Reagent chromic acid

Secondary alcohols are easily oxidized to give excellent yields of ketones. The chromic acid reagent is often used for laboratory oxidations of secondary alcohols. 

The chromic acid reagent is prepared by dissolving sodium dichromate (Na2Cr207) in a mixture of sulfuric acid and water. The active species in the mixture is probably chromic acid, H2Cr04, or the acid chromate ion, HC1O4. Adding chromium trioxide (C1O3) to dilute sulfuric acid achieves the same result. 

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. 

In performing this test, make sure that the acetone used for the solvent does not give a positive test with the reagent. Add several drops of the chromic acid reagent to a few drops of the reagent acetone contained in a small test tube. Allow this mixture to stand for 3-5 minutes. If no reaction has occurred by this time, the acetone is pure enough to use as a solvent for the test. If a positive test resulted, try another bottle of acetone. 

Thus, the chromic acid reagent gives a clear-cut distinction between primary and secondary alcohols and aldehydes on the one hand and tertiary alcohols and ketones on the other. Aldehydes may be distinguished from primary and secondary alcohols by means of Schiff s, Tollens s, Benedict s (Sec. 23.4), and Fehling s tests, and primary and secondary alcohols of lower molar mass may be differentiated on the basis of their rates of reaction with concentrated hydrochloric acid containing zinc chloride—the Lucas reagent (Sec. 25.11B). 

If the chromic acid reagent is not available, prepare it as follows. Add 1 g of chromic anhydride (CrOg) to 1 mL of concentrated sulfuric acid and stir the mixture until a smooth paste is obtained. Dilute the paste cauf/ous/y with 3 mL of distilled water, and stir this mixture until a clear orange solution is obtained. 

In a 3-mL vial or small test tube, place 2 drops of a Uquid unknown (—10 mg if a solid) and 1 mL of spectral-grade acetone. Now add several drops of the chromic acid reagent. 

In small-scale syntheses, a wide variety of oxidants have been employed in the preparation of quinones from phenols. Of these reagents, chromic acid, ferric ion, and silver oxide show outstanding usefulness in the oxidation of hydroquinones. Thallium (ITT) triduoroacetate converts 4-halo- or 4-/ f2 -butylphenols to l,4-ben2oquinones in high yield (110). For example, 2-bromo-3-methyl-5-/-butyl-l,4-ben2oquinone [25441-20-3] (107) has been made by this route. 

Oxidation. Coumarin is not readily oxidized by chromic acid but, by action of the Fenton s reagent, it is converted into 7-hydroxycoumarin (umbeUiferone) [93-35-6] (28). 

Direct oxidation of ethylpyrazines to the corresponding acetylpyrazines may also be carried out in favourable circumstances using hot chromic acid (75JOC1178). Treatment of 2-ethyl-3-alkylpyrazines with chromic acid yields the corresponding 2-acetyl-3-alkyl-pyrazines in yields of 50-70%. In the absence of the 3-alkyl substituent the yields fall dramatically to less than 10%. Acetylpyrazines are more generally prepared by the inverse addition of a Grignard reagent to a cyanopyrazine. 

Lobinanidine is oxidised by chromic acid at 70-80° to lobinanine (XXIII). On more vigorous oxidation by this reagent benzoic acid is formed and also lobininic acid, CgIIj304N, identical with that obtained from lobinine. On hydrogenation it furnishes a-lelobanidine, which closely resembles the -lelobanidine from lobinine (see Table C), but is not identical with it. 

A 8 A solution of chromic acid is prepared by dissolving 26.72 g of chromium trioxide in a mixture of 23 ml of concentrated sulfuric acid and enough water to make the total volume of the solution 100 ml. Rapid dropwise addition of a slight excess of this reagent to an acetone solution (2 % or less) of the hydroxy steroid at room temperature or below with stirring usually results in complete conversion to ketone in less than 10 min. The product is isolated by dilution with water followed by filtration or extraction. 

The chromic acid oxidizing reagent is prepared by dissolving 13.4 g of chromium trioxide in 25 ml of water. To this solution is added 12 ml of concentrated sulfuric acid. An additional minimum quantity of water is added if necessary to dissolve any precipitated salts. 

We can control how far we oxidize by carefully choosing our reagents. If we want to go all the way up to a carboxylic acid, then we just use chromic acid ... 

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