Experiment: Alcohol Reactions Alcohols are important organic molecules characterized by an alkyl group covalently bonded to a hydroxyl group. They may be classified as primary, secondary, or tertiary, depending on the nature of the carbon to which the hydroxyl group is bonded. R-CH 2 OH R R CH OH R R C R OH 1 o alcohol 2 o alcohol 3 o alcohol Some reactions of alcohols vary depending on their classification as 1º, 2º, or 3º alcohols. In today s experiment you will carry out some simple chemical tests for alcohols, first with some knowns and then with an unknown. Solubility The hydroxyl group of an alcohol is capable of hydrogen bonding to water, but the alkyl group is not attracted to water. Thus, there are two opposing solubility forces. Also, the solubilities of isomeric alcohols vary with the extent of branching in the alcohol. Therefore, you will need to carefully record degrees of solubility of the known alcohol samples. From this data you will be able to draw conclusions about the effect of alkyl group length and branching on the solubility of alcohols. Chemical Tests 1. Reaction with Sodium Alcohols have hydroxyl hydrogens which are acidic, but only weakly so. Alcohols are less acidic than water. (What does this say about the relative basicities of alkoxide ions, RO -1, versus hydroxide ions?) In reacting with sodium, the alcohol is not really acting as an acid, but is forming an alkoxide ion as a product along with hydrogen gas. 2ROH + 2Na (s) 2RO -1 Na +1 + H 2 (g) This reaction is really a redox reaction in which Na is oxidized and the H is reduced. Your instructor will probably run this test on a known set of alcohols for you to observe the trends in rate of reaction. Some alcohols will react very quickly, with vigorous gas evolution. Others react very slowly. Record your observations and note any trends in reactivity as it relates to structures of the alcohols.
2. Lucas Test Alcohols will react with an aqueous ZnCl 2 -HCl solution to form alkyl chlorides. This reaction with water as a solvent favors S N 1 mechanisms. Therefore, you can predict the rate of reactions of primary, secondary and tertiary alcohols in this reaction. Because alkyl chlorides are insoluble in water, their formation can be detected by the appearance of cloudiness in the test tube. Thus, the time it takes for cloudiness to appear is a measure of the rate of this S N 1 reaction for various alcohol substrates. Record your observations carefully and compare your unknown results with those of the known samples to gain information about whether you unknown is primary, secondary or tertiary. 3. Oxidation Test Alcohols with a hydrogen attached to the same carbon as the hydroxyl group, in other words primary or secondary alcohols, can be oxidized by many oxidizing agents. C H OH oxidizing agent C O In this experiment you will use an basic solution of potassium permanganate, KMnO 4. The oxidation of the alcohol is accompanied by a reduction of the Mn +7 ion, first to Mn +6 and then further to Mn +4. The test shows visible evidence of reaction because the initial permanganate solution is purple. If oxidation-reduction takes place, the solution changes color to greenish Mn +6 and then becomes the brownish Mn +4. Record your observations for the known samples and compare them to your observations for this reaction done with your unknown alcohol. Unknown Alcohol In addition to running the above chemical tests to classify your alcohol as primary, secondary or tertiary, you will also need to take physical property measurements (BP, RI and density) and to run an NMR and IR to assist you in your identification. As with the other unknown labs, careful recording of observations and organization of your work is important to this lab. Make sure you come to lab with an understanding of the chemistry of the reactions you will run and are clear on the procedures throughout. Pre-lab Preparation Before coming to lab, you must do the following: 1. Read the lab thoroughly to understand the reactions involved and the procedures. 2. In your notebook, write the following:
a. Brief description of solubility trends in alcohols from your textbook or lecture notes. b. Balanced equation for the reaction of alcohols with sodium metal. c. Equations showing the oxidation reaction expected of primary, secondary and tertiary alcohols when treated with KMnO 4. You don t have to balance these equations. Just write the alcohol and potassium permanganate reagent and the correct organic product. 3. Review how to take a boiling point and refractive index. (Appendix 1 and 2). 4. Review IR and NMR operating procedures (Appendix 3). You will be running spectra of your alcohol unknown in this lab. 5. Explain how you could use NMR to determine whether your alcohol is primary, secondary or tertiary. Consider the chemical shifts as well as integration to answer this question. Experimental Procedure! Safety Considerations! Sodium metal is very reactive and requires special handling. Your instructor will demonstrate these reactions.! Potassium permanganate is a strong oxidizing agent. Use gloves to avoid skin contact.! The Lucas Reagent contains acid and irritants. Use gloves to avoid skin contact.! Use boiling stones; never plug sand baths directly into the wall outlet use Variac controllers.! Keep unknown liquids off of skin and avoid breathing the vapors.! Dispose of waste organics in the waste container provided, not in the sink. Carry out all reactions in the hood. Obtain a vial of alcohol unknown and record the number in your notebook. Run the following tests on the known samples indicated below and on your unknown. As you are doing the knowns and observing how they react, you should do your unknown with it so you can compare the behavior of your unknown to that of the knowns. If your compound gives a result similar to that of ethanol, this may be an indication that you have a primary alcohol. A cautionary note is in order here. Just because your unknown gives a result like ethanol doesn't mean that it is ethanol. Nearly all primary alcohols will react similarly in the chemical tests. Your results will only enable to you classify your alcohol, not to definitively identify it. Physical properties and spectra will allow you to complete the identification.
1. Solubility a. Using a test tube, add 1 ml of each of the following alcohols to 5 ml of water. Mix thoroughly with thrumping (tapping the bottom of your test tube with your fingers to swirl the liquids.) Draw conclusions about alcohol solubility in water relative to the length of the alcohol s alkyl group. Known alcohols: ethanol, 1-butanol, 1-pentanol. b. As above, add 1 ml of each of the following alcohols to 5 ml of water and mix. Draw conclusions about alcohol solubility in water relative to the branching of the alkyl group. Known alcohols: 1-butanol, 2-butanol and 2-methyl-2-propanol. c. Again, add 1 ml of your unknown to 5 ml of water and mix. Record your observations about the degree of solubility of your unknown. The results for solubility should not be simply soluble or insoluble, but rather degrees of solubility. In some cases, samples may partly dissolve, indicating slight solubility, which is different than completely insoluble. Notice that the CRC Handbook lists varying degrees of solubility. You should also try to do so in your data recording for this test. Note: It is important to realize that there are primary, secondary and tertiary alcohols that are soluble in water. At the same time, there are primary, secondary and tertiary alcohols that are insoluble. Be careful not to over-conclude regarding your unknown from your solubility data. Chemical Tests Sodium metal Your instructor will demonstrate the following reactions. Add a piece of sodium metal the size of a small pea to 5 ml of absolute ethanol in a test tube. Observe the rate of reaction. Repeat this procedure with 5 ml of 1-butanol, 2-butanol and 2-methyl-2-propanol. What organic product is formed from each of these reactions? What is the effect of branching on the reactivity of the alcohols? Lucas Test To a test tube, add 1 ml of 1-butanol. Repeat this for 2-butanol, 2-methyl-2-propanol and your unknown sample. To each of these tubes, add 3 ml of Lucas reagent (aqueous ZnCl 2 - HCl) and mix thoroughly. Watch for the appearance of cloudiness. Record the time required for each of the alcohols to become cloudy (indicating the formation of alkyl halides). If any samples have not become cloudy after 10 minutes, place them in a beaker of boiling water for 15 more minutes and check for cloudiness. Carefully record all observations.
Oxidation Test 2. Physical tests 3. Spectroscopy Prepare the oxidizing solution by adding 1 ml of 6% KMnO4(aq) to a test tube. Then add about 1 ml of 1-butanol followed by 2 ml deionized water. Shake well and set aside for 25-30 minutes. You can shake it or not during the waiting period. Observe any changes which take place. Repeat this for 2-butanol, 2-methyl-2-propanol and your unknown. Boiling point: Measure the boiling point of your unknown. If you do not recall the procedure, review Appendix 1. Density: Weigh a clean, dry 1.0-mL volumetric flask and stopper accurately to 4 decimal places. Carefully, using an eyedropper, add your unknown to the flask, until the flask is filled to the mark on its neck. Stopper the flask and weigh it. Calculate the density. Refractive index (n D ): Using the Abbe Refractometer, measure the refractive index of the unknown. If you do not recall the procedure, review Appendix 2 and ask your instructor to review the necessary steps with you. This will avoid errors in reading or damage to the instrument. IR Spectrum: Run an IR spectrum of your unknown. If you do not recall the necessary procedures, see Appendix 3 for background information and then check with your instructor before running the IR. NMR Spectrum: Prepare an NMR sample using your unknown and following the instructions in Appendix 3. With your instructors assistance, you will run an NMR spectrum. Again, if you do not recall the necessary procedures, see Appendix 3 for review. Students are not permitted to run an NMR spectrum without direct supervision. Post-Lab and Report Requirements 1. Solubility Trends: Describe the trends you observed in the water solubility testing of the known alcohols. What affect did alkyl group length have on the solubility and why? What effect did branching of the alkyl group have and why? 2. Known Reactions: Write equations or answer the questions listed below for the reactions that you carried out with known alcohols.
a. Reactions with sodium metal: You have already written the general reaction in your pre-lab. For this reaction, just describe any trends in reactivity that you observed for the alcohols tested. 1-butanol 2-butanol 2-methyl-2-propanol b. Reaction with Lucas Reagent: Write one balanced equation and then note any trends in reactivity. Explain why. 1-butanol 2-butanol 2-methyl-2-propanol c. Reaction with KMnO 4 : Describe any trends in reactivity. 1-butanol 2-butanol 2-methyl-2-propanol 3. Data and Discussion Requirements: See Appendix 5 for detailed instructions on the requirements for the unknown reports. You will be following these requirements for all unknown reports, so review this Appendix carefully to be certain that you include all of the required tables and discussion sections.