Chromic acid test

CAUTION. Ethers that have been stored for long periods, particularly in partly-filled bottles, frequently contain small quantities of highly explosive peroxides. The presence of peroxides may be detected either by the per-chromic acid test of qualitative inorganic analysis (addition of an acidified solution of potassium dichromate) or by the liberation of iodine from acidified potassium iodide solution (compare Section 11,47,7). The peroxides are nonvolatile and may accumulate in the flask during the distillation of the ether the residue is explosive and may detonate, when distilled, with sufficient violence to shatter the apparatus and cause serious personal injury. If peroxides are found, they must first be removed by treatment with acidified ferrous sulphate solution (Section 11,47,7) or with sodium sulphite solution or with stannous chloride solution (Section VI, 12). The common extraction solvents diethyl ether and di-tso-propyl ether are particularly prone to the formation of peroxides. 

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.)... 

Chromic acid test. Place into separate clean, dry test tubes (100 x 13 mm), labeled as before, 5 drops of sample to be tested. To each test tube add 10 drops of reagent grade acetone and 2 drops of chromic acid. Place the test tubes in a 60°C water bath for 5 min. Note the color of each solution. (Remember, the loss of the brown-red and the formation of a blue-green color is a positive test.) Record your observations on the Report... 

Chromic acid test. Aldehydes are oxidized to carboxylic acids by chromic acid ketones are not oxidized. A positive test results in the formation of a blue-green solution from the brown-red color of chromic acid. 

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. 

The answer is B. Chromic acid test is a qualitative test that can be used to confirm whether a given unknown compound is an aldehyde or not. Even though aldehydes give a positive response to this test, ketones do not. A sample reaction is shown below ... 

Many chromium (VI) compounds are suspected carcinogens. If you would like to run this test, talk to your instructor first. Most often, theTollens test will easily distinguish between aldehydes and ketones, 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. 

There are numerous other tests used to detect the aldehyde functional group. Most are based on an easily detectable oxidation of the aldehyde to a carboxylic acid. The most common tests are the Tollens, Fehling s, and Benedict s tests. Only the Tollens test is described in this book. The Tollens test is often more reliable than the chromic acid test for aldehydes. 

Cerium(IV) test Acetyl chloride Lucas test Chromic acid test Iodoform test... 

Test for an aldehyde. Perform the chromic acid test outlined in Section 25.7D. 

Flush the aqueous solution remaining in the distillation flask down the drain. Do the same with the aqueous steam distillate once you have completed the extraction. Pour the solution from the test for unsaturation into the container for halogen-containing liquids. Neutralize the solution for the chromic acid test and then pour it into the container for hazardous heavy metals. 

Another method for distinguishing between aldehydes and ketones is the chromic acid test. Because chromic acid is unstable when stored for extended periods of time, the test reagent is prepared as needed by dissolving chromic anhydride in... 

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