Hydrolysis of Acetanilide: Mechanism & Explanation

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

In this lesson, we will get a detailed overview of the hydrolysis reaction acetanilide undergoes under acidic conditions. The primary focus will be on the mechanistic steps and an explanation behind each step of the process.

Breaking that Linkage

Let's say you went mountain climbing and your guide asked you to pick the rope you'd like to use. He laid out three choices in front of you: a single stranded rope, a double stranded rope, and a triple stranded rope. He stressed that on more than one occasion during the climb your life would depend on the rope. Having that knowledge, which rope would you pick to use? Umm. . . I think we'd all take the triple stranded rope right??? You'd pick that one because it was the strongest right? It turns out that some bonds in organic chemistry can be likened to rope in terms of their strength.

The amide bond (a nitrogen atom bonded to a carbonyl group) is a particularly strong bond in organic chemistry. For example, it's used in polymers which form materials like plastic. Even biological systems utilize the amide bond in the form of proteins built from amino acids. Although the bonding is quite strong, it can be broken under forcing conditions. In this lesson, we are going to take a detailed look at the mechanistic steps of how an amide bond is broken under acidic conditions using acetanilide as our model substrate. Let's get started and see how it works!

Steps 1 and 2

It's important to realize that we are under acidic conditions, and we will be using hydrochloric acid (a very strong acid) as our acid of choice. Remember that strong acids completely ionize in solution and so really what we have floating around in solution are hydrogen ions (or H+ ions).

In the first step of the reaction, an acid-base reaction takes place between the oxygen of the carbonyl and one of our hydrogen ions. Notice that in terms of drawing the mechanism arrow to represent this process, the arrow goes from the oxygen (electron donor) to the hydrogen ion (electron acceptor).

First mechanistic step in the hydrolysis of acetanilide

Notice that after the first step we can draw a resonance structure that places a positive charge on the carbon atom. This helps to explain the second step when a molecule of water acts as a nucleophile (electron donor) to attack the carbon. After the water molecule attacks, a second water molecule acts as a base to pull off a hydrogen atom.

Second and third step in the mechanism

Steps 3 and 4

At the start of our discussion, we compared an amide bond to a strong piece of rope. Once we get to our current point in the mechanism, the rope becomes very weak. That's because once the carbon-oxygen double bond is removed, the lone pair of electrons on the nitrogen atom is no longer able to participate in resonance, making it basic and more reactive. Think of it as a stripped down rope that is holding on by a few threads. Now the third step can happen, where the nitrogen atom attacks a hydrogen ion.

Step 3 of the mechanism in which the nitrogen atom accepts a hydrogen ion

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