Step-Growth Polymers: Urethane - Online Tutor, Practice Problems & Exam Prep

Urethane is a carbonate ester formed through the nucleophilic addition of an alcohol to an isocyanate. This reaction involves the carbonyl group of the isocyanate and the hydroxyl group of the alcohol, resulting in a carbamate ester. The nitrogen atom of the isocyanate accepts a hydrogen from the alcohol, while the remaining alkoxy group attaches to the carbonyl carbon, creating the urethane structure. Understanding this formation is essential for grasping the broader concepts of nucleophilic addition and the properties of carbonate esters in organic synthesis.

Step-Growth Polymers: Urethane Concept 1

Hey everyone. So in this video, let's take a look at Urethane formation. Now, Urethane itself is a carbonate ester created from the nucleophilic addition. So, nucleophilic addition of an alcohol to an isocyanate molecule. Now, the carbamate ester itself is just a carbonyl group connected to an amino group.

We're going to say, NR₂, R here could be carbons or hydrogens, and an alkoxyl group which is OR, R here has to be a carbon. So here's our carbonate. So we have an NR₂ group here, and then we have an OR group here. Here we have our isocyanate, and we have an alcohol.

We're going to say the way this works is just imagine that this pi bond is forfeited so that we can attach this alcohol to this isocyanate. We'd say here that hydrogen would go to the nitrogen to give us an NH, and then the RO portion that remains in the alcohol, that'll go to the carbon, thereby giving us this ester portion right here. So that's Urethane, and more specifically, we'd have a carbonate ester. And so, this is how we would approach a basic type of Urethane formation. So just keep that in mind when it comes to this, it's isocyanate reacting with an alcohol.

The H of the alcohol, we can say that it is going to the nitrogen, and the RO portion is going to the carbon, which is our carbonyl group. This is just a very simplistic way of doing it without worrying about a mechanism.

Step-Growth Polymers: Urethane Example 1

In this example, it says, provide the structure of the product created from the reaction of isocyanate and sec-butyl alcohol. Alright. So, isocyanate here, we're going to say we have our nitrogen double-bonded to carbon, double-bonded to oxygen. It's going to react with our sec-butyl alcohol. Remember, sec-butyl alcohol looks like this.

Without the use of the mechanism, we can just say we know that hydrogen would go to nitrogen. We are forfeiting our pi bond that connected the carbon and the nitrogen together so they are now only single bonded to each other. I'm going to draw this hydrogen down like this. The carbon is still double bonded to oxygen. And then the oxygen of the alcohol is going to connect to this carbon.

So it's still connected to this carbon, which is connected to a methyl, which is connected to an ethyl. So, here, this would represent our urethane product. So remember, this would just be a simple carbonate ester product.

Provide the structure of the urethane molecule created from the reaction between phenyl isocyanate and ethylene diol. 

In this practice question, we're told to provide the structure of the urethane molecule created from the reaction between phenyl isocyanate and ethylenediol. So, phenyl isocyanate means our R group is a benzene. We'll connect it to nitrogen, which is double-bonded to carbon, double-bonded to oxygen. Ethylenediol, so that is just OH connected to 2 carbons, and then connected to another OH. We know that one of the hydrogens we have in this reaction goes to the nitrogen. Again, we're not elaborating on the mechanism because we're not asked to do that. We just have to come up with a product. The simplest way of thinking about it is H goes to nitrogen, the rest of the alcohol goes to the carbon to make our ester. We forfeit the pi bond between nitrogen and carbon to do this. Then goes oxygen. So we create this structure, which is our carbonate ester. This is a simplified way of looking at how we produce this product at the end.

Name the alcohol that was used to create the following urethane molecule.

• A. (1S,3S)-3-methyl-1-cyclopentanol
• B. (1S,3R)-3-methyl-1-cyclopentanol
• C. (1R,3S)-3-methyl-1-cyclopentanol
• D. (1R,3R)-3-methyl-1-cyclopentanol