Ch 10 and 11 Chemistry of alcohols and thiols

C h o r o3 2 chromiumvi oxide k

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Unformatted text preview: ium dichromate in aqueous sulfuric acid! C! ! ! ! +! H! !O! r O3 2 Chromium(VI)! oxide! K! ! ! ! ! ! ! 2 Cr 2 O7 Potassium! dichromate! H! !S! ! ! 2 O4 H! !S! ! ! 2 O4 H! !Cr! ! ! ! 2 ! 2 O7 H! !Cr! ! ! 2 !O4 Chromic acid! H! !O! 2 2! ! !Cr! ! ! H2 ! O 4 Chromic acid! 10-13! Chromic Acid Oxidation of ROH! •  Step 1: formation of a chromate ester! OH H Cyc loh exanol fas t and revers ible O + HO-Cr - OH O O- Cr - OH O + H2 O H An alkyl chromate O •  Step 2: reaction of the chromate ester with a base, here shown as H2O! chromiu m(V I) chromiu m(IV) O O H H O H slow, rate Cr -OH determining O H O + O+ H + H Cyclohexan on e O - Cr -OH O 10-14! Oxidation: 1° ROH to RCHO!   Oxidation of a primary alcohol gives an aldehyde or a carboxylic acid, depending on the experimental conditions! •  to an aldehyde is a two-electron oxidation! • to a carboxylic acid is a four-electron oxidation! OH C H3 - C H H A primary alcohol   ! [O] O C H3 - C - H An aldehyde [O] O C H3 - C - O H A carboxylic acid Difficult to stop at aldehyde! 10-15! Chromic Acid Oxidation of RCHO! •  chromic acid oxidizes a 1° alcohol first to an aldehyde and then to a carboxylic acid! •  in the second step, it is not the aldehyde per se that is oxidized but rather the aldehyde hydrate! O R -C- H + H2 O An ald ehyde fas t and revers ible OH R- C-OH H2 CrO4 H An ald ehyde hydrate H2 O O-Cr O3 H R -C- OH H O R -C- OH + HCr O3 - + H3 O+ A c arb oxylic acid 10-16! Oxidation: 1° ROH to RCHO!   ! ! ! Use pyridinium chlorochromate (PCC) to limit the oxidation.! •  A form of Cr(VI) prepared by dissolving CrO3 in aqueous HCl and adding pyridine to precipitate PCC as a solid! pyrid inium ion chloroc hromate ion Cr O3 + HCl + N Pyrid ine ClCrO3 - N H Pyrid inium ch lorochromate (PCC) •  PCC is selective for the oxidation of 1° alcohols to aldehydes; it does not oxidize aldehydes further to carboxylic acids! •  PCC can also be used to oxidize 2° alcohols to ketones.! 10-17! Oxidation: 1° ROH! •  PCC oxidizes a 1° alcohol to an aldehyde! N H CrO3Cl OH O H CH2Cl2! Geraniol Geranial •  PCC also oxidizes a 2° alcohol to a ketone! OH P CC O CH2Cl2! Menthol Menthone 10-18! Alkyl Sulfonates!   Sulfonic acids, like sulfuric acid, are strong acids! •  sulfonate anions are very weak bases (the conjugate base of a strong acid) and are very good leaving groups for SN2 reactions •  sulfonyl chlorides are derived from sulfonic acids O O– O O! R S O– RSO RSO R! ! ! ! ! ! -S-OH O O O– O! A sulfonate anion! A sulfonic acid! (a very weak base and ! (a very strong acid)! stable anion; a very! good leaving group)!   O! R! ! ! ! !! -S-Cl O! A!sulfonyl! chloride! Conversion of an alcohol to a sulfonate ester converts -OH, a very poor leaving group, into a sulfonic ester, a very good leaving group 10-19! Alkyl Sulfonates!   A commonly used sulfonyl chloride is p-toluenesulfonyl chloride (Ts-Cl) ! C H O Cl OSO N p-toluenesulfonyl chloride TsCl, “tos...
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This note was uploaded on 01/27/2014 for the course BIOL 241 taught by Professor Henrychang during the Winter '10 term at Purdue University-West Lafayette.

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