Part 1: Distillation and Analysis of an Unknown Alcohol Mixture

Transcription

1 Experiment DE: Part 1 Experiment DE has multiple goals, including -separation and quantification of a 2-component mixture (fractional distillation and GC); preliminary ID of both components -preparation and purification of an ester using one of the original components (Fisher esterification & simple distillation) -structural identification of the ester (NMR & IR) Part 1: Distillation and Analysis of an Unknown Alcohol Mixture Before beginning, under "Techniques" on the Chem 113A website, review the photos and descriptions for Simple and Fractional Distillation. You should also watch the video showing Simple Distillation (fractional distillation follows a similar technique). See the Figure Fractional Distillation at the end of this document. Key Points for Simple and Fractional Distillations: -always apply a thin coating of grease on all ground-glass joints before assembling the apparatus; wipe off any excess before assembly -DO NOT forget fresh boiling stones before you turn on the heat; IMPORTANT: if you forget to add boiling stones and your solution is already warm to the touch, DO NOT add boiling stones to a hot solution or it will suddenly "flash" and spray hot liquid out of the top! Instead, turn off the heat, and carefully lower the thermowell. Add fresh boiling stones when the solution is lukewarm to the touch. -as the distllation proceeds. Periodically check the joints to make sure ther is no gap where vapors can escape (if you smell a lot of alcohol, either you or a lab mate, may have a leak); adjust your connections as necessary. -any time a distillation is taking place, there is high potential for a water leak or flood; put your books and computers/smartphones on top of the center bench or by the windows! Procedure: Experiment DE Part I: Separation of a 2-Component Mixture by Fractional Distillation. Obtain 50mL of an unknown alcohol mixture from the instructor: write the CODE in your notebook immediately. We will tell you the following about these alcohols: A) both are from the list of possible alcohols below (Table 1) B) both are saturated (no double bonds) and acyclic (no rings) C) they may be straight-chain or branched alcohols D) they may have 3, 4, 5, or 6 carbons Your goals for Part I are to separate the two alcohols by fractional distillation and determine the % composition in the mixture. One of your purified alcohols will be used to prepare an ester in Part II.

2 Fractional Distillation Check out a thermometer and a pair of heat-resistent gloves from the Service Center (both must be returned at the end of the period, or a late fine will apply). Before you begin your distillation, you must calibrate your thermometer using boiling water. The instructor will have a boiling water station ready, immerse your thermometer and note if it reads 100 o C. If your thermometer reads higher or lower than 100 o C, note the difference: this will be your "correction factor" to apply to the distillation temperatures of your 2 alcohols. If the water is not yet boiling, set up your fractional distillation apparatus while waiting. Put 50mL of alcohol mixture and 3-4 boiling stones in a 100mL round bottom flask (the flask with the ground-glass joint, not the boiling flask!). Review the Key Points for Distillation above. Assemble a fractional distillation apparatus as shown in the Figure below. A model will also be available in the lab. Lightly grease all joints and pay particular attention to the position of the clamps, all of which are needed. The vertical column must have steel wool in the inner tube. The purpose of the steel wool is to increase the surface area inside the condensor so that the vapors will undergo many cycles of condensation and evaporation (called "theoretical plates"). The more theoretical plates in the system, the greater the efficiency of the separation. Be careful to not tighten clamp C too tightly, or it may cause the glass to break when it gets hot. Use ring stands (not the fixed "monkey bars"). Note the direction of the water hoses for the condensor.; turn the water on to a slow trickle, not "full blast!" The position of the thermometer bulb is especially important: too high or too low may lead to an inaccurate temperature reading. Ask your instructor to check your apparatus before you turn on the heat. S/he may wrap your vertical condensor with foil to insulate it. Check that your heating mantle is plugged in to the variable transformer. Begin by turning it to no higher than a setting of 4.5. After awhile, you should see the solution boiling vigorously. Distillation will begin only when the vapors reach the top of the stillhead (above the vertical column), then spill over into the water-cooled condenser. This can take awhile, so be patient! As your solution is heating, label your 25 and 50mL Erlenmeyer flasks as "A," "B," "C," and "D". Note the marking for 10mL volumes on each. In your notebook, prepare a table with two columns labeled "Time (min)" and "Temperature ( o C)". Enter values under "Time" starting at 0min, then 2, 4, 6, 8, 10, etc. When the first drops drip into Flask A, note the temperature reading at 0min on the table, then at 2 minute intervals. An ideal distillation rate is 1 drop every 1-3 seconds. If your rate is slower than this, you may wish to slowly raise the heat, but only by small increments (0.1 to 0.2 on the thermowell dial) - it takes several minutes to see a response. [Question: how would your separation be affected if the heating rate is too high?} When Flask A reaches 10mL volume, switch to Flask B - note where the change occurs on your Table. When Flask B reaches 10mL switch to Flask C, and so on until Flask D reaches 10mL.

3 You may observe that when disillation begins the temperature will likely remain fairly stable for awhile (a "plateau"), but at some point, you may notice the temperature change (either go up or down). If the rate of distillation slows noticeably, increase the heating of the thermowell in small increments until distillation resumes at a reasonable rate. (what causes this to happen?) When the volume of Flask D reaches 10mL, turn off the heat. DO NOT let all of the liquid evaporate or your flask will get very hot and may crack. Put on the heat-resistent gloves and carefully lower the ring clamp holding the thermowell from the flask {CAREFUL : Hot!!}. Let the flask cool, then dispose leftover alcohol in the appropriate waste container. It is not necessary to wash any of the distillation glassware; let your glassware dry in the hood and put away in your drawer when it is cool enough to handle. Especially do NOT put any water in the condenser with the steel wool (why not?). Cover the A, B, C, and D flasks with Duraseal TM tightly, secure with rubber bands, and put away in your drawer/locker. Return the thermometer and gloves to the Stockroom on the same day. In your notebook, plot the time vs. temperature data. Use this plot to get a preliminary identification of your 2 unknown alcohols using the boiling points of alcohols given in Table A below. In Part II, you will use spectroscopic data to confirm your unknown alcohol structure. Your plot will also help you to estimate the % composition of your mixture and compare it to GC data you will obtain in Part II. Analysis of Fractions A and D by Gas Chromatography (GC) In order to check the purity of your alcohols, they will be analyzed by gas chromatography (GC). Label two small test tubes as "A" and "D". Put one drop of your A fraction in "A" and one drop of your D fraction in "D" (use a clean pipette for each!). Add 1mL of "GC Dilution Ether"(found in the hood) to each tube, cover with Duraseal TM and put them in a small beaker. Note in most cases we will not be analyzing Fraction B or C, unless this is necessary. Check out a GC syringe from the Service Center (must return to avoid a fine) Your instructor will arrange an order for students in your section to measure their GCs. Check out a GC syringe just before you are scheduled to measure your samples. Our GC instrument can accomodate 2 students at a time, but it will still require time to process the entire class. While you are waiting for the GC, use this time to measure the IR spectra of A and D, and work on spectroscopy problems The GC plot will tell you the purity of your A and D fractions. [Later, you will use GC again on your original mixture to determine the % composition of your 2 alcohols).

4 Table A: List of Possible Alcohols* *Your two alcohols (and of course the alcohols in the reference mixture) are liquid, saturated, acyclic alcohols from among this list: Physical Properties of Various Alcohols** Compound Bp ( C) Mp ( C) Methanol 65 Ethanol 78 2-Propanol 82 2-Methyl-2-propanol Propen-1-ol 97 2-Methyl-2-butanol Methyl-1-propanol Buten-1-ol Butanol Penten-2-ol 119 Isoamyl alcohol Penten-1-ol Pentanol 138 Cyclopentanol Ethyl-1-butanol Methylcyclopentanol Hexanol 156 Cyclohexanol Heptanol 177 **the alcohols used in Experiment DE are saturated and acyclic alcohols, and have from 3 to 6 carbon atoms

5 Experiment DE, Part I - Fractional Distillation Apparatus REMEMBER TO LIGHTLY GREASE ALL JOINTS! thermometer entire thermometer bulb must be below the bend in the adaptor! Kem (spring) clamps water OUT Clamp C - keep LOOSE, don't overtighten! water IN Kem clamp use the West condensor that contains steel wool X - no water Clamp B X - no water Clamp A A B C D 3-4 boiling stones 100mL round bottom flask four 25 or 50mL Erlenmeyer Flasks labelled "A" "B" "C" "D" Place thermowell on a ring clamp attached to a ring stand a few inches above the base Thermowell (NO SAND!) When distillation is in progress, check periodically that all joints are connected.