Organic Chemistry I
Organic Chemistry I CHEM 2210
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Reminder These notes are meant to supplement not replace the textbook and laboratory manual Dehydration of Cyclohexanol l Dehydration is the elimination of water from an alcohol to form an alkene In the course of the reaction cyclohexene forms in the presence of the phosphoric acid catalyst H3PO4 H20 and unreacted cyclohexanol Drierite is anhydrous calcium sulfate It absorbs the water as it is formed preventing it from contaminating the product cyclohexene It also serves as column packing for a fractional distillation 2 Here is the equation for the dehydration of cyclohexanol H3PO4 85 O OH E Q H20 CaSO4 anhydrous The mechanism of this reaction is as follows This is review from the CHEM 2210 lecture GD 9 O OH H3PO4 O OHz H2PO4 El O OHz 09 H20 9 0 H2PO4 Q H3PO4 Here are some things to note about the kinds of mechanisms that express the ideas behind them best and which therefore get full credit on exams l The steps are written as separate balanced equations Nothing is left out and there is no shorthand 2 The equations for the mechanism three in this case when added together give the overall reaction 3 The catalyst H3PO4 in this case reacts in the first step and is made again in a later step This is the way catalysts behave in mechanisms 4 The protonated alcohol H2PO4r and the carbocation appear as reaction products and then react in later steps This means that they are reactive intermediates Sample problem Propose a mechanism for the following reaction The answer is given on the next page OH H3PO4 gt H20 The mechanism of the reaction at the bottom of the previous page is OH QOHz 9 gt H3PO4 gt H2PO4 0H2 q gt gt H20 9 gt gt lt arealrangement step 9 gt lt H2PO4 gtlt H3PO4 Here are some more mechanism hints Ifa carbocation can rea1range by way of a 12 shift to give a more stable carbocation it will More generally if bondline formulas still give you trouble propose your mechanism using condensed or Lewis structures Your instructor won39t mind F 9 H3C OH H3C 0H2 l l l l 9 CH37C7C7CH3 H3PO4 CH37C7C7CH3 H2P04 l l H3C H H3C H G H3C EOHz H3C l l 0 CH37C7C7CH3 a CH3iC7C7CH3 H20 l l l l H3C H H3C H H3C We 9H3 CH37C7C 7CH3 CH37C7C7CH3 arealrangement step l l H3C H H3C H H3C H3C CH3 GD l 9 CHsiCKCiCH3 H2PO4 CC H3PO4 H3C H H3C CH3 Instructors like mechanisms that contain arrows showing electron movement as used in the mechanism version here Here are rough sketches not to scale of setups used for fractional distillation The one used in the current experiment is on the left the one used previously is on the right Hickman still thermometer note placement of bulb cyclohexene collects here r connectors Drierite receiver column packing boiling copper sponge V 39 cyclohexanol and Chips shortnecked phosphoric acid ask sample In the apparatus on the le Drierite anhydrous calcium sulfate absorbs the water formed in this experiment Superheating most often happens when you forget to use boiling stones Boiling of a superheated sample can be irregular and even explosive The Drierite also serves this purpose in this experiment For more information about the apparatus on the right see the Simple and Fractional Distillation notes from the first semester It is necessary to calculate a percent yield for this experiment as it is for any experiment in which you perform a chemical reaction and isolate a product dilTerent from the starting material For instructions and examples on how to do this see the sample problems with solutions on this web site Here are structures of the organic compounds involved in this experiment OH O l l C G CH3CH20H CH3CCH3 ethanol acetone cyclohexanol cyclohexene wash wash madam Pmdua solvent solvent The infrared spectrum of cyclohexanol contains a peak near 3450 cm1 OH stretching vibration this peak is absent from the IR spectrum of cyclohexene The IR spectrum of cyclohexene contains peaks near 3050 cm1 sp2 CH stretch and 1600 cm1 CC stretch both of these are absent from the cyclohexanol spectrum The IR spectrum of a mixture of these two compounds will contain all three peaks Safety considerations for this experiment include Phosphoric acid the catalyst is highly corrosive All of the organic compounds involved in this experiment are highly ammable and at least slightly toxic Handle them with the usual precautions You will use a mercury thermometer to record temperatures during this experiment Handle it with care If it breaks and spills mercury notify your instructor We have a special kit for cleaning up mercury spills ou will use a sand bath during this experiment As you know by now portions of this apparatus become very hot during normal use Handle sand baths with care and don39t touch the hot parts They also use electricity for heating do not use water or other solvents carelessly around them Ifthe wires on any electrical apparatus are frayed the shock and re hazards are multiplied do not use the apparatus Notes for topics that may be included in quizzes given after the experiment 9 To predict the product of a dehydration reaction you will often need to know the mechanism of the reaction This is because rearrangements will occur The mechanism steps are as follows see section 2 above for examples 1 Protonate the alcohol using the acid catalyst 2 Lose H20 to form a carbocation 3 Ifthe carbocation can rearrange by a 12 shi to a more stable carbocation do so 4 Remove a proton from a carbon atom adjacent to the one with the positive charge to form the alkene product Sample problem The alcohol CH32CHCHOHCH3 reacts in the presence of H2804 catalyst to give the alkene CH32CCHCH3 as the major product What other alkene is formed Solution The answer may seem obvious but let s go through the mechanism Here are the rst two steps 1quot Q H30 OH H3C 0H2 l l l l 9 CH37C7C7CH3 H2804 CH37C7C7CH3 HSO4 l H H H H 9 H3O EOHz H3C GD CH37C7C7CH3 a CH3iC7C7CH3 H20 l l l l H H H H The carbocation formed in the second step above is 2 and will rearrange to form a 30 carbocation in the next step H3C H3C H l 9 l l CH37C7C7CH3 CH37C7C7CH3 M 9 l H H H The mechanism is continued on the next page The 30 carbocation can then lose a proton from either carbon adjacent to the positive charge The carbon to the left of the carbon with the positive charge gives the answer the carbon to the right of the positive charge gives the major product structure shown in the statement of the problem H3C H H3C H H3C H l l l l l CH37 7C7CH3 a CH2C7C7CH3 CH37CC7CH3 H H major product more stable answer Note that only the alkene in the center of the last step above is the answer to the question as stated The rest of the mechanism should be on scratch paper only 22 Dehydration of Cyclohexanol Dehydration of an alcohol to form an alkene is one of the fundamental reactions of organic chemistry In this week s example we will transform cyclohexanol into cyclohexene The reaction itself and the procedure are fairly simple but you should realize that several processes take place in the bottom part of the microscale distillation apparatus called a Hickman still Phosphoric acid catalyzes the dehydration of cyclohexanol while granules of Drierite adsorb both the water that forms and that already present in 85 HsPO 4 At the same time granules of Drierite serve the purpose of a pseudo fractional distillation column that allows puri cation of the product The product that condenses and runs down into the collection ring of the Hickman still should be dry and ready for yield determination Almost everything in the procedure is purposefully done in a particular way so it is a good idea to follow it to achieve a good yield of the product A reasonable yield of the alkene product is 40 70 Learn about different parts of a Hickman still before using and handle it with care PRE EXPERIMENT ASSIGNMENT Study this chapter of the manual the lecture notes on the Chemistry Department web site and pages 287 290 in Williamson You will also nd it helpful to study the sections on the dehydration of alcohols reaction in your lecture textbook Do the rst parts of your notebook writeup A student who has prepared for the Dehydration of Cyclohexanol experiment should be able to 1 Identify the process occurring dehydration is an elimination reaction Identify the role of the Drierite in the process 2 Give the equation and the mechanism for the dehydration of cyclohexanol and other alcohols including those that rearrange this is review material from the CHEM 22 10 lecture 3 Give a drawing of and identify the components from a drawing of the Hickman still apparatus used for distillation Explain the function of each part of the apparatus and compare them to the parts of a regular distillation setup that you used for fractional distillation 4 Calculate the theoretical yield and the percent yield for this and similar experiments given the necessary data and perform any of the intermediate calculations required by this process 5 Draw the structure given the name or give the name from the structure of the compounds used in the day s experiment and give the role of each reactant solvent catalyst etc 6 Interpret IR spectra of cyclohexanol and cyclohexene List and assign peaks that are present in the spectrum of cyclohexene and not in the spectrum of cyclohexanol and visa versa 7 Identify and explain safety considerations for this experiment 8 Perform the day s experiment safely and successfully Quizzes given after the experiment has been performed may also include 10 Predict the products of dehydration reactions including those that involve rearrangement For more details see your CHEM 22 10 lecture materials The Reaction It will save time if you turn on your sand bath as soon as you come to lab Start with a setting of about 30 Check the temperature of your sand bath by carefully inserting only the bulb of a mercury thermometer into sand Make sure that the temperature is about 140 150 C by adjusting the setting Load the bottom part of a Hickman still apparatus with granulated Drierite so that you have a little more than a half of the bottom volume lled Make sure that all of the granules are on the bottom and none of them are in the collection ring of the Hickman still Place about 1 mL of 85 H3PO4 in the still with a Pasteur pipette but once again do not contaminate the collection ring A good idea is to wipe the pipette containing H3PO4 from the outside with a piece of a Kim Wipe quickly insert it into the Drierite layer in the bottom part of the Hickman still and add the acid After you mix the Drierite with the acid there will be no visible liquid left Add 07 mL of cyclohexanol to this mixture using the same precautions with a Pasteur pipette and gently mix it with the Drierite and acid using the same pipette Place only the bottom part of the still into a preheated sand bath it is important that temperature is 140 150 C Some of the Drierite should not be heated so do not cover entire bottom of the apparatus with sand Heat the apparatus and observe product condensation in the collection ring The distillation should be completed in about 20 minutes If no distillate is collected after 10 minutes of heating consult with your instructor When you see no more distillate condensing in the collection ring stop heating and let the apparatus cool to room temperature Transfer your product from the collection ring into a pre weighed small shell vial with a slant Pasteur pipette cap it don t forget to include the weight of the cap and determine the yield The Infrared spectrum Take an IR spectrum of a neat sample of your product by placing a small drop of it between two NaCl plates Your instructor will provide you with an IR spectrum of your starting cyclohexanol CLEANUP Used Pasteur pipettes are placed in the broken glass box after use Your product and all used reaction mixtures are poured into the non halogenated organic liquid waste bottle in the hood if your instructor does not collect it POSTEXPERIMENT ASSIGNMENT Write the lab report and have it ready to turn in by the beginning of the next lab Include your IR spectrum in your Data section and calculate the theoretical and percent yields In your Conclusions section interpret the IR spectrum and compare it to the spectrum of the starting cyclohexanol provided by your instructor Which portion of the spectrum is most useful for you to make a conclusion about your experiment Specify which peaks were present in the starting material and not present in your product Specify which peaks are present in your product that were not present in your starting material Draw a conclusion about the purity of your product based on this spectrum Prepare for the dehydration portion of the next quiz