Dehydration of 2-Butanol, an E1 Elimination Reaction

Procedure:

Assemble the gas collection set-up as shown in the diagram. Use a trough for the water bath.To position the 5 ml collection vial, fill it with water, place your finger over the open end, invert it, and place the vial open end down in the trough full of water. Clamp the vial upside down making sure that the open end of the vial stays under the water. The vial should be full of water when you are finished positioning and clamping it. As the gas is produced it will bubble into the 5 ml vial and displace the water.

Place 100 µl of 2-butanol and 50 µl of concentrated sulfuric acid into a clean, dry 3 ml vial with a small spin vane. Screw on the cap containing the septum and the syringe needle attached to the tubing. Place the 3 ml vial in an aluminum block on a hot plate. Place a thermometer in the aluminum block also. Put the end of the tubing in the water bath but not yet up inside the gas collection vial.

Heat and stir the reaction mixture. Allow any bubbles, which inititally should be just air, to come out into the water bath for about a minute. When the temperature of the aluminum block reaches around 110-120˚C put the end of the tubing up inside of the 5 ml collection vial and collect thebutene gases that are the reaction products. Continue collecting gas until it appears that all of the water has been displaced from the collection vial. Remove the tubing from the trough and turn off the hot-plate.

Working underwater, screw a cap with a septum onto the top of the collection vial and remove the collection vial from the water bath. Be sure the cap is on snugly. Wait your turn to inject 500 µl of your gas mixture onto column A of the GC, using the hypodermic syringe and needle provided. Column A is ¼ inch X 8 ft packed with 20% silicone DC-710.

In your notebook record all of the GC parameters that are listed on your handout titled “331 and 332 Laboratory Notebook.”

The first peak that forms is an air peak. You can ignore it. The next 3 peaks are your 3 butene products. The butenes have been determined to elute from the column in the following order: 1-butene, trans-2-butene, cis-2-butene. If the reaction mixture is heated above the recommended temperatures, a rearrangement can occur to yield isobutene, which will be detected as an additional product.

You will be determining the retention time for each peak and the percent composition of the gas mixture based on the area under each peak. Detemination of the area under each peak gives reproducible results of +/- 3-4% when the areas are assumed equal to the peak height x the peak widths at half-height, measured from the baseline of the peak. Area under the peak is directly proportional to the volume of gas coming off the column and for gases under the same conditions, the volume of the gas is directly proportional to the number of moles of gas. So you are actually calculating a mole % composition.

Questions:

  1. GC analysis of a mixture of organic compounds gave the following peak areas in cm2:

hexane = 2.7, heptane = 1.6, hexanol = 1.8, and toluene = 0.5.

a.)Calculate the mole % composition of the mixture.

b.)Calculate the weight % composition of the mixture (moles x molecular weight will give you the relative weight of each component.)

  1. If the mixture is heated too strongly, rearrangement can occur and isobutene (2-methyl-2- propene) can form. Write out a complete mechanism to account for its formation.
  1. When t-pentyl bromide is treated with 80% ethanol t-pentyl alcohol and t-pentyl ethyl ether are formed. But the following amounts of alkene products are also detected on analysis: 8% 2-methyl-1-butene and 32% 2-methyl-2-butene. Explain why the 2-methyl-2-butene is formed in greater yield compared to the terminal alkene.

4. The –SR2+ group is easily removed in elimination reactions, but the –SR group is not.

Explain.

5. Why is sulfuric acid, rather than hydrochloric acid, used to catalyze the dehydration of

alcohols?