| Grignard Reagent |
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Information AboutGrignard Reagent |
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The following reagents can be used in the Grignard reaction:
GRIGNARD REAGENT FORMATION Grignard reagents are formed by reacting the appropriate alkyl or aryl halide with atom, which is otherwise difficult to achieve. Bromide s are most often used, as they react the fastest and are readily available. Iodide s and Chloride s are also used, but Fluoride s are generally unreactive towards magnesium. The Grignard reaction is Exothermic , but because of an oxide layer present on the magnesium, the start of the reaction is sometimes delayed. To start the reaction it is often necessary to squash the magnesium, heavily stir the mixture with glass shards, or to add a small amounts of Iodine , Methyl Iodide , or 1,2-dibromomoethane (which forms Ethene bubbles and can also be mixed with the halide to monitor the reaction). All these methods weaken the oxide layer and expose the magnesium to the halide. Many Grignard reagents such as phenylmagnesium bromide are available commercially in Tetrahydrofuran or Diethyl Ether solutions. Grignard reagents form complex equilibria known as the Schlenk Equilibrium . NUCLEOPHILIC ADDITION REACTIONS In reactions involving Grignard reagents, it is important to ensure that no water is present, which would otherwise cause the reagent to rapidly decompose. Thus, most Grignard reactions occur in solvents such as anhydrous Diethyl Ether or Tetrahydrofuran , because the oxygen of these solvents stabilizes the magnesium reagent. The reagent may also react with oxygen present in the atmosphere, inserting an oxygen atom between the carbon base and the magnesium halide group. Thus, many of these reactions are carried out in Nitrogen or Argon atmospheres. and Grignard reagent — a bromide in this case. Notice that water is required to form the alcohol.]] An example is a key step in the industrial production of Tamoxifen : Grignard reagents react with Formaldehyde to form primary Alcohol s, with other Aldehyde s to form secondary alcohols, and with Ketone s to form tertiary alcohols. Quenching a Grignard with Dry Ice yields the Carboxylic Acid . The Bouveault Aldehyde Synthesis is a one-pot Chemical Reaction that converts a primary Alkyl Halide to an Aldehyde one carbon longer. In the Fujimoto-Belleau Reaction Grignard reagents react with α,β-unsaturated enol-lactones. COUPLING REACTIONS A Grignard reagent can also be involved in a Coupling Reaction . For example nonylmagnesium bromide reacts with an aryl chloride to a nonyl benzoic acid. An iron catalyst is used and not an expensive Palladium catalyst such as used in Heck Reaction s. Acac stands for Acetylacetonate . For the coupling of aryl halides with aryl grignards Nickel Chloride in THF is a very good catalyst. A very effective catalyst for couplings of alkyl halides is Dilithium Tetrachlorocuprate (Li2CuCl4), prepared from mixing Lithium Chloride (LiCl) and Copper(II) Chloride (CuCl) in THF. OXIDATION The oxidation of a Grignard reagent with oxygen takes place through a Radical intermediate to a magnesium hydroperoxide. Hydrolysis of this complex yields Hydroperoxide s and Reduction with an additional equivalent of Grignard reagent gives an Alcohol . The synthetic utility of Grignard oxidations can be increased by a reaction of Grignards with oxygen in presence of an Alkene to an ethylene extended Alcohol . This modification requires Aryl or Vinyl Grignards. Adding just the Grignard and the alkene does not result in a reaction demonstrating that the presence of oxygen is essential. Only drawback is the requirement of at least two equivalents of Grignard although this can partly be circumvented by the use of a dual Grignard system with a cheap reducing Grignard such as n-butylmagnesium bromide. NUCLEOPHILIC SUBSTITUTION Grignard reagents are Nucleophile s in Nucleophilic Aliphatic Substitution s for instance with Alkyl Halide s in a key step in industrial Naproxen production: SEE ALSO
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