How is the Grignard reagent made

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The Grignard reaction

The Grignard reaction is one of the classic organometallic reactions for the synthesis of bonds. In general, a Grignard reaction is understood to mean the reaction of organomagnesium halides (the so-called Grignard compounds, with =,, and =,) with carbonyl compounds, which, after hydrolysis of the magnesium alcoholates formed, yields corresponding alcohols (Fig. 1). As a rule, ethers are used as solvents, in particular diethyl ether and tetrahydrofuran (THF). The reaction and the organomagnesium compounds used are named after their discoverer, Victor Grignard (Nobel Prize in Chemistry 1912).

The bond in the Grignard reagents is strongly polarized. The negatively polarized organic residue formally attacks the positively polarized carbonyl carbon nucleophilically. The negatively polarized carbonyl oxygen coordinates to the positively polarized magnesium. A magnesium alcoholate is formed, which is converted into the corresponding alcohol by hydrolytic processing (Fig. 2).

Mechanism of the Grignard reaction

Apparently different mechanisms occur in the Grignard reaction, depending on the Grignard reagent used. In addition to four- and six-membered transition states, mechanisms with single electron transfer (SET) also come into play (e.g. in the case of sterically hindered Grignard reagents). Typically, however, the reaction appears to proceed through a six-membered transition state. Two molecules of the Grignard compound are involved, but only one of them is used up (Fig. 3).