GrignardLab - Experiment 3 Preparation of Benzoic Acid...

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1 Experiment 3: Preparation of Benzoic Acid INTRODUCTION This experiment is designed both as a preparative and an investigative project. Two outcomes are expected: preparation of benzoic acid from bromobenzene and analysis of the by-products of a Grignard reaction, an integral part of the synthetic procedure. GC/MS analysis of the organic layer, generated in the reaction of a Grignard reagent with CO 2 , will provide experimental evidence for the nature of the by-products. The mechanism of the Grignard reaction has been provided in this hand-out, and students will provide the structure of the by-products in the Pre-Lab assignment. Additionally, students will propose refinements of the experimental procedure designed to minimize the amount of by-products. The knowledge of the side products will help students understand how side reactions affect the yield of the desired material. For example, if you are an R&D scientist in a company, you would want to minimize the amount of by- products. The latter reduce the yield of desired compound, therefore increasing the cost of the product. By-products may contaminate the final material, thus further increasing cost due to need for additional purification steps. PART 1: SYNTHETIC PROCEDURE The experiment begins with the preparation of a Grignard reagent, phenylmagnesium bromide (1) , from bromobenzene and magnesium metal using diethyl ether (“ether”) as the solvent under anhydrous conditions [equation (1)]. The use of an ether, such as diethyl ether, as a solvent for the preparation of Grignard reagents is of key importance because the lone pairs of electrons on oxygen help stabilize the partial positive charge on magnesium in the Grignard reagent and thus facilitate its formation. Grignard reagents do not form in most other “inert” solvents. In general, ethers like diethyl ether or tetrahydrofuran (THF) work well as solvents in many nucleophilic addition reactions because the ether functional group is unreactive with most reactants. Br bromobenzene + Mg anhydrous ether MgBr phenyl magnesium bromide, 1 (1) Phenyl magnesium bromide will then be allowed to react with CO 2 , followed by hydrolysis, to give benzoic acid [equation (2)].
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2 MgBr + CO 2 O (MgBr) O OH O benzoic acid (2) H + The carbon-containing portion of Grignard reagent, 1 , has two characteristics: (1) as a carbanion that serves as a nucleophile for its reaction with carbon dioxide, and (2) as a strong base that reacts with acidic hydrogen atoms. These characteristics are illustrated by the structure of the reagent that bears a strongly polar covalent bond between carbon and magnesium. Ar-----MgBr represented as ArMgBr (where "Ar" is short for aromatic ring) δ δ A. Grignard Reagents as Nucleophiles: Benzoic Acid Of major synthetic interest is the use of Grignard reagents as nucleophiles to form new carbon-carbon bonds, a process that is termed nucleophilic addition . For example, Grignard reagents add to aldehydes to produce secondary alcohols, to ketones to
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