Acid-catalyzed hydrolysis of a nitrile to give a carboxylic...

Acid-catalyzed hydrolysis of a nitrile to give a carboxylic acid occurs by initial protonation of the nitrogen atom, followed by nucleophilic addition of water. Review the mechanism of base-catalyzed nitrile hydrolysis in Section $20.7,$ and then write all the steps involved in the acid-catalyzed reaction, using curved arrows to represent electron flow in each step.

Acid-catalyzed hydrolysis of a nitrile to give a carboxylic acid occurs by initial protonation of the nitrogen atom, followed by nucleophilic addition of water. Review the mechanism of base-catalyzed nitrile hydrolysis in Section
$20.7,$ and then write all the steps involved in the acid-catalyzed reaction, using curved arrows to represent electron flow in each step.

Key Concepts

Acid Catalysis

Acid catalysis refers to the process of increasing the reactivity of a substrate by protonation. In the context of nitrile hydrolysis, the acid donates a proton to the nitrogen atom of the nitrile, making the carbon atom more electrophilic and susceptible to attack by a nucleophile such as water.

Protonation

Protonation is the addition of a proton (H?) to an atom or molecule. In acid-catalyzed reactions, initial protonation of the nitrile nitrogen increases the electrophilicity of the adjacent carbon, facilitating subsequent nucleophilic attack and subsequent transformation into a carboxylic acid.

Nucleophilic Addition

Nucleophilic addition involves the attack of a nucleophile on an electrophilic center. Once the nitrile is protonated, the water molecule acts as a nucleophile, attacking the electrophilic carbon atom, which leads to the formation of an imidic acid intermediate that eventually rearranges to form a carboxylic acid.

Hydrolysis Mechanism

The hydrolysis mechanism describes the sequence of reaction steps that convert a nitrile into a carboxylic acid. It involves a series of protonation, nucleophilic addition, and deprotonation steps, ultimately resulting in the incorporation of water and the formation of the acid functionality.

Curved Arrow Formalism

Curved arrow formalism is a diagrammatic method used in organic chemistry to represent the movement of electrons during chemical reactions. It is used to illustrate each step of the mechanism, showing the transfer of electron pairs during bond making and breaking, which is essential for understanding and communicating the reaction mechanism.

Transcript

00:01 This is the answer to chapter 20, problem number 51 from the mcmurray organic chemistry textbook.

00:09 This problem asks us to write a mechanism for acid -catalyzed nitrile hydrolysis.

00:18 It asks us to review the mechanism of the base catalyzed nitriol hydrolysis, and then to write the mechanism for the acid -catalyzed hydrolysis.

00:31 It also tells us that the first step is going to be protonation of the nitrogen atom, and then that's going to be followed by nucleophilic condition of water.

00:43 Okay, so obviously that's where we should start.

00:47 So the first step, as the problem says, is going to be protonation of the nitrogen atom.

01:11 Okay, so at this point, we're going to have a positive charge on the nitrogen.

01:17 And then as the problem says, the next step is going to be addition of a water molecule that is going to look like this.

01:38 One of these bonds here in the triple bond will break and those electrons will revert to the nitrogen.

02:15 Okay.

02:17 The next step is going to be a proton transfer.

02:22 And so we can just write proton transfer.

02:58 Okay, so that'll get us to this point.

03:01 From here, one of these lone pairs of electrons will add in here.

03:10 One of these bonds will break.

03:33 And so as you can see, what we're working towards here is formation of an amide.

03:40 And so the last step that's going to have to happen to make that possible is going to be deprotonation.

03:51 And so we can just use another equivalent of water.

03:58 I guess i'm trying to write too high up.

04:01 So we can just use another equivalent of water to make that happen.

04:23 Okay.

04:24 And so at this point we have formed an amide.

04:38 Okay.

04:41 So the second part of this mechanism, i'm going to just redraw the amide down here just to give myself some space.

04:50 Not really any point in trying to crowd everything in.

04:54 So we have our amide that we made looks like this.

05:03 Okay.

05:04 And so to kick off the second part of the mechanism, it's again going to begin with a protonation, which makes sense because remember this is acid catalyzed.

05:19 Whenever something's acid catalyzed, it's a pretty good bet that protonation is a key first step...

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