343Expts1and2unksolid_liquid(2)

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Feb 20, 2024

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Organic Lab 343 IDENTIFICATION OF 2 UNKNOWN SOLIDS AND A LIQUID The first two lab sessions will be used to determine the identities of two solid compounds and 1 liquid from a posted list of unknowns. One solid will be provided on a macroscale level and the other will be microscale quantities. Both will be “dirtied up” to make purification more interesting. This means that both compounds will need to be purified before obtaining a reliable, accurate melting point. Both macro- and microscale recrystallization techniques will be used to purify the unknown compounds. A general procedure for identifying your unknown macroscale solid has been provided and it will also serve as a guide for writing both of your final lab reports. Two lab reports worth 100 points each will be submitted. Each report will be due 48 hrs. after the completed lab session. For all unknown solids, be sure to record an unknown letter and the initial mass you obtain. For the unknown liquid, the boiling point will be determined using microscale methodology. You will perform the microscale method often referred to as Siwoloboff’s method , described in Zubrick. The liquid samples provided are pure, so preliminary purification (distillation) is not necessary . Be sure to write down the names of the 4 liquid unknowns before the second session so that you can come prepared with a table of their boiling points . Although the boiling point will be used as the method of identification, what other physical properties may help distinguish the four liquids? Include the answer in your lab report LEARNING TARGETS The primary purpose of this lab is to give hands-on experience with the following basic laboratory techniques: o Melting point determination o Recrystallization (including how to conduct both gravity and vacuum filtrations) o Microscale boiling point determination Other goals include: o Honing observational skills in the lab o Promoting good scientific writing skills (i.e., how to write a well-organized, complete lab report). PLEASE use proper scientific writing for your report which includes NOT using the first- person narrative such as “Then I added hot methanol….” The proper reporting would be “Then hot methanol was added…” Most importantly, try to make the connection between what is observed about physical processes (melting, boiling, crystallization) and what is going on at the molecular level. This should be part of the Discussion in Lab report 2. ORGANIZATION You will organize your work and results in written lab reports. All lab reports contain basic information: date, title, objective, procedure, observations, data, calculations, results, and discussion/conclusion. Please see the rubric for “Writing a Technique Lab”. For each lab session, note the objectives and pertinent physical data in your notebook/laptop before your lab session. Later, you will also include reactions. You must also bring a procedure to follow which you will then record in your notebook as you perform the experiment. Every student’s report is different, but the m easure of a good scientific report is that it contains all the information necessary for someone else to repeat or duplicate your work. You may write procedures and observations side-by-side with procedures described on the left of the page and any observations and initial data on the right. Data will also be organized and presented in a separate section. TAs will discuss the format of lab reports. PROCEDURE 1) A macroscale unknown will be assigned. Record the letter designation of your unknown in your notebook. Weigh out approximately 1.0 g of the unknown and record the exact mass. Describe it: does it appear homogeneous or are there obvious flecks of impurities? What color is it? Is the solid crystalline or amorphous?

Organic Lab 343 2) Obtain a crude melting point and describe what you see. The ramp rate should be 10 o C/minute and the starting temperature should be around 35 o C (Check the list of unknowns). Is the melting point sharp or did it cover a broad temperature range? Report the range. Did all of it melt? Was the phase transition from solid to liquid clean or did discoloration (decomposition) occur? Look at the list of unknowns to know the highest melting point possible (and hence, a stopping point in case some solid doesn ’ t melt). 3) From this first melting point range, consult the list of possible unknowns and narrow down the possibilities as to which unknown you have. The next step is to purify it/recrystallize it. Please read Zubrick on recrystallization very carefully . Test the suggested solvent or solvent combination for recrystallization. To do this, first try dissolving a small amount (microspatula tip-full) of your compound in a small amount (0.5 ml) of solvent in a test tube. If most dissolves and only a little remains insoluble even with w arming that’s a good indication that at least one impurity can be removed by simple gravity filtration. Do it and describe it. (Hint: Heat a beaker full of water and use it to heat the sample in the test tube). Note : often a mixed solvent combination is suggested on the list of solid unknowns posted in the lab. An example would be “methanol/water”. The first solvent (methanol) is the one in which the solid is quite soluble. The second (water) is the one in which it is only spari ngly soluble. BE SURE to read Zubrick’s procedure for recrystall ization using a mixed solvent system. 4) If your solution is discolored after dissolution in your (warm) solvent, it probably contains a soluble colored contaminant. It will be necessary to add a little charcoal (activated carbon), or Norit, to decolorize it. Don’t add too much or it will adsorb your compound. It is often necessary to gravity filter the solution more than once to remove the charcoal. Include a hand-drawing of a gravity filtration set-up. Only 2 treatments with charcoal should be done. If a bit of color remains, it ’ s okay. Move on to the next steps due to time constraints. 5) Once you have a homogenous solution of your solid and solvent: Recrystallize and describe everything you do. What solvent or solvent combo did you use? Was heating necessary for dissolution? Was icing necessary? Did you need to scratch the glass wall of the container to initiate crystallization? How long before crystals appeared? What did the crystals look like in comparison to the initial sample? 6) Isolate your purified product and describe everything. Include a hand-drawn picture of the vacuum filtration set- up. What solvent did you use to wash your crystals? Was it cold or hot? What did the filtrate (mother liquor) look like? Did additional crystals form in the mother liquor? Did you harvest them? What was the appearance of your final, purified product? Are the crystals plate-like, needles, shiny? What was the mass of the final product? Calculate and report percent recovery. What was the melting point of the final product? A range should be reported for the melting point temperature and 2 melting points should be done. STOP HERE FOR FIRST LAB SESSION POST LAB QUESTIONS FOR FIRST LAB REPORT 1. Zahra was collecting her pure crystals via vacuum filtration from a methanol/water mixed solvent system. She poured the solution containing her crystals into the Büchner funnel. Then she rinsed her flask with some warm methanol and poured that over the crystals resting in the Büchner… oops! Tell what happened and why. What should she have done instead? 2. Carleigh was taking the melting point of her crude unknown solid. She noticed some liquid formation at 116 o C – 120 o C and waited for the rest of the solid to melt. At 400 o C, the solid was still present. What could be happening with her sample and what will she likely have to do during the purification process?

Organic Lab 343 3. Klemmer was heating his ethanol solvent in an Erlenmeyer flask on a hot plate to use for recrystallization. The TA noticed he didn’t have a stir stick or boiling chip in the flask and told him to please put one. Klemmer dropped a stick in the flask and suddenly the liquid exploded out of the flask. Oops! What happened? What should he have done instead? Answer the above Post Lab questions at the end of the Lab Report. The first lab report will cover the work done in the first lab session. For the first lab report, we will focus on your procedure/observations, results, and a brief conclusion. Your conclusion should include a description what you did and a comparison between your crude and pure compounds and the identity of your unknown. A discussion will be done in the next lab report and all subsequent reports . Discussions will include the theory behind the techniques being used and what is happening on a molecular level. SECOND SESSION Repeat steps 1-6 in the above procedure with a microscale unknown (use about 100 mg of unknown). Note that microscale unknowns will not require hot gravity filtration. Obtain a liquid Unknown and record the letter of your unknown. Fill the designated test tube with approximately 3.0 cm high of liquid. Follow Siwoloboff ’ s method to determine the boiling point of the liquid. The height of unknown liquid should not exceed the height of the capillary tube. Please note: Sketches of equipment used for the first time a technique is performed (eg, gravity and vacuum filtration, bp determination) are required in lab reports. DO NOT COPY/PASTE the DRAWINGS from this handout . Hand-draw the set ups. Once they ’ ve been sketched in a lab report, you don’t need to do so again in subsequent reports. The second lab report should contain all the elements of a Technique lab report. The Discussion should include (but not be limited to) a discussion on the theory of recrystallization and microscale boiling point determination. What is happening on a molecular level?

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Organic Lab 343 PRECAUTIONS AND CLEANUP You are handling chemicals. Please handle them carefully and wear gloves. Many chemicals are irritants. You should not be afraid of the chemicals and equipment that you are using, but you should always use safe experimental practices. All bottles must be securely capped except when removing reagents. Your purified macroscale and microscale solid unknowns should be placed in the solid waste container. Any residual liquid unknown from the bp determination should be placed in the non-halogenated waste container. Filtrates containing solvents like dichloromethane, ethyl acetate, hexanes, methanol, or ethanol should be placed in the non- halogenated or halogenated waste containers as appropriate. Drying agents, charcoal, used filter papers, etc may be placed in the solid waste container or trash can, check with the TA. Please do NOT leave chemicals on bench tops or by the balances. Dispose of them properly. Keep all bottles capped. HELPFUL HINTS Recrystallization is an art. Use the minimal amount of solvent needed to effect dissolution. Using too much solvent may make it difficult for crystallization to occur. If you think there is too much solvent, then boil it (add a stir stick) to reduce the volume. On the other hand, if the solution is supersaturated, crystallization may occur during gravity filtration. You’ll know this has happened when crystals appear on the paper and down the stem of the funnel. If this occurs, try dribbling hot solvent around the filter paper to get it to dissolve. If this fails, remove the filter paper, place it (charcoal and all) into a beaker and add hot solvent to redissolve. Then try again. Mixed solvents are often preferable. For instance, if your compound is readily soluble in hot methanol, but only sparingly soluble in water, then use hot methanol to dissolve it (gravity filter if needed). Add water dropwise slowly. When the solution reaches the cloud point (described in Zubrick), stop adding water and set it aside to crystallize. Many students have successfully recrystallized their compound and then lost it during vacuum filtration...the ultimate frustration. BE SURE that the solvent you select to rinse your crystals and complete crystal transfer to the Büchner, or H irsch funnel doesn’t dissolve your compound. For instance, if you used hot methanol to recrystallize, don’t use warm methanol to rinse the crystals or they will dissolve; try cold water or icy-cold methanol. A good melting point reading for a pure compound is obtained only If the temperature is increased slowly. You may heat quickly to about 10 degrees below the anticipated melting point, but thereafter, set the ramp rate to 2 degrees celsius/minute. Melting points begin with the first sign of phase change and end when the final hint of solid melts. Very few compounds have sharply defined melting points. Most pure compounds melt over a range of 1-3 degrees. Include answers to the Post Lab questions at the end of your report. Your lab report should be well organized and neat. Post-Lab Questions for Lab Report 2 1. How does the principle of “ likes dissolves likes ” explain the differing solubilities of solutes in various solvents? 2. Explain why the boiling point of your unknown liquid is determined at the point when liquid is sucked back into the capillary tube. A good explanation will explain where all the bubbles were coming from and their likely composition (Also include the term vapor pressure).