Pre-Lab Preparation and Procedure Sheet
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Pre-Lab Preparation and Procedure Sheet

CHEM 212-10Ex4-8Experiment 4How Can a ComplexAlkene be Synthesized?RBr?R Mg Br?R'HO?Introduction:In our retrosynthetic analyses, we used sulfuric acid to accomplish dehydrations of alcohols. Inthis synthesis we have selected a milder reagent, phosphoric acid. As you will find in yourlibrary project reading, there are usually several reagents for carrying out a given reaction step.Usually it is best to use the mildest reagent that will accomplish the desired reaction so thatunwanted side reactions can be minimized. In this case, phosphoric acid is sufficient toaccomplish our goal and as mentioned in this week’s lab discussion, the volatility of the productalkenes allows them (From our retrosynthetic analysis experience we recognize that 2-methyl-3-heptanol is a potential precursor for both 2-methyl-2-heptene and 2-methy-3-heptene.) to bedistilled directly from the reaction mixture as they form.B. What is the Outcome of the Dehydration of an Alcohol?Pre-lab Preparation Assignment:1. Prelab Assignments:Read the remainder of this handout.2. In the Lab Manual:Read in Appendix C, pp. C7-C8 and be prepared to discuss retrosynthetic analyses of structureon p. C9.3. Complete the Prelab activity on the course website by 8:00 PM on Sunday, March 28, 2010.4. In Your Notebook:a. Prepare a Table of Compounds including mol. wt., bp, density and water solubility of 2-methyl-3-hepanol, the desired product 2-methyl-2-heptene and the other likely alkeneproduct 2-methyl-3-heptene.b. Using complete structural formulas of ALL organic compounds used in thisexperiment (R’s are not acceptable as symbols in the structures), write the balanceequations for the dehydration of 2-methyl-3-heptanol forming 2-methyl-2-heptene.c. Write the Waste Disposal Instructions.Category 4: 2-methyl-3-hepanol waste.Category 5: Phosphoric acid and waste from the distillation pot.d. Write the complete procedure for the synthesis of the 2-methylheptenes and a matchingResults section including Table 1.Experimental Procedure.STEP 1: Calculate reagent volumes and masses, and determine the size of the reactionflask:• Measure the volume of your 2-methyl-3-heptanol [If you know the mass and volume fromthe fractional distillation, you do not need to repeat the measurements].• The volume of 85% phosphoric acid to be used equals one-fourth the volume of the alcohol.• If the total volume of the acid plus alcohol is 15 mL or less, use a 50-mL round-bottom;otherwise use a 100 mL roundbottom flask.STEP 2: Dehydration of 2-methyl-3-heptanol:

Experiment 4 – Part BEX4-9• Add your 2-methyl-3-heptanol, the calculated volume of 85% phosphoric acid and twoboiling chips to your reaction flask.• Set up a simple distillation apparatus using your reaction flask.• Heat the reaction mixture to boiling and monitor the vapor temperature.• Carefully distill products regulating the rate of heating to keep the vapor temperature below130˚C. The distillate contains a mixture of the organic products, water and a trace of startingmaterials.• Stop the distillation when only a few milliliters of liquid remain in the round-bottom flask.• Add enough solid sodium chloride to the receiving vessel to saturate the water layer.• Decant the two liquid layers into a 125 mL separatory funnel leaving the solid sodiumchloride behind.• Add 8 mL 0.5 M sodium carbonate, shake and vent the funnel until no more gas evolves.• Separate the layers and test the pH of the aqueous phase. If it is acidic, extract the organiclayer with additional aliquots of 0.5 M sodium carbonate until the pH of the aqueous layerafter the extraction is as basic as the original 0.5 M Na 2 CO 3 .• Transfer the organic layer to an Erlenmeyer flask. Add anhydrous sodium sulfate, stopperand let stand for 15 minutes to dry the organic layer.STEP 3: Product Analysis:• Use a clean dry pipet to carefully transfer the dried organic layer into a pre-weighed vial.• Determine the mass of the product.• Analyze your product mixture by gas chromatography as instructed. Be sure to record theanalytical condition used for this analysis.• Determine the number of alkene compounds in your product mixture and their relativeamounts as indicated below.• Record your results in Table 1, GC Analysis of the 2-Methyl-3-heptanol DehydrationReaction. Calculations should be in your Results section. The GC data sheets should belabeled and placed in your data binder.• Consult the chromatograms of standard compounds 2-methyl-2-heptene, 2-methyl-3-heptanoland 2-methyl-3-heptanone (a by-product of the alcohol synthesis) provided in the lab andrecord their retention times in Table 1.• Identify all peaks that you can in your chromatogram and list these in the “Compound”column of the Product Components section of Table 1. List any unidentified product peak(s)below the identified compounds in the Product Components section of Table 1 as product 2,product 3 etc. and record their retention times.• List in Table 1 opposite their retention times the “mole %'s of the all peaks in thechromatogram” according to the integration provided in your chromatogram.• Calculate the total mole % of products in the reaction mixture and record them in the bottomrow of Table 1. Consider an example where the GC data showed four peaks: 50.% 2-methyl-2-heptene, 25.% unidentified product, 20.% 2-methyl-3-heptanol and 5.% 2-methyl-3-heptanone. The total mole % of products in this reaction mixture is 75. % [50% + 25%].• Calculate the mole % of each product compound among the “total products”. Enter theseresults in Table 1 in the "mole % of the products" column. For the mixture described abovethe results would be: 67.% 2-methyl-2-heptene0.250.50 + 0.25x 100.0.500.50 + 0.25x 100 33 % unidentified product

Experiment 4 – Part BEx4-10Table 1GC Analysis of 2-Methyl-3-heptanol Dehydration ReactionCompounds⇓Standards: (From Chromatogram in thelab)1.2.3.etc.Known Alkene ProductComponents:1.2.3.etc.Unknown Products, Reactants,byproducts:1. 2-methyl-3-heptanol2. 2-methyl-3-heptanone3. Unknown 1 (if present)etc.Total Alkene productsGCRetentionTime(min.)XMole %ofall peaks inthechromatogramXXin thealkeneproductmixtureXXProductMixture TotalTable 2Mass of ProductsCalculated Masses of Known Alkene Products(See Table 1)Mass 1. 2. 3. Etc.Note: Enter names of know alkene from Table 1 in place of numbers in Table 2.

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