Ch11 - Reactions of Alcohols Alcohols are versatile organic...

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Reactions of Alcohols Alcohols are versatile organic compounds since they undergo a wide variety of transformations – the majority of which are either oxidation or reduction type reactions. Oxidation is a loss of electrons Reduction is a gain of electrons. But in organic terms: Oxidation: loss of H 2 ; addition of O or O 2 ; addition of X 2 (halogens). Reduction: addition of H 2 or H - ; loss of O or O 2 ; loss of X 2 . (Neither an oxidation or reduction: Addition or loss of H + , H 2 O, HX). Oxidation-Reduction Chart: R H H H R OH H H R O HR O OH R H R H R OH R H R O R R H R H R OH R R Reduction Oxidation alkane primary alcohol aldehyde acid tertiary alcohol secondary alcohol ketone Ch11 Reacns of Alcohols.doc Page1
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Oxidation of Alcohols Primary and secondary alcohols are easily oxidized by a variety of reagents. Secondary Alcohols The most common reagent used for oxidation of secondary alcohols to ketones is chromic acid, H 2 CrO 4 . R OH R H R O R secondary alcohol ketone Na 2 Cr 2 O 7 , H 2 SO 4 Chromic acid is produced in situ by reaction of sodium dichromate, sulfuric acid and water. Na 2 Cr 2 O 7 + H 2 O + 2H 2 SO 4 2 H 2 CrO 4 + 2 NaHSO 4 Mechanism of oxidation R OH R H O Cr OH HO O chromic acid (CrVI) R R H O Cr O O OH + H 2 O chromate ester R R H O Cr O O OH H 2 O: O R R (CrIV) O Cr OH O- H 3 O + The alcohol and chromic acid produce a chromate ester, which then reductively eliminates the Cr species. The Cr is reduced (VI IV), the alcohol is oxidized. Ch11 Reacns of Alcohols.doc Page2
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Oxidation of Primary Alcohols Primary alcohols are easily oxidized just like secondary alcohols, but the product of oxidation is an aldehyde. R OH H H R O HR O OH primary alcohol aldehyde acid [O] [O] However, the aldehyde can also be easily oxidized to an acid, and this ‘over- oxidation’ is a practical problem. E.g. A common reagent that selectively oxidizes a primary alcohol to an aldehyde (and no further) is pyridinium chlorochromate, PCC. E.g. CH 2 OH Na 2 Cr 2 O 7 , H 2 SO 4 CO 2 H N: CrO 3 , HCl (PCC) OH PCC O H aldehyde Ch11 Reacns of Alcohols.doc Page3
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Tertiary Alcohols These are resistant to oxidation because they have no hydrogen atoms attached to the oxygen bearing carbon (carbinol carbon). Other Oxidizing Reagents Potassium permanganate is a cheaper but stronger oxidizing agent, and conditions must be controlled carefully. OH KMnO 4 , base O OH Thermal dehydrogenation is the cheapest method of oxidation but the high temperatures involved limit the applicability of this method. H OH O CuO, 300 o C Reduction of Alcohols Normally an alcohol cannot be directly reduced to an alkane in one step.
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Ch11 - Reactions of Alcohols Alcohols are versatile organic...

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