Video Transcript
The reduction of a carboxylic acid
with hydrogen and a CuCr2O4 catalyst gives the following product. What structure did the original
carboxylic acid have?
In order to answer this question,
we need to recognize what occurs during the reduction of a carboxylic acid. A carboxylic acid contains the
carboxyl functional group. When a carboxylic acid is reacted
with a reducing agent represented here by a capital H in brackets, the carboxyl
functional group is reduced to a primary alcohol, and water is produced. This is the general equation for
the reduction of a carboxylic acid. In this question, the reducing
agent is hydrogen, typically in the form of hydrogen gas. And copper chromate is added as a
catalyst. We are given the structure of the
alcohol produced. Using this information, we need to
determine the structure of the original carboxylic acid.
Let’s take another look at the
general equation for guidance. Notice that the R group of the
carboxylic acid remains unchanged when forming the alcohol, as does the carbon atom
bonded to the R group. In addition, both molecules have a
hydroxy group bonded in the same position. Thus, this portion of the
carboxylic acid and alcohol, shown here in pink, are the same in both molecules. The difference between the two
structures is that the double-bonded oxygen atom of the carboxylic acid is replaced
by two hydrogen atoms to form the alcohol. This is not exactly what occurs
during the reaction but can help us see how the two molecules are related to one
another.
Now let’s look at the alcohol given
in the question. We know that the alcohol and the
original carboxylic acid will both have a hydroxy group bonded to a carbon atom
which is bonded to the same R group. We saw in the general equation that
the double-bonded oxygen atom of the carboxylic acid was replaced by two hydrogen
atoms when forming the alcohol. Working backwards, we can see that
the two hydrogen atoms in the alcohol must have originally been a double-bonded
oxygen atom in the carboxylic acid. This structure for the molecule
3-methylbutanoic acid is the structure of the original carboxylic acid used in this
reduction reaction.
