Video Transcript
Consider the following
reaction. What product is formed?
In this reaction, a compound
containing an aryl halide and a carboxylic acid is reacted with an alcohol in the
presence of hydrochloric acid, an acid catalyst. Our goal is to determine the
product formed from this reaction. Let’s remove the question and
answer choices for now to give us room to discuss this reaction.
The first thing we need to
determine is which functional group will react with the alcohol under these
conditions. Under certain conditions, alcohols
can react with organohalides. For a reaction to occur, the
alcohol must first be treated with a strong base, like sodium amide, to produce an
alkoxide. Then, the alkoxide can be reacted
with an organohalide, like an alkyl halide. The major product of this reaction
is an ether. This reaction scheme is unbalanced
and only shows the reactants, reagents, and major products of each step of the
reaction.
This overall reaction is known as
the Williamson ether synthesis. The Williamson ether synthesis is a
substitution reaction between an organohalide and an alkoxide to form an ether. There are two main reasons why this
reaction is not the reaction involved in this question. The first reason is that Williamson
ether synthesis requires a strong base, but the reaction given in the question uses
a strong acid catalyst. So, the first step of Williamson
either synthesis would not be able to occur. The second reason is that during
Williamson ether synthesis, the bond between the halogen and the rest of the organic
molecule must break in order to form the ether. The bond between the chlorine and
the benzene ring in an aryl halide is very strong due to resonance and is extremely
unlikely to break under these reaction conditions.
With this information in mind, we
have determined that a reaction between the aryl halide and the alcohol is extremely
unlikely to occur. So, we should focus our attention
on the reaction between a carboxylic acid and an alcohol in the presence of an acid
catalyst. When a carboxylic acid and an
alcohol react in the presence of an acid catalyst, the alkoxy group of the alcohol
replaces the hydroxy group of the carboxylic acid. The products of this reaction are
an ester and water. This reaction is an example of
esterification, a type of chemical reaction where an ester is formed. More specifically, the
esterification of a carboxylic acid with an alcohol in the presence of an acid
catalyst is called Fischer esterification.
With this information in mind,
let’s return to the reaction given in the question. We know that during Fischer
esterification, the alkoxy portion of the alcohol will replace the hydroxy group of
the carboxylic acid. The rest of the molecule shown here
in blue will remain unchanged. The two products of this reaction
are a molecule containing both an aryl halide and an ester, and the second product
is water.
Now that we’ve determined the
products of this reaction, let’s take another look at the answer choices. We can see that the answer choice
which matches the product that we drew is answer choice (B). Thus, the product formed from the
given reaction is answer choice (B).
