2 mol equiv. of the reagent needed Often, an ester R-C(=O)-OR is a by-product. Mechanism: O O H A +6 H A H OH OH HO Cr HO Cr O O O O OH O Cr Cr OH R C + +4 O O Cr HO OH HO OH H oxidation R C R C H step -A H H H H A +3 R CH2OH B aldehyde: Cr Cr+6 doesn't stop here! "chromate ester" Cr+6 -A "chromate ester" A OH H H HO HO HO H HO H OH O Cr O OH O O O R C R C R C R OH C H R C Cr OH O H H H H H H OH O OH O A 2 B O Cr OH oxidation step! HO A HO C +4 R + Cr O +5 2 Cr+4 Cr+3 + Cr , etc. carboxylic acid Chem 215-216 HH W12-Notes – Dr. Masato Koreeda - Page 6 of 13 Date: February 13, 2012 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 aldehyde; 2°-alcohol: gives a ketone 1. O dimethyl H3C S sulfoxide 2. N(CH CH ) O H 2 3 3 O CH3 triethylamine R C R C H Cl H H C O O C Cl oxalyl chloride Mechanism: Cl Cl O O O C C O Cl O C C H3C S C O H3C S H3C S Cl O Cl O CH O C 3 CH3 CH3 Cl Cl This is the species in the solution after step 1. B R H R CO2, CO, Cl O C O C H H H3C S H H3C S H C CH CH3 R H B 3 H H Cl S O C H H CH3 H H N(CH CH ) highly acidic Hs! 2 3 3 pKa ~ 12-13 HN(CH2CH3)3 R O C H R Oxidation step! H C S + 3 H O C C H H3C S "intramolecular H C process" H H H H 1. O Note: H3C S O H CH3 2. N(CH2CH3)3 S O + H3C CH2D Cl D C O O C Cl Chem 215-216 HH W12-Notes – Dr. Masato Koreeda - Page 7 of 13 Date: February 13, 2012 IV 2. Non chromium-based Oxidation Reactions: “greener” methods ii) Sodium hypochlorite (NaOCl): Bleach Usually under acidic conditions (e.g., in acetic acid); HOCl (hypochlorous acid) is the actual oxidant. As HOCl is not stable, it has to be generated in situ in the reaction medium. V. Reactions of alcohols: Direct conversion of alcohols to alkyl halides With SOCl2 (thionyl chloride) or PBr3 (phosphorus tribromide) Mechanism for an alcohol to the chloride with SOCl2/pyridine S H H H H O Cl O Cl O H H H Cl O Cl S S Cl S O BH S Cl O Cl B O or or O highly electrophilic S N N H O R Alternatively, N S that can be formed from SOCl2 Cl + SO2 + Cl and pyridine may be the reagent that puts S(=O)Cl onto H Cl (gas) the OH group.
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