Grignard Reaction Grignard Reaction The Grignard reaction (pronounced /ﾉ｡riﾉｲar/) is an organometallic chemical reaction in which alkyl- or aryl-magnesium halides (Grignard reagents) add to a carbonyl group in an aldehyde or ketone. This reaction is an important tool for the formation of carbon窶田arbon bonds. The reaction of an organic halide with magnesium is not a Grignard reaction, but provides a Grignard reagent. Grignard reactions and reagents were discovered by and are named after the French chemist Franﾃｧois Auguste Victor Grignard (University of Nancy, France), who was awarded the 1912 Nobel Prize in Chemistry for this work.[5] Grignard reagents are similar to organolithium reagents because both are strong nucleophiles that can form new carbon-carbon bonds. Reaction conditions :- In reactions involving Grignard reagents, it is important to exclude water and air, which rapidly destroy the reagent by protonolysis or oxidation. Since most Grignard reactions are conducted in anhydrous diethyl ether or tetrahydrofuran, side-reactions with air are limited by the protective blanket provided by solvent vapors. Small-scale or quantitative preparations should be conducted under nitrogen or argon atmospheres, using air-free techniques.
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Although the reagents still need to be dry, ultrasound can allow Grignard reagents to form in wet solvents by activating the magnesium such that it consumes the water. The organic halide :- Grignard reactions often start slowly. As is common for reactions involving solids and solution, initiation follows an induction period during which reactive magnesium becomes exposed to the organic reagents. After this induction period, the reactions can be highly exothermic. Alkyl and aryl bromides and iodides are common substrates. Chlorides are also used, but fluorides are generally unreactive, except with specially activated magnesium. Magnesium :- Typical Grignard reactions involve the use of magnesium ribbon. All magnesium is coated with a passivating layer of magnesium oxide, which inhibits reactions with the organic halide. Specially activated magnesium, such as Rieke magnesium, circumvents this problem. Grignard Synthesis :- Grignard reactions are actually slow reaction. Initiation of the process occurs with the exposure of reactive magnesium with the organic halides or organic reagents. Alkyl or aryl bromides and iodides are commonly used substrates. Fluorides are generally unreactive. Synthesis of Grignard reagent is really a tedious process and also very dangerous. It is called as Grignard Synthesis. Chances of explosions and burn outs are more. Mechanical method of synthesis of Grignard reagents includes the crushing of Mg pieces in rapid stirring and sonication of the suspension. Few commonly used activating agents are Iodine, methyl iodide and 1,2-dibromoethane. The amount of Mg used by the activating agents is usually very less. The addition of small amount of mercuric chloride will amalgamate the surface of the metal and allow it to react.
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The above methods weaken the passive nature of the MgO on the surface of the Mg and thereby exposing highly reactive Mg to the organic halide. The CaCl2 is to cover the valves of the glass vessels used in the process to absorb all the moisture which would be in the atmosphere inside the vessels. Grignard Reaction Procedure :- The above scheme shows the reaction mechanism for the synthesis of the organometallic compound, Grignard reagent. Grignard reagents are formed by the action of alkyl halide or aryl halide on magnesium metal. The synthetic process is conducted as described below: Magnesium metal is suspended in a suitable solvent. The solvent should be free from moisture and active hydrogen compounds. The Grignard reagents are very reactive towards such compounds containing active proton. So, aprotic solvents like Ether, THF (tetrahydrofuran) are used in the synthesis as solvents. Another reason why ethers are good solvents for Grignard reagents is that the MgX bond is ionic. Thus, it benefits greatly from being effectively solvated. The formation of ions in very nonpolar solvents is very difficult. Ethers are surprisingly good at solvating cations, since the C-O bond is relatively polar, thus allowing the oxygen end of the ether dipole to solvate and stabilize the magnesium ion. To the above mixture, alkyl halide or aryl halides are suspended, which provide ligands required to stabilize the organomagnesium compounds. The reaction between the organomagnesium compounds and the alkyl or aryl halides can be explained by the free radical mechanism as shown above.
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