Video Transcript
Butan-1-ol is strongly heated under
reflux with excess acidified potassium dichromate(VI). What is the product of this
oxidation reaction? (A) But-1-ene, (B) but-2-enal, (C)
butanal, (D) butane, or (E) butanoic acid.
Let’s start by drawing the
structure of butan-1-ol. The prefix but- indicates that this
molecule contains four carbon atoms. A-N indicates that the carbon atoms
are single bonded together. -ol tells that the primary
functional group in this molecule is an alcohol. And the number one indicates that
the hydroxy group of the alcohol is bonded to the first carbon atom of the
chain. We can complete the structure of
butan-1-ol by adding hydrogen atoms so that each carbon atom has four total
bonds. As the hydroxy group is bonded to a
carbon atom that has one alkyl substituent, butan-1-ol is classified as a primary
alcohol.
We are told in the question that
butan-1-ol is strongly heated under reflux with excess acidified potassium
dichromate(VI). Our goal is to determine the
product of this reaction. Acidified potassium dichromate is
an oxidizing agent. So let’s consider the reaction
between a primary alcohol and a generic oxidizing agent, represented here by a
capital O in brackets. Over the course of this reaction,
the hydrogen of the hydroxy group and one of the hydrogen atoms bonded to the same
carbon atom as the hydroxy group will be removed. To make up for this loss of bonds,
a new carbon-oxygen double bond will be formed. This reaction produces an aldehyde
and water. But aldehydes are easily
oxidized.
If there is still oxidizing agent
and water present in the reaction vessel, then the newly formed aldehyde can be
oxidized in a series of complex steps to produce a carboxylic acid. By closely monitoring the reaction
conditions, we can attempt to control which product is ultimately formed, an
aldehyde or a carboxylic acid. If an aldehyde is the desired
product, then the reaction should be carried out with excess alcohol and the
aldehydes should be distilled as it is produced to prevent further oxidation. If the carboxylic acid is the
desired product, then the reaction should be carried out with excess oxidizing agent
under reflux to ensure that any aldehyde produced via the first step is further
oxidized to the carboxylic acid.
Looking at the reaction conditions
given in the question, we can be fairly certain that the primary organic product
produced is a carboxylic acid. To determine the structure of this
molecule, let’s compare the structure of the generic primary alcohol and the
carboxylic acid it produces. We can see that the R group and the
carbon atom bonded to the hydroxy group in the primary alcohol are in the same
location in the carboxylic acid. So we can start to draw the
carboxylic acid by redrawing the R group and the carbon atom that is bonded to the
hydroxy group in butan-1-ol.
Looking at the structure of a
primary alcohol, we see that the carbon atom which is not a part of the R group is
single bonded to a hydroxy group and two hydrogen atoms. But in the carboxylic acid, the
same carbon atom is double bonded to an oxygen atom and single bonded to a hydroxy
group. So to complete the structure of a
carboxylic acid, the carbon atom that is not a part of the R group must be double
bonded to an oxygen atom and single bonded to a hydroxy group.
Now that we know the structure of
the product, we need to name it so that we can choose the correct answer. Like butan-1-ol, the structure
contains four carbon atoms that are joined by single bonds. So the name of this molecule should
begin with butan-: but- meaning four carbon atoms and -an- indicating that the
carbon atoms are single bonded together. The name butan-1-ol ended in -ol,
indicating that the molecule was an alcohol. This product is a carboxylic acid,
so we use the ending -oic acid to indicate the functional group. Unlike butan-1-ol, no position
number is needed in the name, as a single carboxylic acid is always found at the end
of the carbon chain. So we have determined that the
product of the oxidation of butan-1-ol is answer choice (E) butanoic acid.
