Experiment DE: Part II Fisher Esterification and Identification of an Unknown Alcohol Fisher Esterification of an Alcohol (Fraction A) On the Chem 113A website, under "Techniques" and "Videos" review the entries for Gravity Filtration, Extraction, Reflux and Simple Distillation (also see the two Figures at the end of this document). If the GC trace of your Fraction A from Part I shows only one alcohol peak, you may use this for Part II. If not, check with your instructor. Measure an IR spectrum of your chosen alcohol before proceeding. ESTERIFICATION Check out a pair of heat resistant gloves Place 10mL* of Fraction A into a 100mL round bottom flask. Calculate the moles of alcohol you actually used [it's OK if less than 10mL}. Since your alcohol is unknown, for purposes of stochiometry, calculate the number of moles used IF the alcohol were n-butanol (look up the density). This information must be included in your Table of Reagents and Products. Calculate a theoretical yield assuming you are using n-butanol and the actual number of moles you are using. Add twice this number of moles of glacial acetic acid (calculate the volume needed). Add this amount of glacial acetic acid to your flask. (*if you have less than 10mL of Fraction A, measure the actual volume and use this volume to determine the moles of alcohol you will use. Calculate twice the moles of glacial acetic acid. Mix these in your 100mL flask) Add approximately 1 ml (one pipet bulb volume) of concentrated sulfuric acid and three or four boiling chips. Set up for reflux (see Figure 1 below), using a thermowell supported on an iron ring as the heat source. Set the variable transformer to a setting of 3 (or 30). When bubbles appear, start timing for 60 minutes. During the reflux, if you see vapors leave the top of the condensor, turn down your heat setting, and check that water is flowing through your condenser. WORKUP After 60min, turn off the heat source. Wearing heat-resistant gloves, carefully loosen the ring clamp and lower the thermowell. Cool the reaction mixture to nearly room temperature, and decant into a 125 ml separatory funnel. FIRST** wash the organic layer with three 25mL portions of saturated sodium chloride. Then wash the organic layer with two 25mL portions of saturated sodium bicarbonate. **CAUTION: DO NOT wash first with sodium bicarbonate {why?}. If you see excessive amounts of solid appear during the workup, add a little DI water to help dissolve it. Use a spatula to break up any lumps if they clog the stopcock. However, don't keep adding water! A small amount of solid is not a problem and will be removed in the next step.
Place the organic layer (make sure you know which layer this is!) into a 50 ml Erlenmeyer flask and dry over anhydrous magnesium sulfate. Gravity filter directly into a clean, dry 100 ml round bottom flask (use no solvent to rinse). Add three or four boiling chips and set up for simple distillation (see Figure 2 below). ' Collect the first 1-2mL in a test tube and discard. Collect the remaining distillate directly in a tared vial as a single fraction, recording the temperature at the stillhead the boiling point may be a valuable clue for identification. Do not distill to dryness; stop when about 2-3 ml remains in the flask. Weigh and calculate % yield later when you determine the structure and know the molecular weight. Structure Determination of Your Ester Check out an NMR tube with CDCl 3 : Record the IR spectra of the ester product after distillation. Prepare an NMR sample of the ester (one to two or three drops (no more!) of ester in CDCl 3 with TMS in an NMR tube.) The instructor will arrange for a 300 MHz NMR spectrum to be measured for you. Interpret the NMR to determine the structure of the unknown alcohol that was used to prepare the ester. Your instructor will give you some general directions on how to approach this using the data you have collected. Recall that in Part 1, you used boiling points to give a preliminary identification of both alcohols in your mixture. Follow the same approach you used in Experiment C to identify an unknown structure. GC of your Original Alcohol Mixture Check out a GC syringe (must be returned same day) Just before you are ready to measure the GC of your mixture, bring a clean small test tube and the code of your original alcohol mixture to your instructor, who will give you a drop or 2 of your mixture. Add 1mL of GC dilution ether and cover tightly with Duraseal. Check out a GC syringe and measure the GC of your mixture. A reference GC of the 6 possible alcohols will be available. Use this information to determine the % composition of your original mixture. Using Gas Chromatography (GC) to Determine the Percent Composition of the Original 2- Component Mixture of Alcohols The detection method used by our GC is called "Flame Ionization Detection" or FID. Essentially, the sample is burned as it leaves the end of the GC column. A small electrical current results and this is measured as a signal by the detector. Each signal is translated as a peak by the plotter, and a value - the integrated "area" - is measured for every peak on the GC plot. However, different compounds generate a different amount of signal area, even if the amounts used are the same. In Experiment DE, 6 possible alcohols are used (from Table 1). A reference mixture has been prepared of equal amounts of all 6 alcohols. A reference GC will be measured, and you will see that the peaks and areas for each of these peaks are not exactly the same, even though the actual amounts in the mixture are the same. Thus, you will need to determine a correction factor for each peak in the reference mixture, which must be applied to the 2 peaks in your mixture in order to calculate the % composition for your unknown. Each Gas Chromatogram will have a trace of the peaks from the sample, followed by a table that lists parameters for each peak. Among the data for each peak are Retention Time (time in minutes after injection) and Area (a value which gives the integral under each peak). From one GC
measurement to another, the peaks for the same compound are proportional. However, within the same GC, peaks from different compounds are not necessarily proportional, so we need to make corrections before we can assign a % value to each peak. It is important to note that this method will not identify the structures of your alcohols. It is only used to determine % composition in the mixture. The Fractional Distillation will give you a preliminary ID of your alcohols based on the boiling points (note that some of the boiling points are close). The NMR analysis of the one ester you prepare will allow you determine the exact structure of this alcohol. Example of how to use GC data to determine % composition of a mixture Suppose you have the following data for a mixture of equal volumes of compounds A, B and C: Peak ID Ret. Time (min) Area% A 1.89 3.482 B 2.78 4.911 C 3.49 5.044 Note that the Areas for A, B and C are not the same, even though the amounts in the mixture are the same. To determine the correction factors, uou could arbitrarily let the correction factor of any of these be 1.0000. In this example, let's let B be the reference. We now have: c Ret. Time (min) Area% Correction Factor A 1.89 3.482 B 2.78 4.911 1.0000 C 3.49 5.044 We need to find the number by which we must multiply the area of peak A by so that it equals the area of peak B (our mixture has equal volumes). We do so as follows: Correction Factor (A) = 4.911/3.482 = 1.410 Calculate the correction factor for C similarly. We now have our correction factors: Peak ID Ret. Time (min) Area% Correction Factor A 1.89 3.482 1.410
B 2.78 4.911 1.0000 C 3.49 5.044 0.9736 This is how your table for the reference mixture GC should look, except your Table will have entries for the six alcohols (# 1-6) in the reference mixture. Your unknown alcohols should correspond to two of the peaks in the reference GC. Notice that multiplying peak areas by the correction factors gives corrected peak areas which are now proportional to the volume of each component. You will multiply the peak areas of your unknown GC's by the appropriate correction factors to obtain corrected peak areas which now are directly proportional to volumes of the alcohols present. Here is a hypothetical example of how the corrction factors from the results above can be used: Suppose the GC of your mixture has a peak with retention time of 1.89min and area of 10,520, and another peak with retention time of 3.49 and area of 7991. To calculate the corrected areas: Peak 1.89 10520 x 1.410 = 14833 Peak 3.49 7991 x 0.936 = 7478 (14833 + 7478 = 22311) Percent composition of 1.89min peak is 14833/22311 x 100 = 66.48% Percent composition of 3.49min peak is 7478/22311 x 100 = 33.52% FIGURE 1: Reflux
REMEMBER TO LIGHTLY GREASE ALL JOINTS! DO NOT stopper or seal the top! use the West condensor that does NOT contain steel wool Clamp B water OUT Reflux is used to promote a reaction by heating the solution, Unlike distillation, you do not want any vapors to escape from the top of the condensor. If you observe this, reduce the heat, and make sure water is flowing through the condensor as shown. water IN Clamp A 3-4 boiling stones 100mL round bottom flask Thermowell (NO SAND!) Place thermowell on a ring clamp attached to a ring stand a few inches above the base When reflux is in progress, check periodically that the joints are connected. Figure 2: Simple Distillation
REMEMBER TO LIGHTLY GREASE ALL JOINTS! thermometer entire thermometer bulb must be below the bend in the adaptor! Kem (spring) clamps water OUT Clamp A Clamp C - keep LOOSE, don't overtighten! water IN Kem clamp 3-4 boiling stones 100mL round bottom flask Thermowell (NO SAND!) Place thermowell on a ring clamp attached to a ring stand a few inches above the base Discard the first 1-2mL of distillate Collect the rest in a clean tared vial When distillation is in progress, check periodically that all joints are connected.