Textbook Question
What amino acid is formed using the N-phthalimidomalonic ester synthesis when the following alkyl halides are used in the third step?
b. CH3SCH2CH2Br
24. Enolate Chemistry: Reactions at the Alpha-Carbon
Malonic Ester Synthesis
3:20 minutesProblem 40c
Textbook Question
Explain why the following carboxylic acids cannot be prepared by a malonic ester synthesis:
c. 
Verified step by step guidance1
Step 1: Understand the malonic ester synthesis process. Malonic ester synthesis is a method used to prepare carboxylic acids by alkylating malonic ester (diethyl malonate) and then hydrolyzing and decarboxylating the product. The key requirement is that the alkylation step must occur at a position where the malonic ester can form a stable intermediate.
Step 2: Analyze the structure of the given carboxylic acid. The carboxylic acid shown has a cyclopentyl group attached to a carbon adjacent to the carboxylic acid functional group. This structure suggests that the alkylation step would involve introducing a cyclopentyl group.
Step 3: Consider steric hindrance in the alkylation step. The cyclopentyl group is bulky, and introducing it via alkylation of malonic ester would be sterically hindered. This makes the reaction less likely to proceed efficiently.
Step 4: Evaluate the stability of intermediates. During malonic ester synthesis, the intermediate formed after alkylation must be stable enough to undergo hydrolysis and decarboxylation. The bulky cyclopentyl group may destabilize the intermediate, preventing successful synthesis.
Step 5: Conclude why malonic ester synthesis is not suitable. Due to steric hindrance and potential instability of intermediates, the given carboxylic acid cannot be prepared using malonic ester synthesis.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Malonic Ester Synthesis
Malonic ester synthesis is a method for synthesizing carboxylic acids by alkylating malonic ester followed by hydrolysis and decarboxylation. This reaction typically requires a simple alkyl halide and results in a carboxylic acid with a two-carbon extension from the malonic ester. Understanding this process is crucial to determine why certain carboxylic acids cannot be synthesized using this method.
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Structural Constraints
The structure of the target carboxylic acid plays a significant role in determining its feasibility for synthesis via malonic ester synthesis. If the desired acid contains functional groups or structural features that cannot be introduced through the malonic ester pathway, such as cyclic structures or specific stereochemistry, it cannot be synthesized using this method. Analyzing the structure of the acid is essential to identify these constraints.
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Reactivity of Intermediates
In malonic ester synthesis, the reactivity of intermediates formed during the reaction is critical. The malonic ester must be able to undergo nucleophilic substitution with an alkyl halide, and the resulting intermediate must be stable enough to proceed through hydrolysis and decarboxylation. If the intermediates formed from the target carboxylic acid are unstable or reactive in a way that leads to side reactions, the synthesis will not be successful.
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