Chromic acid alternatives

Now let s draw the forward scheme. The starting alcohol is oxidized upon treatment with chromic acid (alternatively, PCC can be used for this step). The resulting ketone is then treated with molecular bromine (Br2) and sodium hydroxide, followed by aqueous acid, to give a carboxylic acid (via a haloform reaction). Finally, the carboxylic acid is treated with ethanol in the presence of an acid catalyst, giving the desired ester (via a Fischer esterification). 

An alternative oxidizing agent similar to chromic acid in Its reactions with or game compounds is potas Slum permanganate (KMn04)... 

The anhydride can be made by the Hquid-phase oxidation of acenaphthene [83-32-9] with chromic acid in aqueous sulfuric acid or acetic acid (93). A postoxidation of the cmde oxidation product with hydrogen peroxide or an alkaU hypochlorite is advantageous (94). An alternative Hquid-phase oxidation process involves the reaction of acenaphthene, molten or in alkanoic acid solvent, with oxygen or acid at ca 70—200°C in the presence of Mn resinate or stearate or Co or Mn salts and a bromide. Addition of an aHphatic anhydride accelerates the oxidation (95). 

Chromic Acid Electrolysis. Alternatively, as shown in Figure 1, chromium metal may be produced electrolyticaUy or pyrometaUurgicaUy from chromic acid, CrO, obtained from sodium dichromate by any of several processes. Small amounts of an ionic catalyst, specifically sulfate, chloride, or fluoride, are essential to the electrolytic production of chromium. Fluoride and complex fluoride catalyzed baths have become especially important in recent years. The cell conditions for the chromic acid process are given in Table 7. 

Note 1. An alternative procedure proceeds by oxidation of the 3/5-hydroxy group with chromic acid-sulfuric acid and subsequent elimination of hydrogen chloride by treatment of the intermediate chloroketone with potassium acetate in methanol. Good overall yields are obtained with this reaction sequence in the androstane series. 

The effect of adding large quantities of acetic acid to the medium is more complicated. The acceleration of the oxidation rate of isopropanol was ascribed initially to a shift of the esterification equilibrium to the right (reaction 29). However, RoCek found that acceleration by acetic acid occurs for oxidations which cannot involve a pre-equilibrium esterification, e.g. those of aliphatic and alicyclic hydrocarbons. The obvious alternative, i.e. that acetic acid combines with chromic acid, viz. 

Redox titrants (mainly in acetic acid) are bromine, iodine monochloride, chlorine dioxide, iodine (for Karl Fischer reagent based on a methanolic solution of iodine and S02 with pyridine, and the alternatives, methyl-Cellosolve instead of methanol, or sodium acetate instead of pyridine (see pp. 204-205), and other oxidants, mostly compounds of metals of high valency such as potassium permanganate, chromic acid, lead(IV) or mercury(II) acetate or cerium(IV) salts reductants include sodium dithionate, pyrocatechol and oxalic acid, and compounds of metals at low valency such as iron(II) perchlorate, tin(II) chloride, vanadyl acetate, arsenic(IV) or titanium(III) chloride and chromium(II) chloride. 

Anthraquinone itself is traditionally available from the anthracene of coal tar by oxidation, often with chromic acid or nitric acid a more modern alternative method is that of air oxidation using vanadium(V) oxide as catalyst. Anthraquinone is also produced in the reaction of benzene with benzene-1,2-dicarboxylic anhydride (6.4 phthalic anhydride) using a Lewis acid catalyst, typically aluminium chloride. This Friedel-Crafts acylation gives o-benzoylbenzoic acid (6.5) which undergoes cyclodehydration when heated in concentrated sulphuric acid (Scheme 6.2). Phthalic anhydride is readily available from naphthalene or from 1,2-dimethylbenzene (o-xylene) by catalytic air oxidation. 

An alternative and more generally used oxidation method employs chromic acid. This process is an exception to our general theme, because here the alcohol is transformed to a carbonyl group by removal of electron density from oxygen rather than from carbon. The first step has been shown to be a rapid equilibrium between the alcohol and its chromate ester, followed by rate-determining decomposition of the ester in the manner shown in Scheme 7.42 It will be noted that the species eliminated from the carbon that becomes the carbonyl carbon is a Lewis acid, not a Lewis base. 

Alternatives.4 Fortunately, there are a variety of alternatives to chromic acid, so its use is really unnecessary. Some homemade alternatives include ... 

Many reagents are available to oxidize a simple secondary alcohol to a ketone. Most labs would have chromium trioxide or sodium dichromate available, and the chromic acid oxidation would be simple. Bleach (sodium hypochlorite) might be a cheaper and less polluting alternative to the chromium reagents. DMP and the Swem oxidation would also work. 

Sodium dichromate in sulfuric acid ( chromic acid, H2Cr04) is the traditional laboratory reagent for oxidizing secondary alcohols to ketones. Bleach (NaOCl) is an inexpensive, chromium-free alternative that also oxidizes secondary alcohols to ketones. Primary alcohols are usually over-oxidized to carboxylic acids under these conditions. 

Primary alcohols and aldehydes are commonly oxidized to acids by chromic acid (H2Cr04, formed from Na2Cr207 and H2SO4). Sodium hypochlorite (bleach, NaOCl) is a chromium-free alternative to chromic acid (Sections 11-2B and 18-19). 

Chromic acid is best avoided if acid-sensitive alcohols are to be oxidized, and an alternative reagent for these is PCC (pyridinium chlorochromate), which can be used in dichloromethane. 

An alternative mechanism has also been proposed in which oxidation at the double bond leads to a ketol derivative, elimination of water from which then gives the unsaturated ketone (Scheme 18a)." Limited kinetic data are available and suggest diat Scheme 17 is obtained for chromic acid oxidations. 

Spontaneous Crosslinking. In the absence of TCPA, evaporated solutions of XVIII can be completely redissolved in fresh solvent in a few minutes. And solutions of 0.05 M XVIII containing 0.007 M TCPA in acetonitrile are stable indefinitely (at least three weeks) at room temperature. However, on evaporation of the solvent under vacuum, the mixture forms a very tough film that is insoluble in acetonitrile or hot DMF, does not melt up to 300°, and is resistant to attack by chromic acid. The IR spectrum is identical to that of XVIII crossiinked BF3 or by irradiation in the presence of TCPA. Apparently as the solution becomes highly concentrated, crosslinking occurs by thermal excitation of charge-transfer complexes of TCPA and XVIII. The IR evidence and the insolubility behavior appear to discount an alternative possibility of simple association of polymer chains facilitated by the presence of the acceptor compound. The spontaneous crossiinking is also observed with the acceptors chloranil and 1,3,5-trinitrobenzene, and presumably would occur with others. 

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