Question

Phenylalanine G can be synthesised from 3-phenylpropanoic acid E through intermediate F using the well-known Hell-Vollhard-Zelinsky reaction shown in Figure 1. CHBO Figure 1 (1) Identify the structure of intermediate F. [2 marks] (1) F from 3- Draw the mechanism for the formation of intermediate phenylpropanoic acid E. [6 marks) (i) Draw the mechanism for the formation of phenylalanine G from intermediate [1 marks) (iv) Outline two drawbacks to this method of amino acid synthesis. [2 marks)

Answer 1

PBrz, Br2 Logo Orlacom OH COH NH3 OH a position NH2 C,H,Bro, The carboxylic acids can be halogenated in their a position, by reaction with Br, catalyzed with PBrz. The brominated carboxylic acids can be transformed into other a-substituted derivatives. Thus, by reaction with ammonia form amino acids. So The structure of intermediate Fis: I OH

Mechanism: Bra-Halogenation (Hell-Volhard-Zelinsky reaction) Br --ot P- Br Per, replaces the hydroxyl (OH) of the carboxylic group with a bromide, resulting in a carboxylic acid bromide. ..- : Br: Br Enol Acyl bromide can tautomerize an enol Br-Br The enol reacts rapidly with Bryhalogenating for the second time at the position of the alpha carbon Exchange between acyl bromide and carboxylic acid. + of EO -- -- -- విధ్యం vH : Br: The carboxylic acid acts as a nucleophile. oation ou + Y Br CH,Bro.

Mechanism: Amino acid formation NH3 -0 :NH3 H-Ó NH2

1) Dialkylation may occur HOY 2) The racemic mixture is obtained OH NH2 D, L Phenylalanine