Video Transcript
In an experiment, a sample of
propanoic acid is heated in the presence of CuCrO4 at 200 degrees Celsius. The reaction scheme is shown. What name does product B have? (A) Butan-1-ol, (B) ethanol, (C)
butan-2-ol, (D) propan-1-ol, (E) propan-2-ol.
The question tells us that the
first reactant is propanoic acid. Propanoic acid reacts with an
unknown reactant A in the presence of a catalyst and high temperature to produce an
unknown product B and water. In this question, our job is to
determine the name of product B. Let’s begin by taking a closer look
at propanoic acid. We can identify propanoic acid as a
carboxylic acid because the name contains the -oic suffix and the word acid.
We can also identify propanoic acid
as a carboxylic acid by examining its structural formula. The molecule contains the carboxyl
group, which is the functional group of a carboxylic acid. It’s helpful to recall that copper
chromate is a catalyst used in reduction reactions. When a reducing agent is present
and the reaction is carried out at high temperature, the complete reduction of the
carboxylic acid occurs. The products of the reaction are a
primary alcohol and water. So what primary alcohol is going to
form?
Let’s start by examining propanoic
acid again. Notice that the carbon-based chain
of propanoic acid contains three carbon atoms. So the primary alcohol produced
will also have a carbon-based chain of three carbon atoms. It’s important to recall that
reducing agents do not react with the carbon backbone. Let’s add the hydrogen atoms to the
first two carbon atoms in the structural formula. In contrast, when the reducing
agent reacts with the carboxyl group, a hydroxy group forms. Recall that in a primary alcohol,
the carbon atom that’s bonded to the hydroxy group is bonded to only one other
carbon atom. To complete our structural formula,
we’ll need to add two hydrogen atoms to the third carbon in the carbon backbone.
Now we’re going to take time to
name the alcohol. The first step in naming a primary
alcohol is to name the longest continuous carbon chain that contains the OH
group. We’ll also omit the final -e. The longest continuous carbon chain
has three carbon atoms, so the name is propane. Prop- indicates the presence of
three carbon atoms, and the suffix -ane tells us that the carbon backbone has only
carbon-to-carbon single bonds. This gives us propan, and knowing
this allows us to eliminate answer choices (A), (B), and (C) because these answer
choices do not contain propan in the name.
The second step is to number the
carbon atoms, so the OH group gets the lowest possible number. By numbering from right to left, we
see that the hydroxy group is located on carbon number one. If we had numbered from left to
right, this would’ve placed the hydroxy group on carbon number three. So we’ll need to go with the first
method. Finally, in step three, we need to
add a position number to the name and add the -ol suffix. Recall that when writing IUPAC
names, a hyphen is used between letters and numbers in the name. The 1 indicates the location of the
hydroxy group. Finally, let’s add the suffix
-ol. This suffix indicates that there is
only one hydroxy group in the molecule. The name of product B is
propan-1-ol, choice (D).
To help us answer the next
question, let’s replace B in the given equation with the structural formula of
propan-1-ol.
What other reactant is needed in
the process? How many moles are needed?
To begin, it would be helpful to
write a chemical equation for the reaction using molecular formulas. The molecular formula for the first
reactant, propanoic acid, is C3H6O2, while the molecular formula of propan-1-ol is
C3H8O. Let’s count up how many of each
type of atom we have on either side of the equation. On the left side of the equation,
we currently have a total of three carbon atoms. And on the right side, we also have
a total of three carbon atoms.
Let’s take a look at the hydrogen
atoms. On the left side of the equation,
there are six hydrogen atoms. On the right side, the first
molecule contains eight hydrogen atoms. The second molecule contains an
additional two hydrogen atoms for a total of 10 hydrogen atoms on the right-hand
side of the equation. Finally, on the left side of the
equation, there are two oxygen atoms. On the right side, the first
molecule contains one oxygen atom, and the second molecule contains an additional
one oxygen atom for a total of two oxygen atoms on the right side of the
equation. By examining the results of the
chart, we see that the carbon atoms are currently balanced. In addition, the oxygen atoms are
also balanced.
However, the hydrogen atoms are not
currently balanced. We would need an additional four
hydrogen atoms on the left side of the equation for this to be a balanced
equation. Copper chromate is considered a
hydrogenation catalyst. In this reduction reaction, the
reducing agent is diatomic hydrogen. The molecular formula of diatomic
hydrogen is H2. Therefore, we would need two moles
of hydrogen per every one mole of propanoic acid to complete the reduction
reaction.
What other reactant is needed in
the process? How many moles are needed? The answer is two moles of H2.
