Video Transcript
An alcohol is heated under reflux in the presence of a sulfuric acid catalyst. The chemical reaction produces cyclohexene and water. What is the structure of the reactant alcohol?
The question describes an experiment where an alcohol was heated under reflux in the presence of a sulfuric acid catalyst. We are given the products of this reaction. We need to use this information to determine the structure of the original alcohol. Let’s remove the answer choices for now so we can discuss this reaction.
An alcohol is a molecule that contains at least one hydroxy group. When an alcohol is heated under reflux in the presence of a sulfuric acid catalyst, the hydroxy group and one hydrogen atom bonded to a carbon atom, two positions away from the hydroxy group, will be removed from the alcohol. And a new carbon-carbon double bond will be formed between the two carbon atoms that each lost a substituent. This produces an alkene and water. The reaction is an example of dehydration, a chemical reaction that involves the loss of water from a compound.
Now that we’ve looked at the dehydration of a generic alcohol, let’s take a look at the reaction described in the question. We know that the reaction described produces cyclohexene and water. To determine the structure of the reactant alcohol, we’ll first need to draw the structure of cyclohexene. The term “hex” indicates that this molecule contains six carbon atoms. The prefix cyclo- indicates that the six carbon atoms are connected in a ring.
So we can start by drawing six carbon atoms connected in a hexagonal structure. The suffix -ene tells us that this molecule is an alkene and must contain a carbon- carbon double bond. There are no position numbers in between the terms “hex” and -ene. This means that the molecule only contains one carbon-carbon double bond.
We can complete the structure by adding hydrogen atoms so that each carbon atom has four total bonds. We can now work backwards to the structure of the original alcohol. Looking at the generic reaction, we can see that the carbon-carbon double bond of the alkene was originally a carbon-carbon single bond in the alcohol. So, we can start to construct the alcohol by redrawing the structure of cyclohexene, except the carbon-carbon double bond will be drawn as a carbon-carbon single bond.
Looking at the generic reaction, we can see that the two carbon atoms that become a part of the alkene each lose a substituent: one loses a hydrogen atom and one loses a hydroxy group. So to complete the structure of the alcohol, we’ll need to add a hydrogen atom to one of the carbon atoms that becomes a part of the alkene and a hydroxy group to the other carbon atom that becomes a part of the alkene. Adding these two groups gives us the structure of the reactant alcohol, cyclohexanol.
Now that we know the structure of the reactant alcohol, we can compare it to the answer choices. We can see that the structure in answer choice (B) matches the one that we drew for cyclohexanol. So, the structure of the reactant alcohol that produces cyclohexene and water when heated under reflux in the presence of a sulfuric acid catalyst is the structure shown in answer choice (B).
