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?
Please explain

The liquid is diethyl ether. The T at 140 produces the ether. Raising the T to 170-180 C will get the alkene.

To understand why a low-boiling liquid was obtained instead of ethylene, we need to examine the reaction mechanism involved in the dehydration of ethanol.

The dehydration of ethanol involves the removal of a water molecule (H2O) from ethanol (C2H5OH). In this reaction, sulfuric acid (H2SO4) acts as a dehydrating agent by providing the necessary protons (H+) for the reaction.

The reaction mechanism proceeds as follows:
C2H5OH + H2SO4 -> C2H5OH2+ + HSO4- (protonation of ethanol)
C2H5OH2+ -> C2H4 + H3O+ (loss of water molecule)

In theory, under high temperature (140°C) and concentrated sulfuric acid conditions, ethylene (C2H4) should be formed as the main product. However, in practice, several factors could lead to the formation of a different product.

The most likely reason for the formation of a low-boiling liquid instead of ethylene is the rearrangement of the intermediate carbocation formed during the reaction.

In the presence of strong acids like sulfuric acid, carbocations can undergo rearrangement reactions, leading to the formation of different products. In this case, the intermediate carbocation formed during the dehydration of ethanol may undergo rearrangement, resulting in the formation of a low-boiling liquid as the major product instead of ethylene.

To obtain ethylene as the desired product, you can consider the following changes in reaction conditions:

1. Use a milder acid: Instead of using concentrated sulfuric acid, a milder acid such as phosphoric acid (H3PO4) or a solid acid catalyst like alumina (Al2O3) could be used. These milder acids are less likely to induce carbocation rearrangements and promote the direct formation of ethylene.

2. Lower the reaction temperature: By reducing the reaction temperature, you can minimize the extent of carbocation rearrangement and favor ethylene formation. Temperature reduction limits the energy available for rearrangement, thereby increasing the yield of the desired product.

3. Shorten the reaction time: Reacting ethanol with sulfuric acid for extended periods can increase the likelihood of carbocation rearrangement. By reducing the reaction time, you can limit the opportunity for rearrangement reactions and enhance the production of ethylene.

In summary, a low-boiling liquid was formed instead of ethylene due to the rearrangement of the intermediate carbocation formed during the dehydration reaction. Adjusting the reaction conditions by using a milder acid, lowering the reaction temperature, and shortening the reaction time can help prevent carbocation rearrangement and increase the yield of ethylene.