Chromic acid alkanes

Metallic oxidants, namely, chromic acid and potassium permanganate, may be used to oxidize alkanes to alcohols or ketones, but these reagents have only limited synthetic value. Alkaline KMnC>4 is rather ineffective, mainly because of the insolubility of alkanes in the aqueous solution of the reagent. Oxidations in acidic solutions such as aqueous CF3COOH,108 or the use of special reagents such as benzyltriethylammonium permanganate109 may give better results. 

The chromic acid test for primary and secondary alcohols exploits the resistance of tertiary alcohols to oxidation. When a primary or secondary alcohol is added to the chromic acid reagent, the orange color changes to green or blue. When a nonoxidizable substance (such as a tertiary alcohol, a ketone, or an alkane) is added to the reagent, no immediate color change occurs. 

Oxidations of alkanes and cycloalkanes are rare and not easy. The rates of oxidation with chromic acid of methyl, methylene, and methine... 

It is important, however, that not all the radicals formed are liberated in the solution, since the oxidation of (+)-3-methylheptane by chromic acid involves the formation of (+)-3-methyl-3-heptanol with retention of configuration to the extent of 70-85%. The normal selectivity has been observed in the hydroxylation of branched alkanes. 

Under vigorous conditions strong oxidizing agents such as chromic acid and permanganate attack alkanes, but the reaction is of little synthetic use for usually mixtures of products are obtained in low yield. The reaction was traditionally used in the Kuhn-Roth estimation of the number of methyl groups in an unknown compound. This depends on the fact that a methyl group is rarely oxidized (the relative rates of oxidation of primary, secondary and tertiary C—H bonds are 1 110 7000) and hence the amount of ethanoic acid formed can be quantified. 

Recall from Section 13.10 that chromic acid (H2Cr04) is astrong oxidizing agent used to oxidize primary or secondary alcohols. Chromic acid does not readily react with benzene or with alkanes ... 

Simple alkyl halides respond to the common characterization tests in the same manner as alkanes they are insoluble in cold concentrated sulfuric acid they are inert to bromine in carbon tetrachloride, to aqueous permanganate, and to chromic anhydride. They arc readily distinguished from alkanes, however, by qualitative analysis (Sec. 2.25), which shows the presence of halogen. 

Since the total collection of faeces corresponding to a given meal is difficult in aquatic animals, apparent digestibilities are measured indirectly by incorporating an inert marker, usually chromic oxide, into the feed. Recently, other markers such as acid-insoluble ash, crude fibre, polyethylene, alkanes, cholestane or rare earth metals have been used successfully. 

Internal or external indicators may be used. Internal indicators are natural constituents of the food such as lignin, acid-indigestible fibre or acid-insoluble ash (mainly silica). More recently, the long-chain hydrocarbons (n-alkanes, C25-C35) found in the waxy cuticle of leaves have been used as internal indicators, especially in grazing studies. External indicators are substances that are added to foods. Chromic oxide (Cr203)... 

---