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The compound tryptophan (an indole amino acid with the side chain -CH2-CH(NH2)-COOH attached at C-3 of indole) is treated with excess SOCl2 followed by methanol (CH3OH). What is the major product of this reaction?

1. Indole derivative where -COOH is converted to methyl ester (-CO2CH3) and the amine is protonated as -NH2*HCl salt
2. Chloro-substituted indole derivative with methyl ester (-CO2Me) and free -NH2
3. Indole derivative with methyl ester (-CO2CH3) and free -NH2
4. Indole derivative where -COOH is converted to acid chloride (-COCl) and -NH2 is free

Correct answer: Indole derivative where -COOH is converted to methyl ester (-CO2CH3) and the amine is protonated as -NH2*HCl salt

Solution

SOCl2 is a highly reactive reagent. It converts -COOH to -COCl (acyl chloride) and simultaneously protonates the -NH2 group to give an ammonium chloride salt (-NH3+ Cl-). When methanol is subsequently added, it attacks the acyl chloride to form the methyl ester (-CO2CH3) with release of HCl. The -NH2 group remains as the HCl salt throughout. The indole ring is not chlorinated under these mild conditions.

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