Video Transcript
The following product results from
the oxidation of an alcohol. Was the alcohol primary, secondary,
or tertiary? Which of the following alcohols
could the reactant be?
Let’s remove the second question
for now to give us some space. Before we discuss the reaction
mentioned in the question, let’s take a look at the difference between primary,
secondary, and tertiary alcohols. An alcohol is a molecule that
contains a hydroxy group. In a primary alcohol, the hydroxy
group is bonded to a carbon atom that has one alkyl substituent. In a secondary alcohol, the hydroxy
group is bonded to a carbon atom that has two alkyl substituents. And in a tertiary alcohol, the
hydroxy group is bonded to a carbon atom that has three alkyl substituents.
We are told in the question that an
alcohol undergoes oxidation to form the product shown. When an alcohol is mixed with an
oxidizing agent, represented here by a capital O in brackets, an oxidation reaction
can occur. Over the course of this reaction,
the hydroxy group and the carbon atom bonded to the hydroxy group will each lose a
hydrogen atom. To account for this loss of bonds,
a new carbon-oxygen double bond will be formed. The products of this reaction are a
molecule which contains a carbonyl group, a carbon atom double bonded to an oxygen
atom, and water. The type of carbonyl-containing
molecule formed depends on the structure of the original alcohol and the reaction
conditions used.
Let’s take a look at the oxidation
of a primary alcohol. The hydroxy group and the carbon
atom bonded to the hydroxy group will each lose a hydrogen atom, and a new
carbon-oxygen double bond will be formed. This produces a molecule in which
an oxygen atom is double bonded to a carbon atom that has one alkyl substituent and
one hydrogen substituent. This structure is an aldehyde. Now, let’s take a look at the
oxidation of a secondary alcohol. The two hydrogen atoms are lost,
and a new carbon- oxygen double bond is formed. This produces a product in which an
oxygen atom is double bonded to a carbon atom that has two alkyl substituents. This structure is called a
ketone.
Now, let’s look at the oxidation of
tertiary alcohols. The hydroxy group has a hydrogen
atom that could be lost during oxidation. However, the carbon atom bonded to
the hydroxy group does not have a hydrogen substituent. Thus, tertiary alcohols do not
undergo oxidation, so we know that tertiary cannot be the answer to this
question. Let’s have a look at the product
provided in the question. We see that this molecule has an
oxygen atom double bonded to a carbon atom that has two alkyl substituents. This is the structure of a
ketone. As primary alcohols oxidize to form
aldehydes and secondary alcohols oxidize to form ketones, this product must be the
result of the oxidation of a secondary alcohol.
Now, let’s return to the second
question.
Which of the following alcohols
could the reactant be?
We’ve already established that
during the oxidation of a secondary alcohol, the hydroxy group and the carbon atom
bonded to the hydroxy group will each lose a hydrogen atom, and a new oxygen-carbon
double bond will be formed, producing a ketone. In this question, we know the
structure of the ketone that is formed, and we want to know the structure of the
original alcohol. To work backwards from the ketone
to the alcohol, we can undo what occurred during oxidation. During oxidation, an oxygen-carbon
double bond was formed. This means that in the original
alcohol, these two atoms were single bonded together. During oxidation, the hydroxy group
lost a hydrogen atom.
To undo this, we need to reattach a
hydrogen atom to the oxygen atom. The carbon atom bonded to the
hydroxy group also lost a hydrogen atom during oxidation. To undo this, we need to bond a
hydrogen atom to the carbon atom bonded to the hydroxy group, so this is the
original structure of the alcohol. Looking at the answer choices, we
can see that the structure that we drew matches the structure of answer choice
(D). Thus, the alcohol that could be the
reactant is (D).
