When a carboxylic acid reacts with phosphorus tribromide and bromine and then water, an α-bromoacid is formed. What is the structure of the product of this reaction if a three-carbon carboxylic acid is used?

Question

When a carboxylic acid reacts with phosphorus tribromide and bromine and then water, an α-bromoacid is formed.  What is the structure of the product of this reaction if a three-carbon carboxylic acid is used?

Quizplus · Accepted Answer

11ecba2d_12bd_b61b_a3bf_2743b2546f4a_MD0008_00 The Hell-Volhard-Zelinsky (HVZ) reaction forms an α-bromoacid by substitution of bromine on the α-carbon (carbon atom adjacent to a carbonyl carbon) of a carboxylic acid. The reaction begins with the addition of phosphorus tribromide (PBr₃), forming an acyl bromide that can tautomerize from the keto to the enol form. The enol form of the acyl bromide then acts as a nucleophile and attacks one of the bromine atoms in Br₂, adding bromine to the α-carbon and forming an α-bromo acyl bromide. When water is added, it acts as a nucleophile and attacks the carbonyl carbon, and the bromine on the carbonyl carbon acts as the leaving group. This forms the final product, an α-bromoacid, which can then be used to synthesize α-amino acids. The α-bromoacid formed from the reaction of a three-carbon carboxylic acid with PBr₃, Br₂, and then water will contain a bromine bonded on the α-carbon. Choice C is the only structure that shows a three-carbon carboxylic acid with a bromine on the α-carbon only. (Choice A) This structure is the α-bromo acyl bromide intermediate formed after addition of PBr₃ and Br₂ to the carboxylic acid. (Choice B) This carboxylic acid contains bromine atoms on both the α- and β-carbons rather than the α-carbon only. (Choice D) This carboxylic acid is a β-bromoacid, meaning bromine is on the β-carbon (two carbon atoms away from the carbonyl carbon) rather than the α-carbon. Educational objective: The α-carbon of a carboxylic acid is the carbon adjacent to the carbonyl carbon and can undergo substitution reactions to form α-substituted carboxylic acids. An α-bromoacid is a carboxylic acid with bromine on the α-carbon and is formed when a carboxylic acid is reacted with PBr₃, Br₂, and then H₂O.

When a Carboxylic Acid Reacts with Phosphorus Tribromide and Bromine