why are Et2O used as solvent for grignard reagent
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Grignard reagents are organomagnesium compounds typically represented as RMgX, where R is an organic group and X is a halogen. They are highly reactive and are used in various organic synthesis reactions. Show more…
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Grignard reagents $(\mathrm{RMg} X)$ are prepared by the reaction of an organic halide and magnesium metal is in ether solvent. $$\mathrm{R}-\mathrm{X}+\mathrm{Mg} \stackrel{\mathrm{R}-\mathrm{O}-\mathrm{R}}{\longrightarrow} \mathrm{R}-\mathrm{Mg} \mathrm{X}=$$ The solvent (usually diethyl ether or tetrahydrofuran) plays a crucial role in the formation of a Grignard reagent. Alkyl halides are more reactive than aryl and vinyl halides. Indeed, aryl and vinyl chlorides do not form Grignard reagent in diethyl ether. However, an alkyl halide containing an alcoholic -OH group can be converted to Grignard reagent by first protecting the -OH group to tert-butyldimethylsilyl ether which is inert to Grignard reagent. The protecting group is finally liberated by treatment with fluoride ion. The function of tetrahydrofuran in the preparation of Grignard reagent is that it (a) Acts as a solvent (b) Helps in maintaning the reactivity of magnesium (c) Both (a) and (b) (d) None of the these
Organic Reaction Mechanisms and Reagents
Level II
Grignard reagents $(\mathrm{RMg} X)$ are prepared by the reaction of an organic halide and magnesium metal is in ether solvent. $$\mathrm{R}-\mathrm{X}+\mathrm{Mg} \stackrel{\mathrm{R}-\mathrm{O}-\mathrm{R}}{\longrightarrow} \mathrm{R}-\mathrm{Mg} \mathrm{X}=$$ The solvent (usually diethyl ether or tetrahydrofuran) plays a crucial role in the formation of a Grignard reagent. Alkyl halides are more reactive than aryl and vinyl halides. Indeed, aryl and vinyl chlorides do not form Grignard reagent in diethyl ether. However, an alkyl halide containing an alcoholic -OH group can be converted to Grignard reagent by first protecting the -OH group to tert-butyldimethylsilyl ether which is inert to Grignard reagent. The protecting group is finally liberated by treatment with fluoride ion. $\mathrm{H}_{2} \mathrm{~N}\left(\mathrm{CH}_{2}\right)_{3} \mathrm{Br}$ cannot be converted into corresponding Grignard reagent because of (a) Reaction between $-\mathrm{NH}_{2}$ and $-\mathrm{Br}$ groups present in the same molecule (b) Strong nucleophilic character of the Grignard reagent (c) Strong basic nature of the Grignard reagent (d) All the three factors
Careful choice of solvent is essential for the successful generation and reaction of a Grignard reagent. Explain why anhydrous diethyl ether and tetrahydrofuran (THF) are common solvents for the generation of Grignard reagents. Show the major product(s) of the reaction of 4-methylphenylmagnesium bromide (prepared in anhydrous diethyl ether) with benzophenone (dissolved in either ethanol, acetone, or diethyl ether). Assume acidic workup in each case. The reaction of PhMgBr with cyclohexanone (C6H10O) followed by addition of sulfuric acid produces 1-phenylcyclohexene (C12H14) as shown below: Draw a plausible electron-pushing mechanism for the formation of the major and minor products starting from bromobenzene.
Adi S.
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