Chemical Tests on Alcohol and Carbonyl Compounds

Transcription

1 hemical Tests on Alcohol and arbonyl ompounds Introduction In this experiment you will run several chemical tests on known compounds which are mainly alcohols or carbonyl compounds. The alcohols are either primary, secondary, or tertiary, depending upon how many other carbon atoms are bonded to the carbon bonded to the group. The carbonyl compounds are either aldehydes or ketones. nce you have tested the known compounds you will run the tests on an unknown compound which may be either one of the knowns or a different alcohol or carbonyl compound. You can then determine by analogy the information the tests tell you about the unknown. An Introduction to the Tests: 2,4-Dinitrophenylhydrazine Test: arbonyl compounds can be distinguished from alcohols using the 2,4-dinitrophenylhydrazine (2,4-DNP) test. Aldehydes and ketones react to form a precipitate but alcohols do not react. The color of the precipitate can vary in color from yellow to red depending on the degree of conjugation in the compound. The precipitate is called a 2,4-dinitrophenylhydrazone. The reactions involved are the following: 2 N N 2 N N 2 N N 2 N N 2 and can be, alkyl or aryl groups. 2 N N N N 2 2 onjugation occurs when an organic compound contains a series of alternating single and double bonds. Thus, an aldehyde or ketone is conjugated if the carbon in the carbonyl group is single bonded to a second carbon that is double bonded to a third carbon. An example is benzaldehyde: When testing your known and unknown compounds it is important to note both the color and time it takes to form a precipitate. hemical Tests on Alcohol and arbonyl ompounds, Page 1

2 The Lucas Test: Primary, secondary, and tertiary alcohols can be differentiated from each other using two tests: the Lucas test and the chromic acid test. The Lucas reagent is a mixture of hydrochloric acid and zinc chloride. It will react with tertiary alcohols immediately, with secondary alcohols within five to ten minutes, but it will not react at all with primary alcohols. The reactions are as follows: l Znl Alcohol " " l Znl 2 l 2 2 Alcohol 2 l Znl 2 No reaction. 1 Alcohol You can tell that the reaction is taking place when the solution becomes cloudy. The alkyl halide is not soluble in water so it forms a second immiscible phase, which is the cloudiness. The rates for the reactions are different because the stabilities of the intermediate carbocations are different. Tertiary alcohols form very stable tertiary carbocations, secondary alcohols form secondary carbocations of intermediate stability, and primary alcohols form very unstable primary carbocations. The Lucas test only works for alcohols with fewer than six carbon atoms because only these alcohols dissolve well enough in the Lucas reagent. This test should only be run on samples which you have previously shown to be alcohols. The hromic Acid Test: The chromic acid reagent is a mixture of potassium dichromate and sulfuric acid. It reacts with aldehydes and with primary and secondary alcohols, but not with ketones or with tertiary alcohols. Primary alcohols are oxidized to aldehydes which are further oxidized to carboxylic acids. Secondary alcohols are oxidized to ketones which do not react further. Thus, it isn t necessary to rely on the timing of the Lucas test to distinguish between secondary and tertiary alcohols. In addition, you can distinguish between aldehydes and ketones. The reactions are: r S 4 3 r 2 (S 4 ) Alcohol r S 4 3 r 2 (S 4 ) Alcohol r S 4 3 r 2 (S 4 ) Aldehyde hemical Tests on Alcohol and arbonyl ompounds, Page 2

3 The Tollens (or Silver Mirror) Test: The Tollens reagent is a mixture of the silver ion and ammonia in basic solution. It can be used to distinguish between aldehydes and ketones or alcohols since aldehydes are easily oxidized while the others are not. When aldehydes are oxidized with the silver ion, the silver is reduced to silver metal which plates out as a shiny mirror on the inside of a test tube. The reaction is: 2 Ag(N 3 ) Ag N 3 Aldehyde 2 Ag(N 3 ) No eaction. Ketone The Iodoform Test: The iodoform reagent is a mixture of iodine and potassium iodide dissolved in water. If a methyl group is adjacent to a carbonyl or a hydroxyl group that can be oxidized to a carbonyl it will react to form iodoform, I 3, which is a yellow precipitate. The reactions are: 3 + I I I I I I I 3 Formation of Iodoform Alcohol Ketone hemical Tests on Alcohol and arbonyl ompounds, Page 3

4 Procedure: Wear safety goggles and gloves for the entire experiment. 2,4-Dinitrophenylhydrazine Test: (This reagent is a suspected carcinogen;) un this test on all known and unknown compounds. Place 6 to 7 drops of 2,4-DNP reagent in one of the depressions of a spot plate. Add one drop of the compound being tested. Stir with a glass stirring rod. ecord whether a precipitate forms or not. If a precipitate forms record its color and the amount of time it takes to form. Dispose of the products in the proper disposal container. hromic Acid Test: (This reagent is a suspected carcinogen.) un this test on all known and unknown compounds. Use caution since this is a hazardous chemical and has a tendency to splatter on mixing. Place five to six drops of acetone on a spot plate followed by one drop of the liquid known or unknown. Stir with a glass stirring rod. Add one drop of chromic acid reagent. Stir and note if there is a color change. To assure that the acetone does not interfere with the test results because of contamination, run a blank test with acetone. (This is a test with the acetone and the chromic acid, but without any known or unknown compound.) Lucas Test: Don t do this test until you have determined which compounds are alcohols. un the test only on alcohols. Place about five drops of the alcohol to be tested in a micro test tube. Add to this about twenty drops of the Lucas reagent. Shake the mixture and allow the solution to stand. Note the time required for the solution to become cloudy and use this information to classify the alcohol as primary, secondary, or tertiary. Tollens (Silver Mirror) Test: Don t do this test except on compounds that you have already shown to be aldehydes or ketones. For each compound you want to test, place 1.0 ml of silver nitrate solution in a micro test tube. Add one drop of sodium hydroxide to the test tube. A brown precipitate will form. Add concentrated ammonia drop by drop with shaking until the precipitate dissolves. (This should take a few drops.) Add one drop of the sample to be tested, stir, and allow the mixture to stand for about 10 minutes at room temperature. If no silver mirror has formed, place the tube in a 40 water bath for about 5 minutes. ecord the results and dispose the products in the proper disposal container. Iodoform Test: (Iodine is one of the reagents used in this test. It is highly toxic and causes burns. un this test on all of the known and unknown compounds. For each compound you want to test place 2 drops of the compound that you are testing in a separate micro test tube. Add 5 drops of aqueous 10 % potassium hydroxide. eat the test tubes in a water bath and add dropwise with the KI/I 2 reagent until the solution is dark brown. (This should take about 20 drops.) Then add 10 % potassium hydroxide dropwise with stirring until the brown color fades. Let the tubes sit in the hot water bath for an additional two minutes, then allow the solutions to cool and note whether a yellow precipitate of I 3 has formed. hemical Tests on Alcohol and arbonyl ompounds, Page 4

5 Alcohol and arbonyl Lab eport Name Predictions: Before you start experimental work predict the results for the known compounds. Enter your predictions in ink, then have your instructor initial your results. ompound 2,4-DNP esults (olor & Time) hromic Acid Test (olor hange) Lucas Test (Time to turn cloudy) Tollens Test (Silver Mirror; yes or no) Iodoform Test (Yellow precipi-tate; yes or no) 1. acetone 2. acetophenone 3. tert-butyl alcohol 4. benzaldehyde 5. benzophenone 6. butanal 7. 1-butanol 8. 2-butanol butanone 10. cyclohexanone hemical Tests on Alcohol and arbonyl ompounds, Page 5

6 esults: ecord your observations as you do the tests and indicate what the tests indicate about each compound. ompound 2,4-DNP esults (olor & Time) hromic Acid Test (olor hange) Lucas Test (Time to turn cloudy) Tollens Test (Silver Mirror; yes or no) Iodoform Test (Yellow precipi-tate; yes or no) 1. acetone 2. acetophenone 3. tert-butyl alcohol 4. benzaldehyde 5. benzophenone 6. butanal 7. 1-butanol 8. 2-butanol butanone 10. cyclohexanone Unknown # hemical Tests on Alcohol and arbonyl ompounds, Page 6

7 Name Problems: 1. What functional group is present in your unknown? 2. Summarize the test results that led you to your conclusion. You should include in your summary reasons why you did or did not perform specific tests on your unknown 3. What other structural features did these tests reveal about the structure of your unknown? (e.g.primary alcohol or methyl ketone.) 4. Write equations for each reaction with your unknown that occurs. hemical Tests on Alcohol and arbonyl ompounds, Page 7

8 5. Draw a chemical structure for each of the known compounds. 6. Did any of your compounds give incorrect test results? If so, list them. hemical Tests on Alcohol and arbonyl ompounds, Page 8

9 5. omplete the following reactions. If no reaction occurs write N.. to indicate that. a. 3 + l Znl 2 b. + Ag(N 3 ) 2 + c I 2 + d. 2 N 2 N N N 2 + e r S 4 hemical Tests on Alcohol and arbonyl ompounds, Page 9