A student wished to prepare ethylene gas by dehydration of ethanol at 140 C using sulfuric acid as the dehydrating agent. A low-boiling liquid was obtained instead of ethylene. What was the liquid, and how might the reaction conditions be changed to give ethylene?
I know that the low boiling liquid is diethyl ether by SN2. I'm just stuck on how the reaction conditions could be changed. It has to do with temp? But should it be lowered? so that the diethyl ether can't be formed?
Why not boost the temperature to 170C?
Dehydration of alcohols to ethers takes place at about 140 degrees C. Dehydration of alcohols to alkenes takes place at 170-180. So I think the answer is to raise the T.
Good job identifying the low-boiling liquid as diethyl ether, which can indeed be formed by the acid-catalyzed dehydration of ethanol. To change the reaction conditions to favor the production of ethylene instead of diethyl ether, one approach would be to adjust the temperature.
At higher temperatures (~140°C), like in the case you described, the formation of diethyl ether is favored due to the elimination of water. However, if you lower the temperature, it will favor the formation of ethylene.
To give ethylene as the main product, you can consider reducing the reaction temperature below the temperature where diethyl ether is formed (around 100°C) to decrease the likelihood of diethyl ether formation. This can be achieved by using a more controlled heating source or even performing the reaction in a cold water bath or using a lower boiling point solvent such as toluene.
By adjusting the temperature to a lower value, you are essentially decreasing the kinetic energy of the reaction, which reduces the likelihood of the formation of diethyl ether but still allows for the formation of ethylene.
Additionally, the reaction time can also affect the product distribution. If you decrease the reaction time by maximizing the contact time between the reactants and the dehydrating agent, it can favor the production of ethylene.
So, to summarize, to obtain ethylene instead of diethyl ether in the dehydration of ethanol, you can lower the reaction temperature below the temperature typically used (around 140°C) and decrease the reaction time to favor ethylene formation.