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Hydrolysis of Aspirin: Mechanism & Reaction

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  • 0:00 Aspirin Stability
  • 0:40 Reaction
  • 1:14 Acidic Mechanism
  • 3:02 Lesson Summary
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Lesson Transcript
Instructor
Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Expert Contributor
Ronald Okoth

Ronald received his PhD. from Brown University in Providence RI. Ronald has taught college level chemistry.

Aspirin is a very popular and well-known over-the-counter drug, but if it is exposed to water, then the reaction makes it ineffective. In this lesson, we will learn how this reaction occurs.

Aspirin Stability

Aspirin is a popular over-the-counter pain and fever reducer. Chances are you've probably used aspirin at some point in your life. When people are in pain, they want to ensure that the pain medication will work as needed. So the stability of medication is an important aspect for producers to consider. If the medication deteriorates then it won't be as effective. One of the main destabilizing factors that aspirin needs to deal with is hydrolysis. Hydrolysis really just means a chemical reaction between a compound and water, which results in the breakdown of that compound.

Reaction

Aspirin can undergo hydrolysis, making it not as effective if it's exposed to water for extended periods of time. The technical name of the active ingredient in aspirin is acetylsalicylic acid. When it reacts with water, we end up with two products, salicylic acid and acetic acid. This is a fairly slow reaction unless it's also heated. Under room temperature conditions, it would take nearly a month for all of the aspirin to be degraded. But at a higher temperature, such as 70 degrees Celsius, it would take less than 10 hours for all of the aspirin to be degraded.

Acidic Mechanism

This reaction can occur with acidic or basic conditions. Let's first take note of the ester on aspirin:

The ester is an important part of this molecule for the hydrolysis reaction
Ester identified

This reaction occurs on the ester, so for simplicity's sake, we will refer to the benzene group as 'R1' and the methane group as 'R2':

Aspirin with R groups

Let's first go through the mechanism under acidic conditions. First, the carbonyl, or the carbon double bonded to the oxygen, is made into a really strong electrophile, or electron deficient atom, by adding a hydrogen to the oxygen:


Hydrogenation


Now that the carbonyl is a good electrophile, the electrons from the oxygen on water can attack:


Water attack


Then a proton transfer occurs in order to make the alcohol with R1 a good leaving group:


Proton transfer


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Additional Activities

Hydrolysis of Aspirin.

Aspirin, also known as acetylsalicylic acid, has an ester functional group. Just like esters, aspirin can undergo hydrolysis under acidic conditions or basic conditions. Now that you are familiar with the mechanism of the hydrolysis of aspirin the following activities are designed to deepen your understanding of the mechanism and apply the knowledge to the hydrolysis of other esters.

Structures of esters.

Which of the following compounds are esters?

Structure of Aspirin.

The structure of Aspirin is given below. Which of the atoms labeled 1-4 is an electrophile?

Base catalyzed hydrolysis of Aspirin.

Consider the first step in the base catalyzed hydrolysis of aspirin shown below;

Which of the following is the structure of the intermediate formed in this step?

Acid Catalyzed hydrolysis of Aspirin

Consider the first step in the acid catalyzed hydrolysis of aspirin shown below;

Which of the following is the structure of the intermediate formed in this step?

The structures of the products of the acid catalyzed hydrolysis of Aspirin are given in the reaction below.

Draw the structures of the products of the acid catalyzed hydrolysis of each of the following esters.

Answer Key:

Structures of esters.

Structure of Aspirin.

The carbon atom labeled 2 is an electrophile.

Base catalyzed hydrolysis of Aspirin.

The structure of the intermediate formed in this step is given below.

Acid Catalyzed hydrolysis of Aspirin

The structure of the intermediate formed in this step is given below.

The structures of the products of the acid catalyzed hydrolysis of each of the esters are given below.

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