Alcohols chromic acid test

Chromic acid test. This test is able to distinguish primary and secondary alcohols from tertiary alcohols. Using acidified dichromate solution, primary alcohols are oxidized to carboxylic acids secondary alcohols are oxidized to ketones tertiary alcohols are not oxidized. (Note that in those alcohols which are oxidized, the carbon that has the hydroxyl group loses a hydrogen.) In the oxidation, the brown-red color of the chromic acid changes to a blue-green solution. Phenols are oxidized to nondescript brown tarry masses. (Aldehydes are also oxidized under these conditions to carboxylic acids, but ketones remain intact see Experiment 31 for further discussion.)... 

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. 

Many chromium(VI) compounds are suspected carcinogens. K you would like to run this test, talk to your instructor first. The Lucas test will distinguish between 1°, 2°, and 3° alcohols, and you should do that test first. If you run the chromic acid test, be sure to wear gloves to avoid contact with this reagent. 

Detection and Determination of the Methyl Alcohol.—(A). Qualitative Test, (a) 25 c.c. of the spirit are slowly distilled and the first 3-4 c.c. of distillate mixed in a conical flask with 25 c.c. of water, 15-20 drops of dilute sulphuric acid and 3-4 c.c. of 1% chromic acid solution, and again distilled. The first 5-6 c.c. of distillate are discarded, the next 10 c.c. being treated with 1 c.c. of 4% phenylhydiazine hydrochloride solution, 0-5 c.c. of 4% ferric chloride solution and 2-3 c.c. of concentrated... 

Jones Oxidation. Dissolve 5 mg of the unknown in 0.5 mL of pure acetone in a test tube and add to this solution 1 small drop of Jones reagent (chromic acid in sulfuric acid). A positive test is formation of a green color within 5 s upon addition of the orange-yellow reagent to a primary or secondary alcohol. Aldehydes also give a positive test, but tertiary alcohols do not. 

They are distinguished from alcohols and other oxygen-containing compounds by their failure to dissolve immediately in cold concentrated sulfuric acid, and from primary and secondary alcohols by their failure to give a positive chromic anhydride test (Sec. 6.30). 

Alphaeucaine hydrochloride gives a yellow precipitate when 5 cc. of a 1 per cent, solution are treated with three drops of a 5 per cent, solution of chromic acid. Cocaine hydrochloride does not. Betaeucaine hydrochloride when rubbed with dry mercurous chloride and then moistened with alcohol yields no color, cocaine hydrochloride when similarly treated turns grayish black. Holocaine hydrochloride gives in aqueous solution a violet precipitate with calcium hypochlorite, cocaine hydrochloride does not. Cocaine and procaine are distinguished by the potassium permanganate test (see Procaine Hydrochloridum). 

Since allyl alcohol is a primary alcohol, it will give a positive chromic anhydride test, whereas allylbenzene will not. Primary and secondary alcohols are oxidized by chromic anhydride, CrOa, in aqueous sulfuric acid. The clear orange solution turns blue-green and becomes opaque within two seconds in the presence of these alcohols. Hence, allyl alcohol and allylbenzene may be distinguished by the former s ability to give the color change in chromic anhydride. 

Although primary alcohols are first oxidized to aldehydes, the aldehydes are further oxidized to carboxylic acids. The ability of chromic acid to oxidize aldehydes but not ketones is taken advantage of in a test that uses chromic acid to distinguish between aldehydes and ketones (see Experiment 52D). Secondary alcohols are oxidized to ketones, but no further. Tertiary alcohols are not oxidized at all by the reagent hence, this test can be used to distinguish primary and secondary alcohols from tertiary alcohols. Unlike the Lucas test, this test can be used with all alcohols regardless of molecular weight and solubility. 

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). 

This test may be used to detect the presence of a hydroxy group, provided that it is shown previously that the molecule does not contain an aldehyde function. The reactions and experimental procedures for this test are given in Section 25.7C. Chromic acid does not distinguish between primary and secondary alcohols because they both give a positive test tertiary alcohols give a negative test. 

Note that a solution of chromic oxide in aqueous sulfuric acid (the Jones reagent) is used as a test reagent for 1° and 2° alcohols. A positive test is observed when the clear orange test reagent gives a greenish opaque solution upon addition of the alcohol (see Chapter 9). 

According to L. KuPberg, MikrochemUy 20 (1936) 244, the sensitivity of this test for chromates with benzidine may be doubled by using a freshly prepared mixture of equal parts of a 1 % alcoholic solution of benzidine and 20 % Hj02. Production of perchromate is responsible for this enhancement, since perchromic acid would convert the reagent into benzidine blue more completely than chromic acid. 

The color change from orange to green that accompanies this change in oxidation state allows chromic acid to be used as a test for primary and secondary alcohols (Section 12.4E). 

In organic chemistry, dilute solutions of hexavalent chromium can be used to oxidize primary or secondary alcohols to the corresponding aldehydes and ketones. Tertiary alcohol groups are unaffected. Because of the oxidation is signaled by a color change from orange to a blue-green, chromic acid is used as a qualitative analytical test for the presence of primary or secondary alcohols. 

As indicated in the Discussion, a solution of chromic oxide in aqueous sulfuric acid is used as a test reagent for 1° and 2° alcohols. 

Procedure. A small amount of the solid test material or a drop of its solution is treated in a micro test tube with several drops of chromic-sulfuric acid (preparation see Vol. II page 65). The open end of the test tube is covered with a disk of filter paper moistened with 2 N sodium hydroxide. The test tube is kept for 5 minutes in boiling water. The disk of reagent paper is then placed on a glass plate, spotted with several drops of permolybdic acid solution (for preparation see Vol. II page 68) and then dried with a current of cold air. If iodine is present, a brown fleck will appear but it fades within a few minutes. The stain is then treated with a drop of 0.1 % alcoholic solution of fluorescein. A positive response is indicated by the development of a red spot. 

The relative ease of oxidation of primary and secondary alcohols compared with the difficulty of oxidizing tertiary alcohols forms the basis for a convenient chemical test. Primary and secondary alcohols are rapidly oxidized by a solution of CrOs in aqueous sulfuric acid. Chromic oxide (CrOs) dissolves in aqueous sulfuric acid to give a clear orange solution containing Cr207 ions. A positive test is indicated when this clear orange solution becomes opaque and takes on a greenish cast within 2 seconds ... 

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