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Preparation of Benzoic Acid

Preparation of Benzoic Acid - The Grignard Reaction...

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The Grignard Reaction: Preparation of Benzoic Acid ________________________________________________________________________ This experiment is based on one included in Pavia, Organic Laboratory Techniques: A Microscale Approach, 4 th ed. NOTES: Review the technique of crystallization in Zubrick (Ch. 13) before coming to lab today. The bottom line on crystallization: Dissolve the compound to be recrystallized in the minimum amount of hot solvent it takes to entirely dissolve the compound. Let this solution cool, undisturbed, on the benchtop. If no crystal growth is apparent after several minutes, scratch the bottom of the flask with a glass stir rod. If after several more minutes there is still no crystal growth, move the flask without disturbing the solution onto an ice bath. Review melting points (Zubrick, Ch. 12, and the handout available on the course website). READ the sections on organometallic and Grignard reagents in Wade, 6 th edition: pp. 432-442. ________________________________________________________________________ Grignard reagents (RMgX) are commonly used reagents in the organic laboratory. They are available commercially as solutions, or can be made in the lab and used immediately. Treatment of an alkyl halide with magnesium metal in an ethereal solvent such as diethyl ether or tetrahydrofuran (THF) generates the Grignard reagent: Br Mg Et 2 O MgBr The carbon-magnesium bond is considered to be polar covalent, giving the carbon atom significant anionic character and making the Grignard reagent both a strong base and a good nucleophile. One of the most frequent uses of these reagents is nucleophilic addition to carbonyl compounds such as ketones and aldehydes: MgBr H O H O MgBr HO H 3 O + The nucleophilic carbon of the Grignard reagent attacks the electrophilic carbonyl carbon, forming the magnesium salt of an alkoxide. This alkoxide ion is protonated in the aqueous workup to give the alcohol product. Grignard reagents will also react similarly with carbon dioxide to provide a convenient synthesis of carboxylic acids. The Grignard reagent attacks the electrophilic carbon,
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forming a magnesium carboxylate. The carboxylate is protonated in aqueous workup to give the carboxylic acid product. Because Grignard reagents are so basic, they react quite readily (sometimes even
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