Exp t 73 Dehydration of 2 Methylcyclohexanol Adapted by R Minard and D Dorisio Penn State Univ from a microscale procedure used by the University of California Irvine in its undergraduate labs The procedure is based on a paper titled Dehydration of 2 Methyl cyclohexanol by R L Taber and W C Champion J Chem Ed 4 4 620 1967 Revised 2 24 02 Introduction The purpose of this experiment is to analyze and to interpret mechanistically the results of acid catalyzed dehydration of 2 methyl cyclohexanol CH3 OH 1 2 Methylcyclohexanol cis and trans D H2O 2 CH2 CH3 CH3 H3PO4 3 4 3 MethylMethylene1 Methylcyclohexene cyclohexene cyclohexane Dehydration of 2 methylcyclohexanol 1 B P 163 166 C with 85 phosphoric acid yields a mixture of three products the main product overall 75 80 is 1 methyl 1 cyclohexene 2 B P 110 111 C also present are 3 methyl 1 cyclohexene 3 B P 104 C and methylenecyclohexane 4 The relative amounts of each product can be determined by gas chromatographic analysis of the mixture Interestingly distillation of the product mixture gives fractions of varying composition where the lower boiling product distills later If all of the products were formed instantaneously then obviously the lower boiling component would be expected to distill earlier that it does not implies that there is either more than one kind of reaction taking place or else there is more than one component in the starting material here it is both The phenomenon is called the Evelyn Effect and is briefly discussed in a recent article by David Todd of Juniata College in the Journal of Chemical Education 71 1994 440 Apparently the alkenes distill nearly as fast as they are formed therefore 1 methylcyclohexene forms faster than 3 methylcyclohexene The reagent supplied by Aldrich is a mixture of approximately 50 50 cis and trans 2methylcyclohexanol In a separate experiment it has been shown that the cis isomer dehydrates about 30 times faster than the trans isomer under identical reaction conditions The 1 methylcyclohexene predominates in all fractions collected but the early fraction is over 90 the fifth fraction as low as 55 The reaction appears to proceed mainly by a trans mechanism E2 mechanism the cis isomer reacting more rapidly However the presence of methylenecyclohexane about 8 of the total alkene product gives evidence that a E1 mechanism is possible as well Prelaboratory Exercises In your chemical data table give the theoretical yield of total methylcyclohexene isomers 1 Why is it important to surround the receiving flask with an ice water bath 2 In gas chromatography components in a mixture can be separated from one another because these components have different solubilities in the non volatile stationary phase Compound X and Compound Y have identical boiling points Compound X has a greater affinity for the non volatile liquid phase Z than does Compound Y When a mixture of X and Y is separated which component should be the first component off the column Cautions 85 phosphoric acid is corrosive to skin and clothing Neutralize any spills with sodium bicarbonate and quickly wash the affected areas of skin with lots of soap and water Synthesis Isolation and Purification Place 2 0 g of 2 methylcyclohexanol 0 5 mL of 85 phosphoric acid HANDLE WITH CARE a boiling chip and a 1 2 stirring bar into a 5 mL round bottom long neck flask Lightly pack the neck with a stainless steel sponge ChoreBoy and carefully mix the reactants thoroughly by swirling gently The reaction should be carried out by a modified version of the Microscale fractional distillation apparatus as shown in the figure below see also the figures in Chapter 5 of the Lab Guide Attach the round bottom flask to the curved distilling side arm Place the thermometer in the top of the side arm so that the thermometer bulb is below the sidearm opening Distill out the products by heating the mixture gently on the sand bath Do not allow the temperature of the distillate to rise above 96 C Continue distilling until approximately 1 6 mL of liquid have been collected DO NOT ALLOW THE DISTILLING FLASK TO BECOME DRY Transfer the distillate to a reaction tube slowly add solid sodium chloride NaCl until the distillate is saturated with sodium chloride and then cautiously with gentle shaking add enough sat d NaHCO3 10 sodium bicarbonate solution to make the distillate neutral to litmus After allowing the organic and aqueous layers to separate transfer the organic layer using a Pasteur pipet to a clean dry reaction tube Dry the product over anhydrous sodium sulfate Na2SO4 and transfer the dried product to a clean dry and labeled shorty vial Screw the lid on tightly as the cyclohexene can evaporate otherwise Gas Chromatographic Analysis Analyze a dichloromethane solution of your product mixture by gas chromatography on a gas chromatograph in Rm 206 Whitmore which has been set up for this purpose Add one drop of your product to 2 mL of dichloromethane and inject only 1 microliter Use the GC instrument and chromatographic conditions defined in the Compendium of GC Conditions in Room 206 Cleaning Up The small amount of phosphoric acid and 2 methylcyclohexanol remaining in the distillation flask and the aqueous layer remaining in the reaction tube can be flushed down the drain with lots of water Final Report Attach your gas chromatogram including the GC analysis parameters sheet to your final report By analyzing your gas chromatogram establish the identity of the peaks and the relative composition of the peaks which represent the products of the dehydration reaction Use the method of triangulation or an electronic integrator to determine the relative amounts of each alkene present in the product mixture Be sure to label each significant peak and to show how you determine composition By referring to your experimental results and the accepted mechanism of reaction for the acid catalyzed dehydration of alcohols offer a simple explanation for the presence of the two products and their relative abundance Answer the following questions at the end of your report Exp t 73 1 The method of spiking is frequently used to positively identify a component in a mixture When this method is used 2 chromatograms are run 1 a chromatogram of the mixture and 2 a chromatogram of the mixture to which has been added a small amount or spike of a known standard compound For example a mixture is known to contain 50 compound A and 50 compound B The first gas chromatogram 1 of this mixture reveals 2