In this lesson we will learn about the Grignard reaction. We will see how the reaction occurs with ketones and aldehydes, and how water and alcohols prevent this reaction from occurring.
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 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 is bonded to an element that is more electronegative than it is (typically oxygen is used).
General Grignard Reaction
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.
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 is 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.
Grignard Reaction with Ketones
The Grignard reaction with ketones is very similar to the reaction with aldehydes, except we end up with a tertiary alcohol instead of a secondary alcohol.
A tertiary alcohol is when the carbon that it is attached to is not attached to any hydrogens (the rest of the bonds are filled with carbons). A secondary alcohol is when the carbon that the alcohol is attached to is attached to only one hydrogen. Then there are also primary alcohols where the carbon is attached to two hydrogens.
In Grignard reactions with ketones, the carbons need to be carefully counted. In the original ketone there are four carbons. Two of the carbons (the black carbons) come before the carbonyl, and one (the green carbon) comes after it. In the final product we still have all four original carbons, as well as the new three carbons.
Notice that the black, green, and red carbons are all attached to the same carbon that the alcohol is on. Since there are three carbons attached to this carbon and no hydrogens, it is a tertiary alcohol.
Grignard Reaction with Alcohols
If there are any alcohols on a carbon chain, then the Grignard reagent will react with the hydrogen on the alcohol before it will react with the carbonyl carbon. This prevents the reaction from properly occurring since now that carbon no longer has a partial negative charge, it is all filled up with hydrogens.
The same thing occurs if water is added to the reaction before the Grignard reagent has reacted with the aldehyde/ketone.
The Grignard reagent will take the hydrogen from the water before it will attack with the carbonyl carbon, also preventing the reaction from occurring.
The Grignard reaction is an important reaction for creating longer carbon chains. A Grignard reagent is a carbon chain attached to magnesium. This creates a partial negative on the carbon atom. This carbanion can react with carbons on ketones and aldehydes since they have a partial positive charge.
The reaction can be prevented from occurring if water or an alcohol reacts with the Grignard reagent before it can react with the carbonyl carbon.