Question
Gabriel Synthesis:
Step #1: Potassium phthalimide is reacted with diethyl bromomalonate in an SN2 reaction to form a substitution product of phthalimidomalonic ester.
Step #2: ____.
Step #3: The deprotonated alkyl imide can now act as a nucleophile, attacking the substrate carbon of a bromoalkane.
Step #4: The created big molecule is now hydrolyzed multiple times with strong base and heat, where you are left with phthalic acid and dicarboxylic acid, with an amine on α-carbon.
Step #5: The dicarboxylic acid formed is then decarboxylated through addition of acid and heat, resulting in the formation of a complete amino acid.
Answer
Gabriel Synthesis:
Step #1: Potassium phthalimide is reacted with diethyl bromomalonate in an SN2 reaction to form a substitution product of phthalimidomalonic ester.
Step #2: Add a base to deprotonate the alkylated imide we formed, allowing it to act as a nucleophile.
Step #3: The deprotonated alkyl imide can now act as a nucleophile, attacking the substrate carbon of a bromoalkane.
Step #4: The created big molecule is now hydrolyzed multiple times with strong base and heat, where you are left with phthalic acid and dicarboxylic acid, with an amine on α-carbon.
Step #5: The dicarboxylic acid formed is then decarboxylated through addition of acid and heat, resulting in the formation of a complete amino acid.
Note ↑: Phthalimide is acidic and exists in solution as a nucleophilic anion. Diethyl bromomalonate contains a secondary carbon bonded to bromine, a good leaving group. This setup should sound much like the SN2 reactions discussed in Chapter 4 of MCAT Organic Chemistry Review. With phthalimide as the nucleophile, the (secondary) substrate carbon as the electrophile, and bromine as the leaving group, this reaction generates a phthalimidomalonic ester.
Note ↑: The carbonyl carbon of an amide is sp2
hybridized and has trigonal planar geometry. A second
resonance structure can be drawn that delocalizes the nonbonded electron pair on the N atom.
Amides are more resonance stabilized than other acyl compounds, so the resonance structure
having the C=N makes a significant contribution to the hybrid.
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