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Grignard reaction[grin′yär rē‚ak·shən]
a general method of synthesizing organic substances using mixed organomagnesium compounds (Grignard reagents) of the type RMgX, where R is an aliphatic or aromatic radical and X is a halogen (usually Br or I, more rarely C1). The Grignard reaction was discovered by the French chemist F. A. V. Grignard in 1900. Grignard reagents are obtained by the action of Mg on alkyl or aryl halides in an ether medium and are usually used in the form of ether solutions. Methods of preparing RMgX using solvents other than ether have been worked out for industrial use. An atom of carbon bound to an atom of magnesium carries a partial negative charge δ-, so that compounds of RMgX are reactive in relation to reagents with positively charged reaction centers:
When RMgX reacts with C02, carboxylic acids are formed. With formaldehyde (CH2=0) primary alcohols are formed; with aldehydes (RCH=0), secondary alcohols; and with ketones (RR’C=0), tertiary alcohols. The use of the Grignard reaction to obtain ketones from carboxylic acids and their derivatives, for example,
RMgX + R’COCI → RCOR’ + MgXCI
is limited. It is difficult to stop the reaction at the ketone-forming stage, and ordinarily tertiary alcohols are formed in considerable quantity.
Grignard reagents react easily with halides of elements, for example.
2RMgX + HgCl2 → R2Hg + 2MgXCl
In this manner it is possible to obtain organic compounds of Be, Cd, B, Si, Ge, Pb, P, and other elements. Grignard reagents react with oxygen and sulfur to form alcohols and thioalcohols, respectively. When reacting with water, alcohols, acids, amines, and other compounds containing a labile atom of hydrogen, the RMgX decompose and hydrocarbons are formed:
RMgX + R’OH → R—H + R’OMgX
If CH3MgI is used, then it is possible to determine the amount of active hydrogen by the amount of methane released (the Chugaev-Tserevitinov method).