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    Gas Chromatography of Alkenes

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    I performed a lab entitled: Acid-catalyzed dehydration of alcohols.
    I have the file attached in word. My problem is I need to know the reaction equations using structure and names from the attached file. I also have a file from the spectrum of butenes. I need the labeling of the 3 dark peaks of butenes cis and trans. My permanganate and bromine test were both positive. PLEASE HELP!!

    Acid-Catalyzed Dehydration of Alcohols
    Organic Chemistry I Laboratory

    One of the easiest organic chemical syntheses is the dehydration of an alcohol using sulfuric acid as a catalyst to produce an alkene(s). The alcohols are first protonated in the acid to form the oxonium ion that dehydrates at elevated temperatures to yield a carbocation. In the second step in the reaction sequence that carbocation loses a beta-proton (a hydrogen atom attached to a carbon atom adjacent to the carbocation) by abstraction with the bisulfate ion to yield an alkene. The alkene forms as a gas or as a layer on the surface of the acid-alcohol mixture (the density of the alkene is less than one); the gas can then be collected over water or the liquid layer can be removed by simple distillation to give the final alkene product. In alcohols where there are more than two kinks of -hydrogens, there will be more than one alkene product formed in the reaction. These alkenes can be separated by gas chromatography using a non-polar column packing.

    For this lab we will be preparing alkenes from a simple starting material, 2-butanol. Another alcohol that is easy to dehydrate is 2-methyl-2-butanol. The reactions and their products are outlined below. These reactions are discussed in Chapter 7 of your lecture text.

    The relative amounts of products formed from these two alcohols depend upon the stabilities of the alkenes formed. In the case of 2-butanol, there are three possible products; 1-butene, cis-2-butene, and trans-2-butene. In lecture you learned which of these is the most stable and why. For 2-methyl-2-butanol, only two products are possible; 2-methyl-1-butene and 2-methyl-2-butene. Again, 2-methyl-2-butene is more stable than 2-methyl-1-butene as it is the more substituted alkene.

    In this lab you will synthesize the alkenes and separate the gaseous products of the dehydration of 2-butanol.

    Laboratory Procedure

    Experiment 1 : 2-butanol

    In each of 2 reaction tubes place 0.40mL (360mg) of 2-butanol and 0.20mL (2 drops) of concentrated sulfuric acid. Mix the reactants well and add a Teflon boiling chip. Using the first tube, assemble the apparatus shown below. Attach the tubing to the reaction tube by inserting a needle from the INSIDE of the septum and then sliding the tubing over the needle. Place the septum oven the reaction tube and use a specially prepared piece of bent glass tubing (provided by the instructor) to direct the tubing into the collection tube. Make sure the Teflon (or polypropylene) tubing fits tightly on the needle and that the needle fits tightly through the septum or you will not recover any product. Clamp the reaction tube above the surface of the sand bath so it is ready to lower into the hot sand once the apparatus is assembled. The collection tube is first filled with water, capped with a finger, and inverted in the beaker of water. Clamp the collection tube in place.

    When you are ready for the reaction to begin, lower the reaction tube into a 100°C hot sand bath and slowly increase the temperature to complete the reaction. Collect a full tube of the butanes, immediately remove the tubing from the water and glass tube and allow the butanes to bubble into the test solutions as described below. When finished, REMOVE THE TUBING FROM THE LIQUID, and then raise the reaction tube out of the sand bath. If the tubing is left in the water bath or a solution, liquid will be forced back into the reaction tube when it cools. Once the reaction tube has cooled, dilute the residue with large volumes of water and wash down the drain.

    Characterization of the Products

    Prior to the collection of your product, prepare two reaction tubes for characterization of the products after the collection of product for GC analysis. In the first tube, place 1.0mL of bromine in methylene chloride. In the second tube, add 1.0mL of cold, alkaline permanganate. After you have collected a tube full of the gaseous alkene products for GC analysis, continue heating the reaction mixture and remove the Teflon tube from under the collection tube and place the end directly into the tube containing Br2 in CH2Cl2. Observe the results after the gaseous alkenes have bubbled into the bromine solution for a minute. Then place the Teflon tube into the cold alkaline permanganate solution. Observe the results after the alkenes have bubbled into the solution of a minute. After these observations have been made, remove the Teflon tube from the solution and then turn off the heat and remove the reaction tube from the hot sand. DO NOT stop heating until you have removed the Teflon tube from any source of liquid. If you cool the reaction tube while the Teflon tube is immersed in a liquid, the liquid will be forced up the Teflon tube and into the hot sulfuric acid reaction mixture. A serious problem! If you do not produce enough product to complete the characterization reactions, stop the reaction, allow the reaction tube to cool, add additional reactants, and make some more product.

    Once you have colleted your product and characterized it by observing the reaction with bromine and cold permanganate, analyze your products (gases) by injecting about 1mL of the gas into the GC. Insert the needle of a 3mL syringe into the septum on the top of your collection tube and withdraw 2-3mL of the gas. Be careful not to withdraw any water into the syringe. Save your remaining product in the tube in case you need to do another chromatogram. Depending upon the GC and settings, it may take more or less than 1mL. Using the areas under the GC curves, determine the relative amounts of the three products and compare these results to the known stabilities of the products.

    Below are some questions you should be able to answer.

    1. What were the GC settings used in the experiment?
    2. What was the basis for your identification of the peaks in the GC.
    3. What were the relative amounts of the three products in the 2-butanol experiment and how do these ratios relate to the relative stability of the products?
    4. What were the results of the test performed on your products?
    5. Write the structure of the three olefins (alkenes) produced by the dehydration of 3-methyl-3-pentanol.
    6. When 2-methylpropene is bubbled into dilute sulfuric acid at room temperature, it appears to dissolve. What new substance has been formed?
    7. From your knowledge of the dehydration of tertiary alcohols, which olefin should predominate in the product of the dehydration of 2-methyl-2-butanol?
    8. What is the maximum volume at STP of the butane mixture that could be obtained by the dehydration of 81 mg of 2-butanol?
    9. What other gases are in the collection tube besides the three butenes at the end of the second experiment?

    Do not turn in the answers to these questions.

    Turn in the Data Sheet attached to this document and attach the labeled chromatograph.

    Revision 07/30/2004
    Organic Laboratory Data Sheet

    Name _________________________________________ Partner _________________________


    I. Reaction equation(s) using structures and names:

    II. Theoretical yield (recovery) and literature melting/boiling point:

    Butenes: Theoretical Yield: Literature BP's:

    III. Data:

    Result of bromine test (Positive or negative - Describe observation):

    Result of cold permanganate test (Positive or negative - Describe observation):

    IV. Chromatogram Analysis - Butenes (Identify peaks and show relative areas. Discuss alkene stabilities.)

    Attach labeled chromatogram to report:

    V. Technique - Provide a self-evaluation and grade yourself on your technique (percentage out of 100%):

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    Solution Summary

    Gas chromatographs of alkenes are assigned and discussed.