Video Transcript
Polyesters can be hydrolyzed by NaOH. Which alcohol product is formed from the hydrolysis of polytrimethylene terephthalate, PTT, the structure of which is shown below? (A) Propane-1-ol, (B) ethene-1,2-diol, (C) ethanol, (D) propane-1,3-diol, (E) butane-1,4-diol.
Let’s begin by taking a closer look at the structural formula of polytrimethylene terephthalate, PTT. PTT is a polymer. Inside the brackets is a single monomer, which is the smallest repeating unit of a polymer. This monomer contains two ester groups. The 𝑛 outside of the brackets indicates that there are many more monomers in this polymer chain.
Let’s draw another monomer to the right of this one. Notice that now there are another two ester groups. Each new monomer added to the polymer chain contains two more ester groups, so this polymer is called a polyester. For this problem, we’re going to focus on only two monomers.
The question tells us that the polyester PTT is hydrolyzed by NaOH. NaOH is a base, and the word “hydrolyzed” refers to the ester hydrolysis reaction. During the reaction, the ester reacts with an aqueous solution of NaOH. And the carbon-oxygen single bonds of the ester groups are broken. The products of the reaction are an alcohol and a carboxylate salt. The portion of the molecule between the carbonyl groups becomes the alcohol, while the portion of the polymer which contains the carbonyl groups and the benzene ring becomes the carboxylate salt.
Let’s now focus on naming the alcohol product. The first step in naming the alcohol is to name the longest continuous chain of carbon atoms that contains the hydroxy groups. In this alcohol, the longest continuous chain of carbon atoms is three carbon atoms in length, so the name of the chain is propane. The prop- part of the name tells us that there are three carbons in the chain, and the -ane portion of the name tells us that there are only single bonds between the carbon atoms in the chain. Knowing this allows us to eliminate choices (B), (C), and (E) because these answer choices do not contain propane in the names.
The second step is to number the carbon atoms of the chain so that the hydroxy groups have the lowest possible numbers. By numbering from the left to right, we see that one hydroxy group is located on carbon number one, and a second hydroxy group is located on carbon number three. Even if we had numbered from right to left, we would have achieved the same result.
Finally, in the third step, we need to add numbers to tell the position of the hydroxy groups. When doing so, we’ll need two separate words and numbers by hyphens. Let’s go ahead and add hyphen, one, comma, three to the name. One and three indicate the location of the hydroxy groups. Now, we’ll need to add a suffix to the name to tell how many hydroxy groups are in the molecule. The alcohol molecule contains two hydroxy groups, so we’ll need to add the suffix -diol to the end of the name.
The name of the alcohol product formed from the hydrolysis of polytrimethylene terephthalate, PTT, is propane-1,3-diol, choice (D).