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Chapter 4: Problem 188

Reactions of Grignard reagents Organic compounds which contain atleast one carbon-metal bond are called organometallic compounds. Grignard reagents are very reactive organometallic compounds which are of great synthetic utility. They are used for the synthesis of alcohols, aldehydes, ketones, carboxylic acids, etc. They are however decomposed by compounds which have active hydrogen.

Short Answer

Expert verified
Answer: The products of the reaction between a Grignard reagent and carbon dioxide (CO2), followed by an aqueous acid workup, are a carboxylic acid (R-COOH) and a magnesium hydroxide salt (Mg(OH)X).

Step by step solution

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1. Introduction to Grignard reagents

Grignard reagents are organometallic compounds containing a carbon-metal bond, specifically with magnesium. They have the general formula R-Mg-X, where R is an alkyl or aryl group, and X is a halogen (Cl, Br, or I). Grignard reagents are very reactive and are powerful tools in organic synthesis, particularly for the preparation of alcohols, aldehydes, ketones, and carboxylic acids.
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2. Preparation of Grignard reagents

Grignard reagents are prepared by reacting an alkyl or aryl halide with magnesium metal in an anhydrous ether solvent, such as diethyl ether or tetrahydrofuran (THF). For example, the reaction of ethyl bromide with magnesium in diethyl ether produces ethylmagnesium bromide, a Grignard reagent: C2H5Br + Mg -> C2H5MgBr It's important to note that the reaction requires an anhydrous environment, as Grignard reagents are sensitive to moisture and can be decomposed in the presence of water or other sources of active hydrogen.
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3. Synthesis of alcohols using Grignard reagents

Alcohols can be synthesized using Grignard reagents by reacting them with various carbonyl-containing compounds (e.g., aldehydes, ketones, or esters). The Grignard reagent acts as a nucleophile, attacking the electrophilic carbon atom of the carbonyl group: R-Mg-X + R'2C=O -> R-R'2CO-Mg-X After the reaction, an aqueous acid (such as hydrochloric acid) is added to the mixture to protonate the alkoxide intermediate, forming the desired alcohol: R-R'2CO-Mg-X + H2O -> R-R'2COH + Mg(OH)X
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4. Synthesis of aldehydes, ketones, and carboxylic acids

Aldehydes can be synthesized by reacting Grignard reagents with formyl halides or formamides, while ketones can be synthesized by reacting Grignard reagents with acyl halides, anhydrides, or esters. Carboxylic acids can be synthesized by reacting Grignard reagents with carbon dioxide (CO2), followed by aqueous acid workup: R-Mg-X + CO2 -> R-COO-Mg-X R-COO-Mg-X + H2O -> R-COOH + Mg(OH)X
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5. Decomposition by compounds with active hydrogen

Grignard reagents are sensitive to compounds that contain active hydrogen atoms (those bonded to highly electronegative elements like oxygen or nitrogen), such as water, alcohols, or amines, which can cause the decomposition of the reagent: R-Mg-X + H2O -> R-H + Mg(OH)X R-Mg-X + ROH -> R-H + Mg(OR)X R-Mg-X + R'NH2 -> R-H + Mg(NR'R)X This sensitivity means that care must be taken in the choice of solvent, the exclusion of moisture, and the compatibility of functional groups present in the starting materials for Grignard reactions.

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