CHM 112 Lab Procedure 3:
ALCOHOLS, ALDEHYD ES, AN D K ETONES ETONES Objectives: (a) to use different chemical tests to differentiate the alcohol, aldehyde, and ketone functional groups. (b) to determine the functional group present in an unknown solution based upon those chemical tests
Introduction: There are several tests that may be performed to distinguish among the different classes of alcohols, aldehydes, and ketones. All these tests tests exploit differences in the type and degree degree of reactivity reactivity of each of the functional groups. You will check the reaction characteristics of five different functional groups (primary alcohols, secondary alcohols, tertiary alcohols, aldehydes, and ketones), and an unknown, and, based upon reaction characteristics, determine the functional group of the unknown. The first step is to identify the structure of each of the functional groups. •
Primary alcohols (1 alcohol) have the group –CH group –CH 2 OH , where the carbon atom with the alcoholic hydroxyl group has at least two additional hydrogens attached to that carbon. The alcoholic hydroxyl group is attached to a primary carbon. Think of a primary alcohol (1°) as one where the carbon with the hydroxyl group is attached to one R group (R stands for any other organic chain or group).
H R
C
OH
H •
Secondary alcohols (2 alcohol) have the group –CHOH , where the carbon atom with the alcoholic hydroxyl group has only o n e additional H atom attached to it. There are two R groups (R stands for any other organic chain or group), and the alcoholic hydroxyl group is attached attached to a secondary carbon. So, a secondary alcohol (2°) is one where the carbon atom with the hydroxyl group is also attached to two R groups.
H R
C
OH
R
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•
Tertiary alcohols (3 alcohol) have the group –COH, where the carbon atom with the alcoholic hydroxyl group has no additional H atoms attached to it. There are three R groups (R stands for any other organic chain or group), and the alcoholic hydroxyl group is attached to a tertiary carbon. So, a tertiary alcohol (3°) is one where the carbon atom with the hydroxyl group is also attached to three R groups. R R
C
OH
R
•
Aldehydes and Ketones are characterized by a functional group called a carbonyl group – a carbon atom double-bonded to an oxygen atom. O C
Carbonyl group
The difference between aldehydes and ketones is in the groups attached to the carbonyl carbon atom. In the case of an aldehyde, there is always at least one H atom attached to the carbonyl carbon atom. An aldehyde has one R group attached. In the case of ketones, there are no H atoms attached to the carbonyl carbon. The ketone has two R groups attached.
H
O
O
C
C
R
R
R Ketone
Aldehyde
(To keep this straight, you might want to think of the word “alde Hyde.” It has an “H” in the name and an H atom attached to the carbonyl carbon atom. Ketone is the one without an “H” in the name and without an H atom in the carbonyl structure.) Now that the structures have been identified, there are several chemical tests that are used to distinguish these functional groups. A. Differences in oxidation: Several different oxidizing agents may be used to cause the oxidation of molecules. Potassium permanganate (KMnO4) or potassium dichromate (K 2Cr2O7) or even atmospheric oxygen (O 2) are oxidizing REV 02/05
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agents. The symbol [O] is commonly used to mean any oxidizing agent. It’s the action of the oxidizing agent, not its identity that is important here. One way to look at oxidation is that oxidation is the reaction that increases the number of C-O bonds and/ or decreases the number of C-H bonds. Therefore, reduction is the opposite and is a reaction that decreases the number of C-O bonds and/ or increases the number of C-H bonds. a. A primary alcohol oxidizes readily, first to an aldehyde, then to a carboxylic acid. These two oxidation steps make sense because the primary alcohol functional group has two C-H bonds that can be broken. O H O [O] [O] C R C OH C H R R OH
H
Aldehyde
Primary alcohol
Carboxylic acid
b. A secondary alcohol only has only one C-H bond that can be broken, so it can only oxidize once, to a ketone. O
R R
C
OH
[O]
C R
R
H
Ketone
Secondary alcohol c. A tertiary alcohol has no C-H bonds that can be broken, so it is NOT oxidized, no matter how strong the oxidizing agent may be. R R
C
[O]
OH
NR (no reaction)
R Tertiary alcohol
d. From this information on alcohols, it is clear that aldehydes will oxidize to carboxylic acids. An aldehyde has one C-H bond that can be broken. Think back to what happens to 1° alcohols. The aldehyde produced by the oxidation of a primary alcohol can be again oxidized to the carboxylic acid, and therefore, any aldehyde can be oxidized to a carboxylic acid. REV 02/05
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O
O [O]
C
C
H
R
R
Aldehyde
OH
Carboxylic acid
e. Any ketone, just like the product of the oxidation of a 2 ° alcohol, has no C-H bonds that can be broken, so ketones cannot be oxidized. O
[O] NR (no reaction)
C R
R Ketone
This lab experiment will use a solution of potassium dichromate (K 2Cr2O7) as the oxidizing agent. The starting solution of potassium dichromate is yellow-orange, but if any oxidation takes place, the solution will turn blue-green. This color change is due to the Cr+6 ion (orange in K 2Cr2O7) being reduced to the Cr +3 ion (blue green). B. The Lucas test for alcohols: This test distinguishes among the three types of alcohols (1°, 2°, and 3°), by the speed with which they react. The reaction is a replacement reaction, where a Cl from HCl (mixed with ZnCl 2) replaces the hydroxyl group on the alcohol. It should be noted that, because aldehydes and ketones do not have a hydroxyl group to replace with Cl, they would both give a negative Lucas test.
ZnCl2 C
OH
+ HCl
C
Cl
Cloudy from the insoluble chloride
Clear solution
Surprisingly enough, 3° alcohols react very fast with the Lucas reagent (HCl mixed with ZnCl2), and will turn cloudy almost immediately. Secondary alcohols do react, but more slowly and must be heated in a water bath in order to react and turn cloudy. Primary alcohols react so slowly (even in a warm water bath) that NO REACTION is observable. A yellow or yellow-orange color without any precipitate is a negative result.
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C. 2,4-Dinitrophenylhydrazine test: This test uses an organic reactant, 2,4dinitrophenylhydrazine, to distinguish the carbonyl compounds (aldehydes and ketones) from the non-carbonyl compounds (the alcohols). The 2,4dinitrophenylhydrazine reagent is a translucent yellow solution, but in the presence of a carbonyl compound, a yellow-colored precipitate will form. In the presence of an alcohol, the solution will remain translucent yellow, forming no precipitate.
O2N
H2N
O2N
O
HN
NO2
C
+
C
N
HN
NO2
+
Carbonyl group
2,4-dinitrophenylhydrazine
Cloudy from the insoluble product
O2N
H2N
HN
NO2
2,4-dinitrophenylhydrazine
C
+
OH
No Reaction -- NR
clear solution
Any alcohol
D. Fehling’s test: This test is very specific for aldehydes; only aldehydes will give a positive result. Any alcohol or ketone will give a negative result. It is based upon the ability of the aldehyde group to reduce the Cu +2 ion of Cu(OH)2 (which is blue) to the Cu +1 ion of Cu2O (which is dark red) in the presence of a base. O
O + 2Cu(OH)2 + NaOH
C R
C
H
R
Aldehyde
Any other group (alcohols, ketones)
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+ Cu 2O + 3H 2O O-
Na+
Carboxylate ion
+ 2Cu(OH) 2 + NaOH
5
NR -- No reaction
H2O
Here is a summary table of all the reactions: Reaction
Chemical
Positive result (seen)
Negative result (seen)
Groups that give positive result
Potassium dichromate Oxidation Lucas test (to distinguish alcohols)
K 2Cr2O7
Orange to green, blue or blue/green Clear to cloudy
Remains orange
2,4dinitrophenylhydrazine test (aldeydes, ketones from alcohols) Fehling’s Test (aldehydes from ketones)
2,4dinitrophenylhydrazine
Form yellow/ orange solid
No solid formed
1° and 2° alcohols, aldehydes Secondary alcohol- slow Tertiary alcohol-fast Aldehydes, Ketones
Cu(OH)2 and OH-
Blue to something else (red, green yellow)
Remains blue
Aldehydes
ZnCl2 and HCl
Remains clear
Groups that give negative result Ketones, 3° alcohol Primary alcohol, Aldehydes, ketones All alcohols
Ketones All alcohols
Experimental Procedure: You will perform each test indicated on 6 solutions (an aldehyde, a ketone, a primary alcohol, a secondary alcohol, a tertiary alcohol and the assigned unknown), and fill the results in on the tables provided on the Report Sheet. Based on the results of the tests with the known solutions and the unknown, you deduce which functional group (not which compound) is in the unknown. As part of the report , write a paragraph explaining the reasoning used to identify the functional group present in the unknown. This paragraph should discuss the tests performed, the results of each test, and how the results confirmed or ruled out certain functional groups, allowing identification the functional group in the unknown. There are also four questions at the end of the report to be answered. Procedure: Use clean test tubes for all these tests – w ash them out between tests so that cross-conta min ation does not occur. DO NOT HEAT STOPPERED TUBES -- REM OVE STOPPER S BEFORE HEATIN G! DO NOT rinse out tubes with acetone! Since acetone is a known solution in this procedure, rinsing out tubes w ith acetone will contamin ate the tubes and invalidate results. Wash tubes out with soap and water, using deionized w ater as a final rinse. Tubes do not need to be dry for these procedures. Except for testing the assigned unknown solution (which you must do individually), you may work in pairs for this experiment unless directed otherwise by your instructor. REV 02/05
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Before starting the procedure, fill a 600-mL beaker slightly less than halfway with tap water to begin heating for the water baths needed on two parts of the procedure Remem ber the definition of “precipitate”: it isn’t just particulate or “clumpy” particles; a change in the cloudiness (becoming m ore cloudy) is also indicative of a precipitate – the cloudiness represents very fine particles that do not settle out! Use the bottle marked “ethanol” as a know n primary alcohol solution; use the bottle marked “95% ethanol” as the solvent in the 2,4dinitrophenylhydrazine test (given in the directions for that test). A. Oxidation by K 2 Cr 2 O 7 : Place 3 mL (about 60 drops) of 5% aqueous K 2Cr2O7 in a 50 mL beaker. Carefully add 1 mL (about 20 drops) of concentrated sulfuric acid (if a precipitate appears at this point, carefully swirl until it dissolves). Slowly (a few drops at a time with mixing) add 2 mL of the solution to be tested. If the color of the solution changes to green or blue-green, then oxidation has taken place (the Cr+6 ion has been reduced to the Cr +3 ion) and the reaction is positive; if no color change takes place, then the reaction is negative. This procedure must be performed for each of the 5 known solutions and the unknown, and the results recorded in your notebook for the Report Sheet. When you have completed these tests, be sure to place the chromium waste into the proper waste bottle. Then rinse out beaker with w ater (tap w ater is acceptable) and add this to the waste bottle. Chromium is very toxic, and should NEVER go down the sink! B. Lucas test: Place 10 drops of the solution to be tested into a labeled test tube. Add approximately 30 drops of Lucas reagent. Stopper with a cork and shake vigorously for a few seconds. Allow mixture to stand at room temperature and observe how long it takes for any precipitate (cloudiness) to form. If no cloudiness is observed after 15 minutes, warm the unstoppered test tube in a 60-75 °C water bath for 15 minutes. Observe the results. Record the results, including the amount of time taken for the reaction to occur, and temperature at which the reaction occurred. Sample results might include: immediate cloudiness (immediate positive reaction), cloudiness after 5 minutes at room temperature (slightly delayed positive reaction), cloudiness after 3 minutes in 60 ° C water bath (delayed positive reaction), no cloudiness after 15 minutes in 60 °C water bath (negative reaction). A yellow or yellow-orange color without any precipitate is a negative result. This procedure must be performed for each of the 5 known solutions and the unknown, and the results recorded in your notebook for the Report Sheet. C. 2,4-dinitrophenylhydrazine test: Place 5 drops of the solution to be tested into a labeled test tube. Add 10 drops of 95% ethanol (as a solvent) and then add 5 drops of the 2,4 dinitrophenylhydrazine reagent to the tube and mix. Observe the results. A positive result is the formation of a yellow/orange precipitate. A negative test result is the absence of a precipitate, and the persistence of the translucent REV 02/05
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solution. This procedure must be performed for each of the 5 known solutions and the unknown, and the results recorded in your notebook for the Report Sheet. When you have completed these tests, be sure to place the 2,4dinitrophenylhydrazine w aste into the proper waste bottle. Then rinse out tubes with w ater (tap water is acceptable) and add this to the waste bottle. 2,4-dinitrophenylhydrazine is very toxic, and should NEVER go down the sink! D. Fehling’s test: Place 15 drops of Fehling’s reagent (Part A) and 15 drops of Fehling’s reagent (Part B) into a labeled test tube and mix. Add 10 drops of the solution to be tested; stopper the tube and shake for a few seconds. Remove the stopper and place the tubes in a boiling water bath for five minutes. Observe the results. A positive result is the formation of a brick red precipitate. A negative result is the absence of a precipitate, and the persistence of the blue solution. This procedure must be performed for each of the 5 known solutions and the unknown, and the results recorded in your notebook for the Report Sheet. To figure out your unknown you may construct a table (like this one) in your notebook to help keep track of results filling by “positive” or “negative” depending upon your results in lab. If you use a table like this, put it in your notebook and fill in results there. Don’t fill out this sheet and NOT put this information in your notebook. Test: Chromate Lucas test: 2,4Fehling’s test oxidation dinitrophenylhydrazine 1° alcohol (ethanol) 2° alcohol (2propanol) 3° alcohol (2-methyl2-propanol) Aldehyde (formaldehyde) Ketone (acetone) Your unknown:
The pattern of positive and negative results for your unknown should match the pattern for one of the known solutions to identify the functional group of your unknown. If no patterns match, an error in testing/reporting was made, either with one of the known solutions or the unknown solution in one of the tests. You can determine the error and repeat the erroneous tests. Your instructor can help you determine which tests to repeat.
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Lab partner(s) ________________________ Date ______________________
Notebook points/initials __________ Name ___________________________
REPORT FOR LAB PROCEDURE 3: ALCOHOLS, ALDEHYDES, AND KETONES The unknow n assigned to you is number______ _____ ____ _ You will be testing six solutions (five with known functional groups and one will be an unknown functional group) with each of the four tests. Record all experimental data in your notebook; this report sheet is for your final report only.
Draw structures of these molecules which illustrate the FIVE known functional groups tested here (draw the structure above the name).
(A) ethanol (1º alcohol)
(B) 2-propanol (2º alcohol)
(D) formaldehyde (aldehyde)
(C) 2-methyl-2-propanol (3º alcohol)
(E) acetone (ketone)
Test #1: Oxidat ion by K 2 Cr 2 O 7 Observation of test results
Conclusion
(Did the color or odor change? Did the solution become cloudy or clear? How fast did any changes occur?)
(is test positive or negative?)
Solution tested: ethanol
2-propanol
2-methyl-2-propanol
formaldehyde
acetone
assigned unknown
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Test #2: Lucas test Observation of test results
Conclusion
(Did the color or odor change? Did the solution become cloudy or clear? How fast did any changes occur?)
(is test positive or negative?)
Solution tested: ethanol
2-propanol
2-methyl-2-propanol
formaldehyde
acetone
assigned unknown
Test #3: 2,4-Dinitrophenylhydrazaine test Observation of test results (Did the color or odor change? Did the solution become cloudy or clear? How fast did any changes occur?)
Solution tested: ethanol
2-propanol
2-methyl-2-propanol
formaldehyde
acetone
assigned unknown
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Conclusion (is test positive or negative?)
Test #4: Fehling’s t est Observation of test results
Conclusion
(Did the color or odor change? Did the solution become cloudy or clear? How fast did any changes occur?)
(is test positive or negative?)
Solution tested: ethanol
2-propanol
2-methyl-2-propanol
formaldehyde
acetone
assigned unknown
The functional group of your unknow n is:_________ _________ Write a paragraph explaining how you arrived at the conclusion concerning the functional group of the unknown. This should include what tests you performed, the results, and how the results confirmed or ruled out certain functional groups, so that you could conclude the type of functional group in the unknown.
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Answer the following questions: 1. Is ethanol a primary, secondary, or tertiary alcohol? Write out the structures of ethanol and also any oxidation products of ethanol. If there is more than one oxidation product, give the structure of each of the products.
2. Is 2-propanol a primary, secondary, or tertiary alcohol? Write out the structures of 2-propanol and also any oxidation products of 2-propanol. If there is more than one oxidation product, give the structure of each of the products.
3. Is 2-methyl-2-propanol a primary, secondary, or tertiary alcohol? Write out the structures of 2-methyl-2-propanol and also any oxidation products of 2-methyl-2propanol. If there is more than one oxidation product, give the structure of each of the products.
4. 2-Propanol is the IUPAC systematic name of this alcohol. It has a common name by which it is much better known (You’ll see it in the grocery store or pharmacy). Give that common name.
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