Textbook Question
Show how you would accomplish the following multistep syntheses, using the indicated starting material and any necessary reagents.
(a) hept-6-en-1-ol → ε-caprolactone
(b) methoxybenzene → p-methoxybenzamide
22. Carboxylic Acid Derivatives: NAS
Carboxylation
4:37 minutesProblem 59
Textbook Question
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.]
Verified step by step guidance1
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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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Reaction Mechanisms
Understanding reaction mechanisms is crucial in organic chemistry as it describes the step-by-step process by which reactants transform into products. Each mechanism outlines the movement of electrons, the formation and breaking of bonds, and the intermediates involved. A clear grasp of these mechanisms helps chemists predict the outcomes of reactions and troubleshoot when desired products are not formed.
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Functional Groups
Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Recognizing the functional groups present in reactants and products is essential for predicting reactivity and understanding how changes in these groups can affect the outcome of a reaction. This knowledge is vital when suggesting modifications to achieve the desired product.
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Stereochemistry
Stereochemistry involves the study of the spatial arrangement of atoms in molecules and how this affects their chemical behavior. The three-dimensional orientation of functional groups can significantly influence reaction pathways and product formation. Understanding stereochemistry is important for addressing issues related to selectivity and yield in organic reactions, particularly when the desired product has specific stereochemical requirements.
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