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10.6 Oxidation of Alcohols 459 10_BRCLoudon_pgs5-0.qxd 12/5/08 1:34 PM Page 459 10.6 OXIDATION OF ALCOHOLS 459 10.6 OXIDATION OF ALCOHOLS A. Oxidation to Aldehydes and Ketones Primary and secondary alcohols can be oxidized by reagents containing Cr(VI)—that is, chromium in the 6oxidationstate—togivecertaintypesofcarbonyl compounds (compounds + ) that contain the carbonyl group, C) A O ). For example, secondary alcohols are oxidized to ketones: OH O S Na2Cr2O7 CH3"CHCH2CH2CH2CH2CH2CH3 CH3CCH2CH2CH2CH2CH2CH3 (10.35) aqueous H2SO4 2 h 2-octanol 2-octanone (94% yield) OH O S " CrO3 (10.36) pyridine e` e` (89% yield) Several forms of Cr(VI) can be used to convert secondary alcohols into ketones. Three of 2 2 these are chromate (CrO4_), dichromate (Cr2O7_), and chromic anhydride or chromium trioxide (CrO3). The first two reagents are customarily used under strongly acidic conditions; the last is 3 often used in pyridine. In all cases, the chromium is reduced to a form of Cr(III) such as Cr |. Primary alcohols react with Cr(VI) reagents to give aldehydes. However, if water is pre- sent, the reaction cannot be stopped at the aldehyde stage because aldehydes are further oxidized to carboxylic acids: O O S S K2Cr2O7 K2Cr2O7 CH3CH2CHCH2OH CH3CH2CHCH CH3CH2CHC OH aqueous H2SO4 aqueous H2SO4 "CH3 "CH3 "CH3 2-methyl-1-butanol 2-methylbutanal 2-methylbutanoic acid (10.37) For this reason, anhydrous preparations of Cr(VI) are generally used for the laboratory prepa- ration of aldehydes from primary alcohols. One commonly used reagent of this type is a com- plex of pyridine, HCl, and chromium trioxide called pyridinium chlorochromate, which goes by the acronym “PCC.” This reagent is typically used in methylene chloride solvent. CrO3 HCl O ( N ) S PCC CH3(CH2)8CH2OH CH3(CH2)8CH (10.38) CH2Cl2 (solvent) 1-decanol decanal (92% yield) Water promotes the transformation of aldehydes into carboxylic acids because, in water, aldehydes are in equilibrium with hydrates formed by addition of water across the CAO double bond. 10_BRCLoudon_pgs5-0.qxd 12/5/08 1:34 PM Page 460 460 CHAPTER 10 • THE CHEMISTRY OF ALCOHOLS AND THIOLS O OH O SSCr(VI) R CH H2O R "CH OH R C OH (10.39) L + LL L L an aldehyde an aldehyde hydrate a carboxylic acid Aldehyde hydrates are really alcohols and therefore can be oxidized just like secondary alcohols. Be- cause of the absence of water in anhydrous reagents such as PCC, a 1,1-diol does not form, and the reaction stops at the aldehyde. Tertiary alcohols are not oxidized under the usual conditions. For oxidation of alcohols at the a-carbon to occur, the a-carbon atom must bear one or more hydrogen atoms. The mechanism of alcohol oxidation by Cr(VI) involves several steps that have close analo- gies to other reactions. Consider, for example, the oxidation of isopropyl alcohol to the ketone acetone by chromic acid (H2CrO4). The first steps of the reaction involve an acid-catalyzed displacement of water from chromic acid by the alcohol to form a chromate ester. (This ester is analogous to ester derivatives of other strong acids; Sec. 10.3C.) O H3C OO H3C $ $ S H3O | A O (10.40a) $CH OH @Cr @ (several steps) $CH O Cr H2O L L L + HO OH + H3C ) ) H3C "OH isopropyl alcohol chromic acid a chromate ester After protonation of the chromate ester (Eq. 10.40b), it decomposes in a b-elimination reaction (Eq. 10.40c). H3C O H3C O $ $ S S $CH OHHCr A O1 O| 2 $CH OHHCr A O| O1 2 (10.40b) L LL1 1 L L 1 + 1 H3C "OH H3C "OH CH O O O 3 H3C S $ S S E2 H C "C O Cr A OH| C A O Cr1 A OH| Cr1 OH1 H O1 (10.40c) 3 $ _ 3 | L L L 1 + 1 L 1 + H3C ΄ ΄ "H "OH "OH "OH acetone Cr(IV) H O1 1 L chromium "H accepts electrons This last step is essentially an E2 reaction. Because Cr(VI) is particularly electronegative, es- pecially when protonated, the chromium readily accepts an electron pair and is thus reduced. The reaction is so rapid that the Brønsted base in the reaction can be very weak; in fact, water is the base in Eq. 10.40c. In the resulting H2CrO3 by-product, chromium is in a 4 oxidation 3 + state. The ultimate by-product is Cr | because, in subsequent reactions, Cr(IV) and Cr(VI) react to give two equivalents of a Cr(V) species, which then oxidizes an additional molecule of alcohol by a similar mechanism. Cr(IV) Cr(VI) 2Cr(V) (10.41a) + LT Cr(V) (CH3)2CH OH Cr(III) (CH3)2CAO 2H| (10.41b) + L LT + + 10_BRCLoudon_pgs5-0.qxd 12/5/08 1:34 PM Page 461 10.6 OXIDATION OF ALCOHOLS 461 The Breath Test for Alcohol Law enforcement officers can use any of several devices to determine a person’s blood alcohol level, and two of these are based on ethanol oxidation to acetic acid. All of the devices are based on the fact that, in the lungs, a known small fraction of ethanol in the blood escapes into the air that is sub- sequently exhaled, and it is the alcohol content of this exhaled vapor that is analyzed.In the original measuring device,called a “breathalyzer,”the exhaled air is collected and allowed to react with acidic 3 potassium dichromate (K2Cr2O7). The alcohol reduces the Cr(VI) to Cr |.The resulting change in 3 color of the chromium from the yellow-orange of the Cr(VI) oxidation state to the blue-green of Cr | is detected by a simple spectrometer.The amount of Cr(VI) reduced is calibrated in terms of percent blood alcohol. (See Problem 10.49 at the end of the chapter.) A more recent device uses electro- chemical technology. The alcohol is oxidized at a platinum electrode to produce acetic acid along with four protons and four electrons.(See Study Problem 10.2 on p.452.) The resulting electron flow in the electrochemical cell is detected and calibrated in terms of blood alcohol concentration. A third device uses infrared spectroscopy (Chapter 12) to detect ethanol. B. Oxidation to Carboxylic Acids As noted in the previous section (Eq. 10.37), primary alcohols can be oxidized to carboxylic acids using aqueous solutions of Cr(VI), such as aqueous potassium dichromate (K2Cr2O7) in acid. Another useful reagent for oxidizing primary alcohols to carboxylic acids is potassium permanganate (KMnO4) in basic solution: O O S S KMnO HCl CH CH CH CH CHCH OH 4 CH CH CH CH CHC O CH CH CH CH CHC OH 3 2 2 2 2 OH 3 2 2 2 _ 3 2 2 2 _ L L "C2H5 "C2H5 "C2H5 2-ethyl-1-hexanol a carboxylate ion 2-ethylhexanoic acid (conjugate base of the (74% yield) carboxylic acid) (10.42) As shown in this equation, the immediate product of the permanganate oxidation is the conju- gate base of the carboxylic acid because the reaction is run in alkaline solution. Isolation of the carboxylic acid itself requires addition of a strong acid such as HCl or H2SO4 in a second step. The manganese in KMnO4 is in the Mn(VII) oxidation state; in the oxidation of alcohols, it is reduced to MnO2, a common form of Mn(IV). Because KMnO4 reacts with alkene double bonds (Sec. 11.5A), Cr(VI) is required for the oxidation of alcohols that contain double or triple bonds (see Eq. 10.36). Potassium permanganate is not used for the oxidation of secondary alcohols to ketones be- cause many ketones react further with the alkaline permanganate reagent. PROBLEMS 10.26 Give the product expected when each of the following alcohols reacts with pyridinium chlorochromate (PCC). (a) CH2CH2OH (b) HO CH2CH2CH2 OH L L L H3C V OH 10_BRCLoudon_pgs5-0.qxd 12/5/08 1:34 PM Page 462 462 CHAPTER 10 • THE CHEMISTRY OF ALCOHOLS AND THIOLS 10.27 From which alcohol and by what method would each of the following compounds best be prepared by an oxidation? (a) (CH3)2CHCH2CH2CH2CO2H (b) O S CH3CH2CCH2CH3 (c) OH O (d) S A (CH3)2"CCH2CH2CH2CH %CH O 10.7 BIOLOGICAL OXIDATION OF ETHANOL Oxidation and reduction reactions are very important in living systems. A typical biological oxidation is the conversion of ethanol into acetaldehyde, the principal reaction by which ethanol is removed from the bloodstream. biological oxidation CH3CH2OH CH3CHA O (10.43) ethanol acetaldehyde The reaction is carried out in the liver and is catalyzed by an enzyme called alcohol dehydroge- nase. (Recall from Sec. 4.9C that enzymes are biological catalysts.) The oxidizing agent is not the enzyme, but a large molecule called nicotinamide adenine dinucleotide, abbreviated NAD|; the structure of NAD| and a convenient abbreviated structure for it are shown in Fig. 10.1. When H O H S CONH NH " 2 " C 2 L M M r r N N abbreviated structure | | for NAD "R | CH2 O O O H H M" P) " " H " " H NH2 O _ "OH" OH O N " O N r P N N O _ $O CH2 O L " H H ""H " " H "OH" OH NAD| Figure 10.1 Abbreviated and full structures of NAD|.The colored portion of the full structure is abbreviated as an R-group..
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