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Grignard Reagents

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GRIGNARD REAGENTS. Magnesium is unique amongst the chemical elements in the application which it finds in syn thetic organic chemistry by virtue of its power of dissolving readily in solutions of many organic compounds which contain chlorine, bromine, or iodine as a constituent atom in their molecules. Thus, if magnesium is added to methyl iodide dissolved in pure dry ether, a vigorous ebullition soon develops, and after a short time the magnesium completely disappears. The solution obtained is termed a Grignard reagent, so named after its discoverer V. Grig nard who first carried out this reaction in 1900. In the previous year P. Barbier had found that magnesium may be used to bring about the synthesis of the organic compound dimethylheptenol. and this suggested the above experiment to Grignard. Magnesium will dissolve in ethereal solutions of a wide range of substances which are constituted similarly to methyl iodide (alkyl and aryl halides and substituted derivatives of these), but all halogen com pounds of this class do not react with equal facility. In all cases success in the preparation of these reagents depends on the use of pure materials; the presence of water must be carefully avoided, or immediate decomposition of the reagents will be brought about.

Grignard reagents have proved of the greatest utility in organic chemistry as a means of effecting the syntheses of an extensive variety of substances. The preparation of the Grignard reagent methyl magnesium iodide, and the synthesis of phenylmethylcar binol by means of this afford a typical illustration of the manner in which these reagents are prepared and used. Methyl iodide dissolved in ether is slowly added to magnesium covered with ether. Reaction soon commences and is carefully regulated by efficient cooling. When this stage of the reaction is complete, ben zaldehyde dissolved in ether is added drop by drop to the well cooled and vigorously stirred solution. Each drop causes a hissing sound on striking the Grignard solution and an additive compound between the reagent and benzaldehyde shortly begins to separate. Decomposition of this by means of water and dilute acid yields phenylmethylcarbinol dissolved in the ethereal layer, from which it is separated, after drying, by distillation. As benzaldehyde and methyl magnesium iodide can react to give products other than phenylmethylcarbinol (viz., benzyl alcohol, acetophenone, phenyl styryl ketone, dibenzoylmethane, and aa'- diphenyldiethyl ether), it is necessary in the experiment above described to adhere to exact conditions with regard to quantities and details of working.

Symbolically the preparation of phenylmethylcarbinol is repre sented as follows: Although the Grignard reagent is represented as in (1), each molecule is actually combined with some of the solvent ether. Thus by evaporation of the ethereal solution under specified con ditions, a compound of the composition may be obtained, but the exact structure of this remains a matter for varied opinion. By heating more strongly, may be pre pared. The ethereal solution is, however, employed for synthetic purposes, and the equation is represented as above, since the prod ucts formed by the use of Grignard reagents may be accurately accounted for whilst ignoring the combined ether of the reagent.

The reaction given is one of a general type, namely, the reaction between Grignard reagents and compounds containing the carbonyl grouping (> C=0). By such reactions, alcohols, unsaturated hydrocarbons, and acids may be readily prepared. Frequently in addition to the carbonyl group a second grouping may also under go reaction as in the formation of tertiary alcohols from esters. Many reactions of types quite different from these -3 known, however, and the examples here cited provide only the most frag mentary indication of the application of Grignard reagents; for detailed summaries of original investigations see Alex. McKenzie, British Association Report, 1907, p.273; Henry Wren, "Organo metallic Compounds of Zinc and Magnesium" (1913) ; Chemical Society (London), Annual Reports, 1915, vol. xii., p. 97; 1925, vol. xxii., p. 121. (A. M. W.)

magnesium, ether, reaction, iodide, reagent, methyl and solution