The Acid-catalyzed Dehydration of a secondary alcohol like 3-methylcyclohexanol follows an E1 mechanism in which a 2° carbocation is formed as an intermediate. Two sequential hydride shifts could result in the formation of the more stable 3° carbocation. This rearrangement would require one 2° carbocation to rearrange to another 2° carbocation: a relatively high energy carbocation rearrangement. We will investigate the extent to which this rearrangement takes place by analyzing the products of this dehydration by Gas Chromatography.
In order to prepare for this lab, you must study the microscale procedure given for Experiment 11.2 in your "Modern Projects and Experiments" textbook. Also review your "Techniques" textbook for a review on microscale distillation (11.3c), extraction (8.7) and Gas Chromatography (16).
Once you have read the text, you will need to prepare your notebook with the following:
The purpose of this lab is to investigate this alcohol dehdration reaction and to gain further insight to the kinetics of carbocation rearrangement in acidic aqueous solution.
This is an important part of your pre-lab. Be sure to include each possible intermediate in sequence and use curved arrows to show the movement of electrons in every step including proton transfer steps.
Table of Reactants/ Products/ Solvents
Calculate the yield of all combined cyclohexene isomers.
Make this an outline of what you will do, not how you will do it.
Leave room to the right of the procedure for notes/ observations/ changes on the day of the experiment.
During the lab:
You will be distilling the alkene product as it forms in order to shift the equilibrium in its direction. The reaction works best when stirring is provided, so check out a micro-stirbar from the stockroom. Take care not to lose it- you will need to return the stirbar to the stockroom and will be charged if it is not recovered. It is important to collect the alkene product into a vial cooled in an ice-water bath as it forms. The alkenes are relatively volatile and can easily escape following the distillation. Be sure to weigh the vial before and after the distillation in order to estimate the percent yield.
As always, take care during the extraction to save all layers until you are certain that you have isolated the alkene product. Once you have the product, you will store it in the refrigerator in a tightly-sealed, well labeled vial, until the next laboratory period. Label the vial with your name, the contents of the vial (methylcyclohexene) and the date.
If you find yourself running short on time on the first day, you may stop after the distillation and before the extraction, but you must store your product in a tightly sealed, well labeled vial in the refrigerator until the next laboratory period.
The alkene product (1-2 μL) will be injected NEAT onto the GC column during the next lab period (we will not be using a capillary GC for this separation, and so you will NOT dilute the product with diethyl ether). You will need the results of peak enhancement to confirm the identities of the alkene products. Do not leave until you have obtained the result of these experiments and have sketched the results in your notebook.
Measure the areas of each product signal by measuring the height of each signal and multiplying it by the width at half the height. If necessary, expand your GC trace on a copy machine to minimize your error. Remember that 3-methylcyclohexene and 4-methylcyclohexene will not be resolved by our packed-column GC, so you may treat their overlapping signals as representative of "unrearranged" product.
Complete the summary sheet (.doc) and attach it to the top of your data and a copy of your GC trace.
This lab is due one week from the day that the GC is scheduled.
|©Kathleen Armstrong||May, 2009|