Grignard Reaction with Alcohol, Ketone & Aldehyde

Lesson Transcript
Instructor: Laura Foist

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

In this lesson, we'll learn about the Grignard reaction. We'll see how the reaction occurs with ketones and aldehydes, as well as how water and alcohols prevent this reaction from occurring.

Grignard Reaction Definition

Reactions that make a carbon-carbon bond are important because they are how longer chains are formed. The Grignard reaction is a reaction that uses an organometallic to create this carbon-carbon bond. These reactions are called Grignard reactions after Francois Auguste Victor Grignard, whose work developing this reaction led him to be awarded the Nobel Prize in chemistry.

An organometallic is a carbon that is bonded to a metal. Since the carbon is more electronegative than the metal, the carbon can pull electrons away from the metal. This leaves a partial negative charge on the carbon. Once the metal becomes attracted to another strong negative charge, in the form of halides like bromine, this compound can act like a carbanion. A carbanion is a carbon with a negative charge. Grignard reagents use magnesium as the metal.

The negative charge on the carbon can then react with carbons that have a partial positive charge on them. A carbon can have a partial positive if it's bonded to an element that is more electronegative than it is (typically oxygen is used).

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  • 0:04 Grignard Reaction Definition
  • 1:10 Grignard Reaction Description
  • 1:28 Grignard Reaction With…
  • 2:25 Grignard Reaction With Ketones
  • 3:26 Grignard Reaction With…
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Grignard Reaction Description

The Grignard reaction occurs with the carbon attaching to the aldehyde or ketone. Then, after adding water, we end up with a longer carbon chain attached to an alcohol.

The general Grignard reaction adds the carbon chain from the Grignard reagent onto the aldehyde or ketone
Grignard reaction

In this example, we see that the red 'R' group from the Grignard reagent gets attached to the aldehyde or ketone.

Grignard Reaction With Aldehyde

An aldehyde is a carbon chain, and the last carbon on the chain is double bonded to an oxygen. Since this carbon is double bonded to an oxygen, it has a partial positive charge. This is the perfect spot for the carbanion on the Grignard reagent to react.

The Grignard reagent can react with an aldehyde to create a secondary alcohol and a longer carbon chain
Aldehyde-Grignard reaction

Here, we see the red carbon from the Grignard reagent attack the black carbonyl carbon. Those electrons can now jump up to the electronegative carbon. Once water is added, the blue hydrogen is attached to the oxygen. So we end up with the original three black carbons, three red carbons from the Grignard reagent, and the alcohol (the original black oxygen and the new blue hydrogen).

It's important to note that the water needs to be added after the Grignard reagent has finished reacting with the aldehyde. If the water is added before the Grignard reagent has reacted with the aldehyde, then the reaction won't occur. We will discuss why this happens later in this lesson.

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