Alkene Formation: Dehydrohalogenation and Dehydration

chemical equation showing the dehydrohalogenation of 1- and 2-bromobutane

chemical equation showing the dehydration of 1- and 2- butanol

During this activity students investigate E2 and E1 reactions by reacting 1- and 2-bromobutanes with ethoxide and by reacting 1- and 2-butanol with sulfuric and phosphoric acid. The product distributions are investigating by examining chromatograms collect from gas chromatographic analysis of the gaseous products.

The reaction of 1-bromobutane with potassium ethoxide.

a chemical eauation showing the production of an 1-butene from the reaction of 1-bromobutane and potassium ethoxide.
Since the setup for the four reactions is extremely similar on one reaction sequence is presented below.

A solution of potassium ethoxide in ethanol (3 mL) is added to a 5-mL conical vial.

1-Bromobutane (0.32 mL) is added to the 5-mL vial. The ground glass joint on the gas-delivery tube is greased with silicone grease and connected to the vial.

The apparatus is inserted into an aluminum block that has already been heated to 80 to 90 °C, an previously prepared, water-filled, inverted test tube is placed over the gas-delivery tube, and the gas being pushed out of the 5-mL vial is collected. A after a few moments bubbles can be seen collecting in the inverted test tube.

After collecting approximately 4 mL of gas, the test tube is removed and a gas collection tube capped with a septum is placed at the end of the gas-delivery tube.

After collecting approximately several milliliters of gas, a syringe is used to collect a sample of gas for analysis on a gas chromatograph.

Gas Chromatograms of the Product Mixtures from the Dehydrohalogenation and Dehydration Reactions

Gas chromatogram of the gases collected from the reaction of 1-bromobutane with potassium ethoxide

a chemical equation showing the production of 1-butene from the reaction of 1-bromobutane and potassium ethoxide.
A gas chromatogram of the gases collected from the reaction of 1-bromobutane with potassium ethoxide. The chromatogram shows two peaks. The first peak due to air in the sample appears at a retention time of 0.471 min with a relative area of 86.87. The second peak due to 1-butene appears at a retention time of 2.863 min with a relative area of 13.13.

Gas chromatogram of the gases collected from the reaction of 2-bromobutane with potassium ethoxide

a chemical equation showing the production of 1-butene and Z- and E-2-butene from the reaction of 2-bromobutane and potassium ethoxide.
A gas chromatogram of the gases collected from the reaction of 2-bromobutane with potassium ethoxide. The chromatogram shows four peaks. The first peak due to air in the sample appears at a retention time 0.485 min with a relative area of 53.90. The second due to 1-butene appears at a retention time of 2.938 min with a relative area of 10.88. The third peak due to E-2-butene at a retention time of 3.500 min with a relative area of 27.27. The fourth peak due to Z-2-butene appears at a retention time of 4.065 minutes with a relative area of 7.95.

Gas chromatogram of the gases collected from the reaction of 1-butanol with sulfuric and phosphoric acids

a chemical equation showing the production of 1-butene and Z- and E-2-butene from the reaction of 1-butanol with sulfuric and phosphoric acids.
A gas chromatogram of the gases collected from the reaction of 2-bromobutane with potassium ethoxide. The chromatogram shows five peaks. The first peak due to air in the sample appears at a retention time 0.254 min with a relative area of 14.68. The second due to an impurity (perhaps 2-methylpropene) appears at a retention time of 1.061 min with a relative area of 2.42. The third peak due to 1-butene at a retention time of 1.298 min with a relative area of 9.60. The fourth peak due to E-2-butene appears at a retention time of 1.487 minutes with a relative area of 42.32. The fifth peak due to Z-2-butene appears at a retention time of 1.734 minutes with a relative area of 30.99.

Gas chromatogram of the gases collected from the reaction of 2-butanol with sulfuric and phosphoric acids

a chemical equation showing the production of 1-butene and Z- and E-2-butene from the reaction of 2-butanol with sulfuric and phosphoric acids.
A gas chromatogram of the gases collected from the reaction of 2-bromobutane with potassium ethoxide. The chromatogram shows four peaks. The first peak due to air in the sample appears at a retention time 0.264 min with a relative area of 48.88. The second peak due to 1-butene at a retention time of 1.452 min with a relative area of 8.18. The third peak due to E-2-butene appears at a retention time of 1.686 minutes with a relative area of 26.59. The fourth peak due to Z-2-butene appears at a retention time of 1.941 minutes with a relative area of 21.35.