Video Transcript
Excess ethanol was reacted with
concentrated sulfuric acid at 130 degrees Celsius to produce a product with the
molecular formula C4H10O. The product was found not to be an
alkene or an alcohol. What class of molecule might the
product be?
To answer this question, we need to
consider the reaction between ethanol and sulfuric acid. The ending “ol” indicates that
ethanol is an alcohol. “Eth-” indicates that the molecule
contains two carbon atoms, and “an” tells us that the atoms are single bonded
together. With the information the name
provided, we can draw this structure for ethanol. When alcohols are reacted with
concentrated sulfuric acid, a dehydration reaction can occur. A dehydration reaction is a
reaction that involves the loss of water. Over the course of this reaction,
the hydroxy group and a hydrogen atom bonded two positions away from the hydroxy
group are removed.
To make up for this loss of bonds,
a new carbon-carbon double bond is formed. The products of the dehydration of
ethanol are ethene and water. Ethene is an alkene, an unsaturated
hydrocarbon that contains at least one carbon-carbon double bond. We are told in the question that
the product was found not to be an alkene. So a different reaction must have
taken place.
Let’s look at the structure of
ethanol again. The hydroxy group is bonded to a
carbon atom that has one alkyl substituent. Alcohols with this bonding patterns
are known as primary alcohols. Primary alcohols can dehydrate to
form alkenes like we’ve shown. But this occurs at temperatures
around 180 degrees Celsius. The reaction described in the
question occurs at a much lower temperature.
When excess ethanol is reacted with
concentrated sulfuric acid at 130 degrees Celsius, a reaction occurs between two
ethanol molecules. This reaction will produce water as
a product so it can still be classified as a dehydration reaction. But it is also often classified as
a substitution reaction. A substitution reaction is a type
of reaction where a part of a molecule is removed and replaced with something
else. In this substitution reaction, the
hydroxy group of one ethanol molecule is replaced with the alkoxy group of the other
ethanol molecule.
This reaction produces a
symmetrical product that has an oxygen atom single bonded to two carbon chains. This molecule has four carbon
atoms, 10 hydrogen atoms, and one oxygen atom. So the molecular formula of
ethoxyethane matches the molecular formula of the product given in the question.
We want to know what class of
molecule this product is. Molecules which contain an oxygen
atom bonded to two alkyl groups are known as ethers. So the class of molecule produced
when excess ethanol reacts with concentrated sulfuric acid at 130 degrees Celsius is
an ether.
