benzoic2 - THE GRIGNARD REACTION. PREPARATION OF BENZOIC...

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Benzoic Acid: Page 1 THE GRIGNARD REACTION. PREPARATION OF BENZOIC ACID THE PROBLEM TO BE INVESTIGATED : Benz oic acid is synthesized by the use of an organometallic reagent, C 6 H 5 -Mg-Br, with CO 2 in a Grignard reaction . BACKGROUND INFORMATION : Organometallic compounds are important in life processes. One of the significant discoveries in organic chemistry is a series of reactions involving the Grignard reaction. Much research elucidating the structures, mechanisms, and general application in organic synthesis has been expended. Classified as an addition reaction , a Grignard reagent and methanal (form aldehyde ) produces - after workup - a primary alcohol . With any other aldehyde, the Grignard reaction yields a secondary alcohol , while a ketone or ester will afford a tertiary alcohol : CARBONYL REACTANT (electrophile) PRODUCT formaldehyde primary alcohol aldehyde secondary alcohol ketone tertiary alcohol ester tertiary alcohol carbon dioxide carboxylic acid While benzoic acid may be obtained by alternative methods; e.g. , permanganate or dichromate oxidation of benzyl alcohol or an alkyl-substituted benzene, or hydrolysis of benzonitrile, the Grignard reaction is a non- oxidative process. THE NATURE OF THIS INVESTIGATION : Grignard reagents must be produced under anhydrous conditions and used immediately after preparation. The presence of protic media such as water or alcohol will decompose the Grignard reagent resulting in the formation of the corresponding hydrocarbon ( Eq. 3 ). Addition of the nucleophilic R - (or, in this case, Ar - ) group to the electrophilic carbonyl group (O=C=O) also requires anhydrous conditions. The importance of a protic-free (anhydrous) environment can not be overemphasized; for, failure in generating the Grignard reagent is often associated with the presence of protic material ( Eq. 3 ). As in this experiment, a common impurity, biphenyl ( R-R = C 6 H 5 -C 6 H 5 ) may also result from a coupling reaction to form a different hydrocarbon ( Eq. 4 ).
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Benzoic Acid: Page 2 ether (solvent) H 2 O + R -Mg-Br )))))))))))))) R -H + HO-Mg-Br Eq. 3 (or any labile protic medium) ether (solvent) R -Br + R -Mg-Br )))))))))))))) R-R + MgBr 2 Eq. 4 PROCEDURE Caution . All ethers are extremely flammable. Observe caution in handling chemicals. Until the Grignard reagent is reacted with the electrophile, all apparatus and chemicals must be absolutely dried!! Anhydrous diethyl ether must be used in preparation of the Grignard reagent. After the addition reaction , t-butyl methyl ether is employed in extraction. Preparation of phenylmagnesiumbromide
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This note was uploaded on 11/14/2010 for the course CHEM CHEM266L taught by Professor Christine during the Fall '10 term at Waterloo.

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benzoic2 - THE GRIGNARD REACTION. PREPARATION OF BENZOIC...

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