Which of the following structures is not a functional group isomer of C3H6O2? Five structural formulas are provided below.
In this question, we are provided the molecular structures of five different molecules. And we need to determine which of the molecules is not a functional group isomer of C3H6O2. First of all, a functional group is a part of a molecule made of particular atoms with a specific bonding arrangement. Functional groups tend to behave in specific, predictable ways. For example, molecules with the same functional group may have similar physical and chemical properties. Functional group isomers are molecules with the same molecular formula but different functional groups.
To solve this problem, we should examine each structural formula to identify the functional groups present and to write a molecular formula. The molecule that does not have the provided molecular formula is not a functional isomer of C3H6O2. Let’s begin with molecule (A). The functional group in this molecule is a carboxyl group, which means that the molecule is a carboxylic acid. The name of this carboxylic acid is propanoic acid. We won’t be going over naming carboxylic acids in this video, but we can recognize that the stem prop- indicates that there are three carbon atoms present in the carbon chain. In addition, the suffix -oic plus the word acid are used in the names of all carboxylic acids.
Now, let’s determine the molecular formula of this molecule. There are three carbon atoms, six hydrogen atoms, and two oxygen atoms in this molecule. Therefore, its molecular formula is C3H6O2. Because this molecular formula is the same as the one provided in the question, this molecule is an isomer.
Molecule (B) contains two functional groups: an aldehyde functional group and an alcohol functional group known as a hydroxy group. The name of this molecule is 3-hydroxypropanal. In this name, hydroxy indicates the presence of the hydroxy group, and this group is located on the third carbon atom in the carbon chain, and the ending A-L signifies the presence of the aldehyde group. The molecule contains three carbon atoms, six hydrogen atoms, and two oxygen atoms. Therefore, like molecule (A), it has a molecular formula of C3H6O2. However, because this molecule has different functional groups than molecule (A), it is a functional group isomer.
Molecule (C) also has two functional groups: a carboxyl group and a carbon-to-carbon double bond. The name of this molecule is 2-propenoic acid, although it’s also often written as prop-2-enoic acid. Either way, the E-N in the name indicates the presence of the carbon-to-carbon double bond in the carbon chain, and the number two tells us that the carbon-to-carbon double bond begins on the second carbon atom in the carbon chain. We can also recognize here that the suffix -oic plus the word acid tells us the molecule is a carboxylic acid. The molecule contains three carbon atoms, four hydrogen atoms, and two oxygen atoms. Therefore, its molecular formula is C3H4O2. This molecular formula is different than the one provided in the question. Therefore, this molecule cannot be a functional group isomer of C3H6O2.
Let’s take a moment to examine molecules (D) and (E) to find out why they are functional group isomers of C3H6O2. Molecule (D) has a total of three functional groups, two hydroxy groups, and one carbon-to-carbon double bond. The name of this molecule is propene-1,1-diol, although it’s also often written as prop-1-ene-1,1-diol. Molecule (E) has the ester functional group, and its name can be written as methyl ethanoate or methyl acetate. The rules for naming these molecules is beyond the scope of this video. It’s more important to recognize that both of these molecules have the molecular formula provided in the question, so they must be functional group isomers of C3H6O2.
In summary, all of the molecules in this question differ by the types of functional groups present. However, only molecule (C) does not have the molecular formula C3H6O2. Because molecule (C) has a different molecular formula, it is not a functional group isomer of C3H6O2.