hem 215 F07 otes Dr. Masato Koreeda - Page 1 of 5. Date: September 12, 2007 hapter 13. lcohols, Diols, and Ethers verview: hemistry and reactions of sp 3 oxygen groups, particularly oxidation of an alcohol, ether formation, and reactions of oxirane (epoxide) groups. I. What are alcohols, phenols, and ethers? lcohols (-) Primary alcohols (1 -alcohols) pka ~167 IUP names ommon names 3 methanol methyl alcohol 3 2 ethanol ethyl alcohol 3 2 2 1-propanol n-propyl alcohol 3 2 2-methyl-propanol isobutyl alcohol 3 3 2 2,2-dimethyl-propanol neopentyl alcohol 3 3 Secondary alcohols (2 -alcohols) pka ~178 3 3 3-2 3 2-propanol 2-butanol isopropyl alcohol sec-butyl alcohol Tertiary alcohols (3 -alcohols) pka ~19 3 3 3 2-methyl-2-propanol tert-butyl alcohol Phenols (Ph-) pka ~102 Ethers (--) 3 2 -- 2 diethyl ether 3 Ph--= 2 phenyl vinyl ether -: alkoxy Ph-: aryloxy 3 3 2 Ph methoxy ethoxy phenoxy II. xidation xidation historical use of the term: (1) oxide (oxyd/oxyde) the acid form of an element 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) ote: The loss of electron (oxidation) by one atom or compound must be matched by the gain of electrons (reduction) by another.
hem 215 F07 otes Dr. Masato Koreeda - Page 2 of 5. Date: September 12, 2007 III. xidation state (or number) counting (see Ege s, pp 513-514) e ep 1-2 2 "the contribution of an to the oxidation number of the is " "the contribution of a to the oxidation number of the is 1" Therefore, The oxidation number of this is -4. -2 0 ther examples of oxidation numbers: 1. 1 1 "reducing agent" "oxidizing agent" 1 1-2-2 r r r r 2. 0 2 Zn r r r Zn r ydrocarbon oxidation-reduction spectrum: -4-2 0 2 4 "reduction" (hydrogenation) methane methanol formaldehyde (methanal) formic acid (methanoic acid) carbonic acid 4 2 (oxygen insertion) (also: dehydrogenation) increasing %; decreasing % decreasing %; increasing % "reduction" note: used in biochem.: oxidase = dehydrogenase enzyme
hem 215 F07 otes Dr. Masato Koreeda - Page 3 of 5. Date: September 12, 2007 IV. xidation of alcohols 1 -alcohol 2 -alcohol - 2 aldehyde - - 2 ketone 3 -alcohol " not easily oxidized xidation methods: There are hundreds that differ in experimental conditions, but these follow basically the process shown below. difficult to remove as LG X LG "E2 - type" reaction - - LG "converting this to a leaving group" LG: leaving group istorically, most common reagents involve high-valency metals. 1. r (VI)-based reagents - all r(vi) reagents have toxicity problems r 3 r chromium trioxide (chromic anhydride) "to be oxidized" alancing the oxidation-reduction reaction "to be reduced" 2 r 3 oxidizing agent r 3 3 x - 2 2 2 e ("two-electron" oxidation) 2 x r 3e r 3 verall, 3-2 2 r 3 6 2 r 3 "stoichiometry"
hem 215 F07 otes Dr. Masato Koreeda - Page 4 of 5. Date: September 12, 2007 1a hromic acid [ r 3 2 2 r 4 ] (hydrous conditions) r problems: 1. acidic conditions 2. can't stop at the stage of an aldehyde because of the presence of water Jones reagent: r 3 / 2 S 4 / 2 historically, one of the most commonly used chromium -based reagents for the oxidation of alcohols hromate: Dichromate: a 2 r 4 (sodium chromate)/ 2 S 4 / 2 a 2 r 2 7 (sodium dichromate)/ 2 S 4 / 2 a 2 r 2 7 a r r 1b. nhydrous r r 3 pyridine pyridinium chlorochromate (P): one of the most widely used oxidants! pyridinium dichromate (PD) [(py ) 2 r 2 7-2 ] a r l pyridinium chlorochromate (P) prepared from pyridine, l, and r 3 xidation 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 P: - more electrophilic than the r in r 3 because of l r l r r 4 (chromium is reduced!) oxidation step r 4 becomes r 3 through redox-disproportionation. : often referred to as "hromate" ester r still r r l l -l : extremely acidic! r still r l
hem 215 F07 otes Dr. Masato Koreeda - Page 5 of 5. Date: September 12, 2007 The oxidation of primary alcohols with Jones reagent: 1 -alcohol r 3, 2 S 4 2 acetone (solvent) more than 2 mol equiv. of the reagent needed aldehyde not isolable faster step ften, an ester -(=)- is a by-product. Mechanism: r - r 2 r r r "chromate ester" oxidation step aldehyde: doesn't stop here! r 4 r 3 r "chromate ester" r oxidation step! r r - 2 r 4 r 3