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grignard_instr - Grignard Synthesis of Benzoic Acid...

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2 R-Mg-Br R-Mg-R + MgBr 2 Grignard Synthesis of Benzoic Acid Organometallic compounds are versatile intermediates in the synthesis of alcohols, carboxylic acids, alkanes, and ketones, and their reactions form the basis of some of the most useful methods in synthetic organic chemistry. They readily attack the carbonyl double bonds of aldehydes, ketones, esters, acyl halides, and carbon dioxide. The use of organometallic reagents can produce the synthesis of highly specific carbon-carbon bonds in excellent yields. Among the most important organometallic reagents are the alkyl- and arylmagnesium halides, which are almost universally called Grignard reagents after the French chemist Victor Grignard, who first realized their tremendous potential in organic synthesis. Their importance in the synthesis of carbon-carbon bonds was recognized immediately after the report of their discovery in 1901. Grignard received the 1912 Nobel Prize in chemistry for applications of this reagent to organic synthesis. The Grignard reagent is easily formed by reaction of an alkyl halide, in particular a bromide, with magnesium metal in anhydrous diethyl ether. Although the reaction can he written and thought of as simply R – Br + Mg R – Mg – Br (RMgX) it appears that the structure of the material in solution is rather more complex. There is evidence that dialkylmagnesium is present and that the magnesium atoms, which have the capacity to accept two electron pairs from donor molecules to achieve a four-coordinated state, are solvated by the unshared pairs of electrons on diethyl ether: Grignard reagents, like all organometallic compounds, are substances containing carbon- metal bonds. Because metals are electropositive elements, carbon-metal bonds have a high degree of ionic character, with a good deal of negative charge on the carbon atom. This ionic character gives organometallic compounds a high degree of carbon nucleophilicity. δ δ + δ R – Mg – R The Grignard reagent is a strong base and a strong nucleophile. As a base it will react with all protons that are more acidic than those found on alkenes and alkanes. Thus, Grignard reagents react readily with water, alcohols, amines, thiols, etc., to regenerate the alkane. Such reactions are generally undesirable and are referred to as reactions that “kill” the Grignard.
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In the absence of acidic protons, Grignard reagents undergo a wide variety of nucleophilic addition reactions, especially with compounds containing polar C=0 bonds. The resulting carbon-carbon bond formation yields larger and more complex molecules; and because a variety of different organic (R or Ar) groups can be introduced into organic structures, a wide array of organic compounds can be produced. Some reactions of Grignards are shown below.
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grignard_instr - Grignard Synthesis of Benzoic Acid...

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