One of the first reactions of Grignard reagents we learn is the one with aldehydes and ketones, where secondary and tertiary alcohols are formed:
These are classic examples of nucleophilic addition reactions to carbonyl compounds, and the good news is that organolithiums (RLi) react identically, following the same mechanism:
The last reaction is an example of how esters react with organolithiums, and we cover this in a separate article, which you can find here.
So, what is the mechanism of the reaction of organolithiums with aldehydes and ketones? Let’s start with the aldehyde.
The Mechanism of the Reaction of Organolithiums with Aldehydes
The two reactive species are the electrophilic carbon atom of the carbonyl group and the highly nucleophilic carbon fragment of the organolithium:
Therefore, the reaction starts with a nucleophilic addition of the R group to the carbonyl, forming a tetrahedral alkoxy intermediate, which is protonated in the last step with an acidic aqueous workup to give the secondary alcohol:
The Mechanism of the Reaction of Organolithiums with Ketones
Ketones have an extra alkyl, aryl, or other carbon fragment attached to the carbonyl group; therefore, their reaction with organolithiums and Grignard reagents gives tertiary alcohols. Other than this, the mechanism is identical to what we have seen for aldehydes reacting with organolithiums:
To summarize the reaction of organolithium (RLi) reagents with aldehydes and ketones, remember that they react identically to how Grignard reagents do. In the case of aldehydes, secondary alcohols are formed, whereas ketones, because of the extra alkyl group, give tertiary alcohols.
Below are some practice problems on the reaction of Grignard reagents, which, once again, are mostly identical to those of organolithiums.
