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Aldehyde Reactions: Formation of Alcohol from Aldehyde, Ketone using Grignard Reagents

Secondary and Tertiary alcohols can be formed from aldehydes and ketones (respectively) using Grignard Reagents:

Aldehyde Reactions: Formation of Alcohol from Aldehyde, Ketone using Grignard Reagents image3.png

The carbon chain from the Grignard reagent (R) is added to the aldehyde/ketone. This is helpful for synthesis reactions where you need to add an alkane chain however, the “R” group is usually pretty small (think ethane to cyclohexane in size):

Ethane Addition Example

Aldehyde Reactions: Formation of Alcohol from Aldehyde, Ketone using Grignard Reagents image2.png

Cyclohexane Addition Example

Aldehyde Reactions: Formation of Alcohol from Aldehyde, Ketone using Grignard Reagents image1.png

The reaction mechanism is depicted below:

Aldehyde Reactions: Formation of Alcohol from Aldehyde, Ketone using Grignard Reagents image4.png

In the first step, the Mg-R bond breaks, sending that pair of electrons to attack the carbon atom that makes up the aldehyde/ketone bond. This attack pushes electrons from the carbon-oxygen bond to the oxygen atom, allowing the R to bond.

In the second step, the newly acquired lone pair electrons on the oxygen atom attack a nearby H3O+ molecule, stripping away its proton.

There are many variations of Grignard reagents but all Grignard reagents will have an alkane chain, a Mg atom, and a halogen (Cl, Br).