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Question Video: Determining the Nature of the Reactants and Products of the Reaction of Propanoic Acid in the Presence of CuCrO₄ Chemistry

In an experiment, a sample of propanoic acid is heated in the presence of CuCrO₄ at 200°C. The reaction scheme is shown. (a) What name does product B have? (b) What other reactant is needed in the process? How many moles are needed?

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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.

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