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Ch. 18 - Nucleophilic Acyl Substitution 1: Carboxylic Acids
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Non è quello che usi tu?Cambia libro di testo
Capitolo 17, Problema 59

When a chemist attempted the following reaction sequence, the desired product was not formed.
(a) Why?
(b) Suggest a solution to the problem. [Think about chemistry from the end of Chapter 13.]
Chemical reaction sequence showing reactants and products, with "not formed" highlighted, indicating an unsuccessful reaction.

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The reaction sequence involves the formation of a Grignard reagent from the bromide compound. The first step is the reaction of the alkyl bromide with magnesium in diethyl ether (Et₂O) to form the Grignard reagent. This reagent is highly reactive and can be used to form carbon-carbon bonds.
The presence of the hydroxyl group (OH) in the starting material is problematic because Grignard reagents are strong bases and will react with the acidic hydrogen of the hydroxyl group, leading to the formation of an alkoxide rather than the desired Grignard reagent.
To solve this problem, the hydroxyl group should be protected before the formation of the Grignard reagent. A common method is to convert the alcohol into a silyl ether using a reagent like trimethylsilyl chloride (TMSCl) in the presence of a base such as imidazole.
Once the hydroxyl group is protected, the Grignard reagent can be formed without interference from the acidic hydrogen. The Grignard reagent can then react with carbon dioxide (CO₂) to form the carboxylate intermediate.
Finally, the carboxylate intermediate is quenched with an acid, such as H₃O⁺, to yield the carboxylic acid product. After the reaction is complete, the protecting group can be removed to regenerate the hydroxyl group, yielding the desired product.

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