Nucleophilic Addition Reactions of Aldehydes & Ketones

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson, we'll examine the nucleophilic addition reactions of aldehydes and ketones, including addition of water and alcohols. First, we'll review what aldehydes and ketones are, as well as the basics of nucleophilic addition reactions.

What Are Aldehydes and Ketones?

If you walk into your kitchen on an average day, chances are you'll find some aldehydes laying around. From the vitamin A in your carrots to the formica on your countertops, aldehydes might sound complicated, but they are actually part of our everyday lives. Aldehydes can also be found in other vitamins, such as vitamin B6, hormones, and the all important sugar, glucose. Aldehydes are used in manufacturing as well, as intermediates to make dyes, pharmaceuticals, and plastics.

But, what about ketones? Ketones have an equally important place in our lives. These molecules show up in sugars and hormones as well, and are important byproducts of metabolic reactions. One common ketone used for industrial purposes is acetone, like the substance you find in nail polish remover. But it's used for far more than that. Ketones like acetone are used to form lacquers, varnishes, resins, paints, and even explosives.

Aldehydes and ketones are unique because they have a carbonyl group, or a carbon attached to an oxygen with a double bond. Aldehydes have a carbon in the carbonyl group bound to an oxygen, another carbon and a hydrogen.

Structure of an aldehyde
aldehyde

Ketones, however, have a carbon in the carbonyl group bound to an oxygen, and two additional groups, but no hydrogens.

Nucleophilic Addition

Because oxygen is very electronegative it forms a polar bond with carbon, where the electrons are shared unequally. The oxygen gets a partial negative charge as a result, and the carbon atom gets a partial positive charge.

Since the carbon atom is relatively electron poor, this opens it up to attack by a nucleophile, or an atom that can donate electrons. Aldehydes and ketones have no good leaving groups, so when the carbonyl carbon is attacked by a nucleophile, the nucleophile simply pushes the electrons toward oxygen and forms a new bond with the carbon atom.

Since the oxygen atom now has a negative charge, it can pick up a hydrogen ion from solution, forming an alcohol on the carbonyl carbon. This type of reaction is called a nucleophilic addition reaction and is characteristic of aldehydes and ketones.

Nucleophilic addition of an aldehyde
nucleophilic addition

Let's look at two examples of nucleophilic addition reactions.

Water

Nucleophilic addition of water to aldehydes or ketones forms hydrates, or compounds that have water chemically bound to them. Let's look at how this happens. In the presence of an acid or base, the oxygen atom in water is able to act as a nucleophile, attacking the carbonyl carbon. The water molecule adds on as a hydroxyl group, and the oxygen in the carbonyl group picks up a hydrogen ion from solution. This forms a hydrate with two hydroxyl groups attached to the previous carbonyl carbon.

Formation of a hydrate through nucleophilic addition of water with an aldehyde
hydrate formation

The reaction is reversible and proceeds back and forth. In fact, isolation of the hydrate product is usually difficult, because in most cases the aldehyde or ketone is more stable.

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