Video Transcript
Two different experiments were
performed under different reaction conditions with propan-1-ol. Details on each experiment are
shown in the scheme below. What are the names of products A
and B? (A) A is propanone; B is
propanal. (B) A is propanal; B is
propanone. (C) A is propanone; B is propanoic
acid. (D) A is propanoic acid; B is
propanal. Or (E) A is propanal; B is
propanoic acid.
To answer this question, we need to
determine the names of products A and B. A and B are both made from the same
starting material, propan-1-ol. Looking at the structure of
propan-1-ol, we see a hydroxy group bonded to a carbon atom that has one alkyl
substituent. So propan-1-ol is a primary
alcohol. Both A and B are produced when
propan-1-ol reacts with potassium dichromate, an oxidizing agent. Let’s clear some space for now to
take a closer look at these reactions.
When a primary alcohol is reacted
with an oxidizing agent, represented here by a capital O in brackets, the hydrogen
of the hydroxy group and one of the hydrogen atoms bonded to the same carbon as the
hydroxy group will be removed. To make up for this loss of bonds,
a new carbon-oxygen double bond will form between the two atoms that each lost a
hydrogen atom. This produces an aldehyde and
water. Aldehydes are easily oxidized. If there’s still oxidizing agent
and water present in the reaction vessel, the newly formed aldehyde will 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 run with excess alcohol and the aldehyde should
be distilled as it is produced to prevent it from oxidizing further. If a carboxylic acid is the
preferred product, then the reaction should be run with excess oxidizing agent under
reflux to ensure that any aldehyde produced is further oxidized to the carboxylic
acid.
If we look at the reaction
conditions provided in the scheme, we can see that the first set of reaction
conditions indicates that product A is an aldehyde. And the second set of reaction
conditions indicates that product B is a carboxylic acid. We can use the general equations to
help us draw the structures of A and B. Looking at the first equation, we
can see that the R group and the carbon atom bonded to the hydroxy group in the
primary alcohol are in the same position in the product aldehyde. So, we can start drawing structure
A by redrawing the R group and the carbon atom that is bonded to the hydroxy
group.
In the primary alcohol, we see that
the carbon atom that is not a part of the R group is single bonded to a hydroxy
group and two hydrogen atoms. When the aldehyde is formed, this
carbon atom becomes double bonded to an oxygen atom and single bonded to one
hydrogen atom. So, to complete our structure, we
need to replace the hydroxy group and two hydrogen atoms of the primary alcohol with
one double-bonded oxygen atom and one single-bonded hydrogen atom.
Now that we know the structure of
A, we can determine the structure of B. Looking at the second reaction
equation, we can see that the aldehyde and carboxylic acid have the same basic
structure except the position occupied by a hydrogen atom in the aldehyde is
occupied by a hydroxy group in the carboxylic acid. So, to draw structure B, we can
redraw the structure of A, then replace the hydrogen atom with a hydroxy group.
Now that we have identified the
structures of A and B, let’s take another look at the answer choices. We can see that each name provided
in the answer choices starts with “propan,” prop- meaning three carbon atoms and A-N
indicating that the carbon atoms are single bonded together. We can see that the structures of A
and B both contain three carbon atoms that are joined by single bonds.
While the beginning of each name
provided in the answer choices is the same, the endings are different, with the
names ending in either -one, -al, or -oic acid. Each ending indicates a different
functional group. -one indicates that the structure
contains a ketone, -al is used to indicate the presence of an aldehyde, and -oic
acid indicates that the molecule is a carboxylic acid. Since product A is an aldehyde, we
should expect the name to end in A-L. Thus, the name of product A is
propanal. As product B is a carboxylic acid,
we should expect the name to end in -oic acid. Thus, the name of product B is
propanoic acid.
Therefore, the answer choice which
correctly names the products of the oxidation of propan-1-ol is answer choice
(E). A is propanal; B is propanoic
acid.
