Alcohols, Phenols and Ethers
5.0 Grignard reagent
5.1 Reaction & Mechanism
5.2 Product of Grignard reagent
5.3 Planning a Grignard synthesis
5.4 Restriction of the use of Grignard reagents
5.0 Grignard reagent
5.2 Product of Grignard reagent
5.3 Planning a Grignard synthesis
5.4 Restriction of the use of Grignard reagents
Organomagnesium halides were discovered by French chemist Victor Grignard in 1900. Grignard received the Noble prize for his discovery in 1912 and organomagnesium halides are now called as Grignard Reagents in his honor. Grignard reagents have great use in organic synthesis.
Grignard reagents are usually prepared by the reaction of an organic halide and magnesium metal (turnings) in an ether solvent.
The order of reactivity of halides with magnesium is also $RI > RBr > RCl$ (very few organomagnesium fluorides have been prepared. Aryl Grignard reagents are more easily prepared by Aryl bromides and aryl iodides than from aryl chlorides which reacts very sluggishly.
The general formula for Grignard reagents is $R—Mg—X$. Grignard reagents are seldom isolated but are used for further reactions in ether solution.
A Grignard reagents forms are complex with its ether solvent, the structure of the complex can be represented as,
Complex formation with molecules of ether is an important factor in the formation and stability of Grignard reagent.
Grignard reagents are very strong bases. They react with any compounds that have a hydrogen atom attached to an electronegative atom such as oxygen, nitrogen or sulfur.
The reactions of Grignard reagents with water and alcohol are nothing more than acid-base reactions, they lead to the formation of the weaker conjugate acid and weaker conjugate base. The Grignard reagents behave as if it contains an anion of an alkane as if it contains carbanion.
Not only are Grignard reagents strong bases they are also powerful nucleophiles. Reactions in which Grignard reagents acts as a nucleophile are by far the most important.
Grignard reagents carry out the nucleophilic attack at a saturated carbon when they react with oxiranes. The nucleophilic alkyl group of the Grignard reagents attacks the partially positive carbon of the oxirane ring. Because it is highly strained, the ring opens and the reaction leads to the salt of primary alcohol. Subsequent acidification produces the alcohol.
Grignard reagent reacts primarily at the less substituted ring carbon atom of substituted oxiranes. The most important reactions of Grignard reagents are those in which these reagents act as nucleophiles and attacks on unsaturated carbon especially the carbon the carbon of a carbonyl group.
