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Grignard reagent in organic chemistry

April 2, 2019

In organic chemistry, students during second semester learn about organometallic reagents: grignard and organolithium.

 

Preparation of Grignard reagent:

We must start with an alkyl halide (any hydrocarbon molecule with a halogen attached, such as bromine). It then reacts with Mg  and ether, making MgBr bond. We can rewrite our grignard reagent. The MgBr will dissociate from the rest of the molecule leaving a carbon anion (negative charge). Make sure to put the negative charge on the carbon that had the bond with MgBr.

 

Reactions of Grignard:

Grignard reagent is a strong base. Thus it will attack any electrophile and can participate in a variety of reagents. Grignard reacts with water, alcohols, and acids getting protonated. Therefore, in order for grignard to attack a specific molecule we can not put water in the same step. We have to either number steps or put it on a separate arrow. Grignard is also known for attacking carbonyl compounds, such as ketones, aldehydes, and esters. When grignard reagent reacts with ketones and aldehydes, it attacks once forming an alcohol. With esters, grignard attacks twice forming a tertiary alcohol. 

 

Organolithium reagent is very similar to Grignard. To prepare it we need an alkyl halide that reacts with 2Li. Lithium replaces the halogen in the molecule, making a carbon lithium bond. Organolithium reagent is a strong base as well and participates in all of the reactions that were described above for grignard. 

 

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