CHMA2000 EXPT 7: The Physical and Chemical Properties of Alcohols Objectives: At the end of this experiment you should be able to: 1. Understand the physical and chemical properties of alcohols 2. Understand the classification of alcohols 3. Prepare an alcohol derivative 4. Perform a micro scale boiling point determination 5. Identify an unknown alcohol based on various tests Prelab Exercises: Before coming to lab you should: 1. Read about alcohols and the experiment and its background 2. Prepare a flow chart outlining the lab exercises 3. Research the various structures of the alcohols used in this experiment. Prepare a table with their classifications. 4. Answer the questions at the end of the report. This will become part of the final report The alcohols differ widely from their corresponding hydrocarbons in both physical and chemical properties. For example, the proton of the OH group is sufficiently acidic to hydrogen bond with oxygen of another alcohol molecule. This leads to the formation of loosely bonded molecular aggregates. Consequently, alcohols have much higher boiling points and greater heats of vaporization than their hydrocarbon counterparts. Intermolecular hydrogen bonding also takes place with water molecules and leads not only to water solubility for the alkanols through C 5, but in some instances results in the formation of constant boiling mixtures named azeotropes. Thus 95% ethyl alcohol has a constant boiling point and will distill unchanged in composition through even the most efficient distillation apparatus. We may summarize some of the chemical properties of the alcohols as follows: a) Reactions involving the acidic hydrogen b) Replacement (substitution) of the hydroxyl group c) Oxidation d) Elimination (to yield an alkene)
From a classification standpoint, there are three main classes of alcohols, namely: primary, secondary and tertiary. [Make sure you know what this terminology means.] All alcohols possess some acid properties but primary alcohols are the most acidic and tertiary alcohols the least acidic. One of the easy ways to show the presence of acidic hydrogens is to cause a displacement with an active metal to yield hydrogen gas. Since alcohols are such weak acids they react at room temperature with only the alkali metals. The alcohols also undergo replacement of their OH group with the appropriate reagents. The tertiary alcohols undergo these reactions most easily while the primary alcohols are the least reactive toward these reagents. Since dehydration to an alkene (olefin) involves loss of the OH functional group, tertiary alcohols yield olefins most readily; even mild reagents such as iodine serve as a catalyst. On the other hand, 1 o alcohols are more resistant to elimination reactions and will require more drastic reagents (e.g. H 2 SO 4 ) The three classes of alcohols also differ in ease of attack by oxidizing agents. Tertiary alcohols are most resistant, as oxidation of the alcohol group would involve cleavage of the C-C bond and thus only occurs under the most drastic conditions. In contrast, 1 o and 2 o alcohols are oxidized with reasonable ease to carbonyl compounds. You will work in pairs for this experiment. Each pair will determine the identification of an unknown alcohol and will complete the exercises outlined to understand the physical and chemical properties of alcohols. A. Physical Properties: Examine methanol, ethanol, ethylene glycol, 1-pentanol and your unknown alcohol for odor and solubility in distilled water (3-4 drops per ml of distilled water). Record your results. What do they mean? B. Variation of Solubility with Branching of the Carbon Chain Place 2mL of distilled water in a clean test tube and add 1-butanol drop wise while shaking the tube until insolubility is observed (use a maximum of 20 drops). Repeat this experiment using tertiary butyl alcohol (2-methyl -2-propanol) comparing the number of drops needed with each. What is the effect on solubility from branching of the carbon chain? Now try your unknown alcohol. Record your results. What do they mean?
C. Acidity of the Hydroxylic Hydrogen Atom Sodium, potassium and other active metals can replace the hydrogen atom of the hydroxyl group in alcohols. The evolution of hydrogen gas, as evident by visible bubbling, confirms that a reaction is taking place. The apparatus MUST BE SCRUPULOUSLY DRY since water also has active hydroxylic hydrogen atoms; and so reacts with these reagents too. Place 2mL of the following in separate DRY test tubes. Tube 1: 1-butanol Tube 2: 2-propanol Tube 3: 2-methyl-2-propanol (tertiary butyl alcohol) Tube 4: unknown alcohol To each tube add a very small piece of pure metallic sodium (obtain from the instructor). Keep away from fellow students and eyes! Observe the rate of evolution of hydrogen gas. Give equations for the reactions. What conclusions may be drawn regarding the acidity of the three classes of alcohols? Now add 2 drops of phenolphthalein indicator (colorless in acid; pink in alkali) to the solutions above. Record your results. What do they mean? D. Combustion Working in a fume hood, burn a few drops of the following on a watch glass. Make sure that the glass is cleaned after each experiment. Note and record all results. 1. ethanol 2. 1-pentanol 3. benzyl alcohol 4. unknown alcohol Write balanced equations for the reactions that took place. E. Lucas Test The Lucas reagent (ZnCl 2 in concentrated HCl can cause severe burns, and inhaling the vapors is harmful. Use gloves and a do the experiment in a fume hood; avoid contact with skin and do not breathe the vapors.) consists of zinc chloride dissolved in concentrated HCl. The test is based on the different rates at which primary, secondary and tertiary alcohols are converted into insoluble chlorides. The general equation is: R ZnCl OH + HCl 2 R Cl + H 2 O
Primary alcohols are not converted to chlorides for several hours at room temperature. Secondary alcohols dissolve to give a clear solution, then form chlorides that make the solution cloudy in about five minutes. If R has too many carbon atoms in the chain (more than C 6 ) this often interferes with the reaction. Tertiary alcohols react instantly, giving an insoluble alkyl chloride which appears as a cloudy dispersion or sometimes as a separate layer. Place 1mL of Lucas reagent in each of 5 test tubes, and then add 3 drops of the following alcohols: Tube 1: 1-butanol Tube 2: 2-propanol Tube 3: cyclohexanol Tube 4: 2-methyl-2-propanol Tube 5: your unknown alcohol Record the length of time that is required for the mixture to become cloudy or separate into two layers. Relate your observations to the structure of the alcohols. F. Determination of the Boiling Point of Your Unknown Alcohol Place 1-2mL of your unknown alcohol in a test tube. To this tube add a melting point tube (open end down). Secure the assembly to a thermometer by means of a rubber band. Attach this CAREFULLY and securely to a ring stand. Place the thermometer and tube into a beaker containing glycerol (see Figure #1). Gently heat the glycerol on a hot plate until you see a rapid, continuous stream of bubbles emerging from the melting point tube. Immediately remove the heat source and let the bath cool slowly until the bubbling stops: then record the temperature when the liquid just begins to enter the melting point tube. Allow the temperature to drop a few degrees so that the liquid partly fills the melting point tube. Heat again very slowly until the first bubble of vapor emerges from the mouth of the melting point tube. Again record the temperature. The two temperatures represent the boiling point range. They should be within a degree or two of one another. Record the barometric pressure so that you can make a pressure correction. Compare this range to the alcohols listed in Table #1.
Figure #1: Boiling point assembly G. Preparation of Alcohol Derivative: A derivative is a solid crystalline material that is prepared from a compound by a standardized procedure. The methodology used is well tried and tested over decades or even centuries. The purpose of preparing a derivative is so that a second physical property (in this case the mp) can be determined. By having this second temperature (along with the bp of the alcohol in this experiment) it is usually possible to narrow down the identity of an unknown compound to a few possibilities or even a single compound. The prepared product after purification (usually by recrystallization) will give a very accurate melting point. For alcohols, the most commonly prepared derivative is the 3,5-dinitrobenzoate ester. The following shows a balanced equation for the reaction. Can you identify the functional groups in the reactants and organic product? O O C Cl + ROH C O R + HCl 3,5-dinitrobenzoyl chloride 3,5-dinitrobenzoate ester To a clean DRY test tube add 0.30 grams of 3,5-dinitrobenzoyl chloride. Why does the test tube need to be dry? To this add approximately 0.5-1.0 ml of your unknown alcohol. Gently heat this mixture in a water bath. Make sure that the mixture does not boil too vigorously or you will lose your entire sample and you will have to start again. Gently heat the solution for a minimum of 15 minutes. Remove the solution from the heat and allow the mixture to cool. SLOWLY add 2mL of 5% sodium carbonate and 2mL of water and mix vigorously using a vortex mixer. At
this point you should see solid material forming. Place the test tube in an ice bath for a minimum of 5 minutes. Collect the product (alcohol derivative) by vacuum filtration and wash with cold water. Recrystallize the derivative from ethanol-water. To do this, place the solid product in a test tube and add small portions (0.5-1.0mL) of hot ethanol to it until the solid dissolves. It is suggested that you place the tube in a hot water bath after the addition of each small portion of ethanol (this will increase the rate at which the derivative will dissolve). Once the derivative has completely dissolved slowly add cold water dropwise until cloudiness persists. Reheat the mixture and add a few more drops of water until you again get a persistent cloudiness. Then place the tube in an ice bath for a minimum of 10 minutes. Filter the recrystallized product using vacuum filtration. Dry a small portion of the purified derivative on a ceramic tile and determine the melting point. Compare the melting point to those listed in Table #1. Place the rest of your derivative in a glass sample vial and label it appropriately. Identify your unknown alcohol.
Table #1: Derivatives of Alcohols Boiling point ( o C) Alcohol MP of 3,5-dinitrobenzoate ( o C) 65 methanol 109 78 ethanol 94 82 isopropanol (2-propanol) 122 83 tert-butyl alcohol (2- methyl -2-propanol) 101 97 1-propanol 75 97 allyl alcohol 50 100 sec-butyl alcohol (2-butanol) 76 102 tert-amyl alcohol (tert-pentyl alcohol) 117 108 isobutyl alcohol (2- methyl -1-propanol) 88 116 3-pentanol 100 118 1-butanol 64 119 2-pentanol 61 129 ethylene chlorohydrin 92 130 isoamyl alcohol (3-methyl-1-butanol) 62 138 1-pentanol 46 141 cyclopentanol 115 156 1-hexanol 58 161 cyclohexanol 113 170 furfuryl alcohol (2-Furylmethanol) 81 176 1-heptanol 48 176 2-octanol 32 177 tetrahydrofurfuryl alcohol (Tetrahydro-2-84 furanmenthanol) 182 phenol (carbolic acid) 146 194 1-octanol 62 195 ortho-cresol (1-Hydroxy-2-methylbenzene) 138 205 benzyl alcohol 113 214 1-nonanol 52 231 1-decanol 57 Last updated May, 2016. J. Hoyle & M. Tate, 2016