Investigating Organic Functional Groups

Transcription

1 Austin Peay State University Department of hemistry hem 1041 autions l and a are corrosive and toxic Purpose In this lab you will use traditional chemical tests to identify a variety of functional groups in known chemicals. You will use those positive results to identify the functional groups present in an unknown compound. Introduction hemists today are trained to use elaborate technologies and instrumentation to determine chemical structure and makeup. owever, many years ago, these technologies were not yet available, and there were very few ways for chemists to determine the composition of any chemical. hemists would use a series of tests to characterize the structure and makeup of a chemical. You will be doing some of these tests in this lab, in particular, ones that indicate functional groups present on a molecule. Functional groups can be defined as the parts of organic molecules that are prone to chemical reactions. Saturated hydrocarbons (carbon chain molecules with all single bonds and the maximum number of hydrogen atoms) are not especially susceptible to reactions, but chemicals that contain oxygen, nitrogen, halogens (Br, l, F, and I) and double or triple bonds between carbon atoms can readily react in specific conditions. Alkenes, alcohols, aldehyde, ketone, and alkyl halide compounds will be studied. Alkenes Alkenes are chemicals that contain a carbon-carbon double bond as the functional group. They react with the magenta-colored chemical potassium permanganate (KMn 4 ) to product manganese dioxide (Mn 2 ), which is a brown solid. In other words, you can determine if a chemical has a double bond by adding KMn 4 and looking for the development of the brown solid Mn 2. Example 1: xidation of 1-butene by KMn KMn Mn 2 +2K Alcohols An alcohol is any organic compound that has a hydroxyl functional group (-) bonded to a carbon atom, which is usually connected to other carbon or hydrogen atoms. Alcohols are oxidized with Jones reagent (chromic acid ( 2 r 4 ) and sulfuric acid ( 2 S 4 )) to give a carbonyl (=) and a green chromium compound. Positive identification of an alcohol will be evident when the addition of chromic acid results in a green solution. This reaction is quick, working on the order of 2-3 seconds. Aldehydes also react with chromic acid to produce the green chromium compound, but the reaction takes a bit longer, seconds or so. Revision S10 Page 1 of 7

2 Austin Peay State University Department of hemistry hem 1041 Example 2: Reduction of 2-propanol (iso-propanol) r r Ketones and Aldehydes Ketones and aldehydes both contain the same functional group, a carbon-oxygen double bond. In a ketone, the carbon atom is also attached to two other carbons. In an aldehyde, the carbon is bonded to another carbon and a hydrogen. Both ketones and aldehydes react with a chemical called 2,4-dinitrophenylhydrazine (2,4-DP) to form a yellow or orange solid within a few minutes. Example 3: Reaction of propanone (a ketone) with 2,4-DP ,4-DP Example 4: Reaction of propanal (an aldehyde) with 2,4-DP ,4-DP Alkyl alides Alkyl halides are alkanes that have a halide ion (such as chloride or bromide) bonded to carbon in the usual place of a hydrogen atom. alides in organic compounds can be identified by way of a Beilstein test. A copper wire is heated under a flame to from a layer of copper(ii) oxide (u) on the wire. Dip the coated wire in your compound and then place under the flame. If a halide is present in the organic compound, it reacts with copper(ii) to form a copper halide, which burns green in the flame. Example 5: Identification of methyl bromide by Beilstein test u(s) flame 3 Br u(s) ubr 2 Revision S10 Page 2 of 7

3 Austin Peay State University Department of hemistry hem 1041 Procedure Keep the resulting test tubes from your standard tests until the end of the experiment for comparison to your unknown compound. 1. Testing for alkenes: Set up two test tubes in the test tube rack. In tube one, put in 4 drops of hexane. In tube two, put in 4 drops of pentene. Add 2 ml of ethanol to each tube. Add two drops of the permanganate solution, stopper, and shake (be careful you know which tube is which!) Record the results on the data sheet. 2. Testing for alcohols: Set up two new test tubes in the test tube rack. In tube one, put in 1 drop of hexane, and in tube two put in one drop of ethanol. Add 1 ml of acetone to each tube followed by 1 drop of the chromic acid reagent. Gently swirl the tubes. Record the results on the data sheet. 3. Testing for ketones: Set up two test tubes in the test tube rack. In tube one, put in 2 drops of hexane, and in tube two, put in 2 drops of acetone. Add 2 ml of ethanol to each tube followed by 3 ml of the 2,4-DP reagent. Stopper and shake well. Let the solution stand for up to 15 minutes. You can go onto the next test in this time. Record the results on the data sheet. 4. Testing for aldehydes: You will be using two tests for aldehydes, the chromic acid test as well as the 2,4- DP test. Because you already know what the negative tests for these look like, you will only be doing the positive tests. Set up two test tubes in the test tube rack. In tube 1, put in 1 drop of hexanal, 1 ml of acetone, and 1 drop of chromic acid. Swirl the tube. In tube 2, put in 2 drops of hexanal, 2 ml of ethanol, and 3 ml of the 2,4-DP reagent. Stopper and shake the tube. Wait up to 15 minutes for a reaction in tube 2. ompare the tests to the positive and negative tests in 2 and 3 above. Record the results on the data sheet. 5. Testing for alkyl halides: Take a small length of copper wire and brush it clean using the sand paper. old it over a flame for about 30 seconds. Take it out of the fire and let it cool down. Dip the wire in hexane and hold over the flame. Repeat the procedure using methylene chloride. Record the results on the data sheet. 6. Testing your unknown: btain your unknown and perform the tests described in steps 1-5. Make careful observations and record your results on the data sheet. ompare to the results to those for the known chemicals to determine the functional groups present in your compound. Waste Disposal ollect all chemicals in the appropriate waste container. lean-up Wash all glassware with soap then rinse 3 times with tap water, and once with deionized water. Revision S10 Page 3 of 7

4 Austin Peay State University Department of hemistry hem 1041 Data Sheet 1 ame: Lab Partner: Test Sample bservations Alkene Test exane Propene Alcohol Test exane Ethanol Ketone Test exane Acetone Aldehyde Tests Reaction with hromic acid Reaction with 2,4-DP Alkyl alide Test exane Methylene hloride Revision S10 Page 4 of 7

5 Austin Peay State University Department of hemistry hem 1041 Data Sheet 2 Unknown umber: Test bservations Positive (+) or egative (-)? Alkyl alide Test Aldehyde Test Ketone Test Alcohol Test Alkene Test Revision S10 Page 5 of 7

6 Austin Peay State University Department of hemistry hem 1041 Post-lab Assignment ame: 1. Which functional groups are present in your unknown chemical? Explain your reasoning using the results of the tests performed on your unknown. 2. Using what you have learned in this lab, is there a way to identify a chemical that has an alcohol and an aldehyde functional group on it? If yes, which tests would you use and why? 3. Propose a way to identify a compound with both an aldehyde and a ketone functional group. Explain your answer. Revision S10 Page 6 of 7

7 Austin Peay State University Department of hemistry hem 1041 Pre-lab Assignment ame: 1. Draw the Lewis-dot structures of the compounds listed below that you will be using in this lab. ircle and name the functional group. a) exane d) Ethanol b) Acetone e) exanal c) Propene f) Methylene chloride 2. ame and circle the functional groups shown in the compounds below. Which test would you use for the following chemicals? 3 a) b) 3 Revision S10 Page 7 of 7