Reactions of Aldehydes and Ketones With Grignard Reagents

This page looks at the reaction of aldehydes and ketones with Grignard reagents to produce potentially quite complicated alcohols. It is mainly a duplication of the information on these same reactions from a page on Grignard reagents in the section on properties of halogenoalkanes.

Note: If you want to read more about these and other reactions of Grignard reagents you might like to follow this link.

What are Grignard Reagents?

A Grignard reagent has a formula RMgX where X is a halogen, and R is an alkyl or aryl (based on a benzene ring) group. For the purposes of this page, we shall take R to be an alkyl group.

A typical Grignard reagent might be CH₃CH₂MgBr.

The Preparation of a Grignard Reagent

Grignard reagents are made by adding the halogenoalkane to small bits of magnesium in a flask containing ethoxyethane (commonly called diethyl ether or just "ether"). The flask is fitted with a reflux condenser, and the mixture is warmed over a water bath for 20 – 30 minutes.

\text{CH}_3\text{CH}_2\text{Br} + \text{Mg} \xrightarrow{\text{ethoxyethane}} \text{CH}_3\text{CH}_2\text{MgBr}

Everything must be perfectly dry because Grignard reagents react with water.

Warning! Ethoxyethane (ether) is very dangerous to work with. It is an anaesthetic, and is extremely inflammable. Under no circumstances should you try to carry out this reaction without properly qualified guidance.

Any reactions using the Grignard reagent are carried out with the mixture produced from this reaction. You can't separate it out in any way.

Reactions of Grignard Reagents With Aldehydes and Ketones

These are reactions of the carbon-oxygen double bond, and so aldehydes and ketones react in exactly the same way – all that changes are the groups that happen to be attached to the carbon-oxygen double bond.

It is much easier to understand what is going on by looking closely at the general case (using "R" groups rather than specific groups) – and then slotting in the various real groups as and when you need to. The "R" groups can be either hydrogen or alkyl in any combination.

In the first stage, the Grignard reagent adds across the carbon-oxygen double bond:

Dilute acid is then added to this to hydrolyse it.

Note: Almost all sources quote the formation of a basic halide such as Mg(OH)Br as the other product of the reaction. That's actually misleading because these compounds react with dilute acids. What you end up with would be a mixture of ordinary hydrated magnesium ions, halide ions and sulfate or chloride ions – depending on which dilute acid you added.

What you need to learn about this depends on what your examiners want. The only way to find that out is to look at old exam papers and mark schemes. If you are a UK A-level student and haven't got copies of these, find out how to get hold of them by going to the syllabuses page to find your Exam Board's web address.

An alcohol is formed. One of the key uses of Grignard reagents is the ability to make complicated alcohols easily.

What sort of alcohol you get depends on the carbonyl compound you started with – in other words, what R and R' are.

The Reaction Between Grignard Reagents and Methanal

In methanal, both R groups are hydrogen. Methanal is the simplest possible aldehyde.

Assuming that you are starting with CH₃CH₂MgBr and using the general equation above, the alcohol you get always has the form:

Since both R groups are hydrogen atoms, the final product will be:

A primary alcohol is formed. A primary alcohol has only one alkyl group attached to the carbon atom with the -OH group on it.

You could obviously get a different primary alcohol if you started from a different Grignard reagent.

The Reaction Between Grignard Reagents and Other Aldehydes

The next biggest aldehyde is ethanal. One of the R groups is hydrogen and the other CH₃.

Again, think about how that relates to the general case. The alcohol formed is:

So this time the final product has one CH₃ group and one hydrogen attached:

A secondary alcohol has two alkyl groups (the same or different) attached to the carbon with the -OH group on it.

You could change the nature of the final secondary alcohol by either:

changing the nature of the Grignard reagent – which would change the CH₃CH₂ group into some other alkyl group;
changing the nature of the aldehyde – which would change the CH₃ group into some other alkyl group.

The Reaction Between Grignard Reagents and Ketones

Ketones have two alkyl groups attached to the carbon-oxygen double bond. The simplest one is propanone.

This time when you replace the R groups in the general formula for the alcohol produced you get a tertiary alcohol.

A tertiary alcohol has three alkyl groups attached to the carbon with the -OH attached. The alkyl groups can be any combination of same or different.

You could ring the changes on the product by

changing the nature of the Grignard reagent – which would change the CH₃CH₂ group into some other alkyl group;
changing the nature of the ketone – which would change the CH₃ groups into whatever other alkyl groups you choose to have in the original ketone.

Questions to test your understanding

Questions on reactions of aldehydes and ketones with Grignard reagents

Answers

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