Acetal Formation: Mechanism

Instructor: Danielle Reid

Danielle has taught middle school science and has a doctorate degree in Environmental Health

Acetals are molecules derived from either a ketone or aldehyde group and are chemically important in carbohydrate synthesis. In this lesson, you'll learn about the process of acetal formation and steps involved to form an acetal molecule.

What Is an Acetal Compound?

Most likely, you've heard of the sugar molecular glucose. But did you know glucose can make acetals? They certainly can! As simple carbohydrates, they utilize the acetal formation mechanism to make more complex carbohydrates. This is a classic example of how an acetal plays an important role in the chemistry of carbohydrates. But before we get ahead of ourselves with the acetal formation mechanism, let's define the term acetal.

Acetals are organic compounds classified as diether products. They are formed when an aldehyde or ketone reacts with alcohol in the presence of an acid catalyst. There are a lot of terms to digest in this description: aldehyde, ketone, and acid catalyst. Let's look at each one carefully.

Aldehyde, Ketone, and Acid

There are two cousins in organic chemistry who often get mistaken for one another: aldehyde and ketone. An aldehyde is composed of an R group and hydrogen atom bonded to a carbonyl group (CO). Ketones contain two R groups bound to a carbonyl group (CO). When you see the term 'R group', think of this as a spot any molecule or atom can occupy.

As you can see in Diagram 1, the structure of a ketone is quite similar to an aldehyde; hence, it makes perfect sense why these two cousins often get mixed up!

Diagram 1: Structure of an (a) Aldehyde and (b) Ketone

Acid catalysts are acids that speed up the rate of a reaction. Always remember that catalysts are never eaten alive in a chemical reaction. They simply play the role as a super boost, ensuring the reaction reaches completion in record time. Now that we know what acetal is, as well as how to define and describe them, let's look at the mechanism of acetal formation.

Acetal Formation: Understanding the Mechanism

The Acetal Formation Mechanism
acetal formation

There are seven steps involved in acetal formation. As we work through each step, keep in mind the terms that we defined earlier. Also, note that in organic chemistry reactions, the arrows indicate the movement of electron pairs. In this example, we'll use a ketone as the reacting molecule with alcohol.

Step One: Protonate the carbonyl group on the ketone molecule (a) using the proton from an acid catalyst. Protonation refers to the transfer of a proton (H) to a molecule or atom in order to form a bond.

Step One of Acetal Formation
step 1

Step Two: Take an alcohol and use it to perform a nucleophilic attack on the product formed in step one (b). A nucleophile is a species that graciously donates its electron pair (two dots) in order to form a chemical bond.

Step Two of Acetal Formation
step 2

Step Three: Deprotonate, or remove the proton (H), the product formed in step three, to form a hemiacetal (c) and hydronium ion (d). A hemiacetal is a molecule formed when an alcohol (shown in blue) is added to an aldehyde/ketone (shown in red), as highlighted in the diagram.

Step Three of Acetal Formation
step 3

Step Four: Take hemiacetal and protonate the alcohol group using the hydronium ion (e). This will ensure that, given its unstable charge, the alcohol will be eager to leave the molecule in step five.

Step Four of Acetal Formation
step 4

Step Five: Move the electron pairs (dots) from the other oxygen atom on the product in step four to form a double bond and bump the alcohol group off the product (f). The end result of this alcohol group leaving is the formation of a protonated ketone (g) and loss of water (h).

Step Five of Acetal Formation
step 5

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