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The Sigma Insight: Alcohols, Phenols, Ethers
The reaction of ethanol () with concentrated sulfuric acid () is a beautiful example of how temperature controls the pathway of a chemical reaction. Depending on the heat supplied, the same reactants can yield entirely different products!
The Room Temperature Reaction When ethanol and concentrated sulfuric acid are mixed at room temperature, the reaction is relatively mild
The sulfuric acid acts as a strong acid, protonating the alcohol. This leads to the elimination of a water molecule and the formation of ethyl hydrogen sulphate.
Turning Up the Heat: If we increase the temperature to , the reaction dynamics change
At this temperature, the protonated ethanol molecules undergo intermolecular dehydration. Two molecules of ethanol combine, losing a single water molecule between them, to form diethyl ether.
Extreme Heat: Heating the mixture even further to triggers a different mechanism
Instead of two molecules reacting together, a single ethanol molecule undergoes intramolecular dehydration. It loses a water molecule from within its own structure, resulting in the formation of a double bond and yielding ethylene.
Why Not Acetylene? We have seen the formation of ethyl hydrogen sulphate, diethyl ether, and ethylene
But what about acetylene ()? Acetylene contains a carbon-carbon triple bond. To form acetylene from ethanol, we would need to remove four hydrogen atoms and one oxygen atom, which corresponds to an extreme level of dehydrogenation. Concentrated sulfuric acid is a powerful dehydrating agent, but it is not capable of such extensive dehydrogenation under these conditions. Therefore, acetylene cannot be formed in this reaction.
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