Hell-Volhard-Zelinskii

By application of the Hell-Volhard-Zelinskii reaction,an a-hydrogen of a carboxylic acid 1 can be replaced by bromine or chlorine to give an a-bromo-or a-chlorocarboxylic acid 2 respectively. 

I Alpha Bromination of Carboxylic Acids The Hell-Volhard-Zelinskii Reaction... 

The Hell-Volhard-Zelinskii reaction is a bit more complex than it looks and actually involves substitution of an acid bromide enol rather than a carboxylic acid enol. The process begins with reaction of the carboxylic acid with PBr3 to form an acid bromide plus HBr (Section 21.4). The HBr then catalyzes enolization of the acid bromide, and the resultant enol reacts with Br2 in an cr-substitution reaction to give an cv-bromo acid bromide. Addition of water hydrolyzes the acid bromide in a nucleophilic acyl substitution reaction and yields the a-bromo carboxylic acid product. 

Problem 22.6 If methanol rather than water is added at the end of a Hell-Volhard-Zelinskii reaction, an ester rather than an acid is produced. Show how you could cany out the following transformation, and propose a mechanism for the ester-forming step. 

Hell-VoLhard-Zelinskii (HVZ) reaction, in which an acid is treated with Bc2 and PBr3. The a-halogenated products can then undergo base-induced E2 elimination to yield a,j6-unsaturated carbonyl compounds. 

When an optically active carboxylic acid such as (jR)-2-phenylpropanoic acid is brominated under Hell-Volhard-Zelinskii conditions, is the product optically active or racemic Explain. 

Amino acids can also be prepared by a two-step sequence that involves Hell— Volhard-Zelinskii reaction of a carboxylic acid followed by treatment with ammonia. Show how you would prepare leucine, (C bCHCl-HClKNPfylCC H, and identify the mechanism of the second step. 

Hell-Volhard-Zelinskii (HVZ) reaction (Section 22.4) The reaction of a carboxylic acid with Br2 and phosphorus to give an -bromo carboxylic acid. 

Heat of combustion, 113 Heat of hydrogenation, 186 table of, 187 Heat of reaction, 154 Helicase, DNA replication and, 1106 Hell-Volhard-Zelinskii reaction, 849 amino acid synthesis and. 1025 mechanism of, 849 Heme, biosynthesis of, 966 structure of, 946 Hemiacetal, 717 Hemiketal, 717 Hemithioacetal, 1148 Henderson-Hasselbalch equation,... 

In order to introduce an S03 group into long-chain fatty acids, direct sulfonation with sulfur trioxide had been suggested in 1962 [29], similar to the Hell-Volhard-Zelinskii reaction of the a-bromation studied by Watson [34], Ten years previously de Boer had reviewed this mechanism among others for lower aliphatic sulfocarboxylic acids [35]. The sulfonation process was proposed as a two-step mechanism. The first step is the rapid addition of the S03 into... 

Halogenation of carboxylic acids (Hell-Volhard-Zelinskii) and acid... 

The Hell-Volhard-Zelinskii reaction involves formation of an intermediate acid bromide enol, with loss of stereochemical configuration at the chirality center. Bromination of (/ )-2-phenylpropanoic acid can occur from either face of the enol double bond, producing racemic 2-bromo-2-phenylpropanoic acid. If the molecule had a chirality center that didn t take part in enolization (Problem 22.33), the product would be optically active. 

A malonic ester synthesis is used to form 4-methylpentanoic acid. Hell-Volhard-Zelinskii bromination of the acid, followed by reaction with ammonia, yields leucine. The last reaction is an Sn2 displacement of bromide by ammonia. 

The Hell-Volhard-Zelinskii method and the phthalimide method. 

Alpha bromination (Hell-Volhard-Zelinskii reaction) (Sec. 22.4)... 

Hell-Volhard-Zelinskii reaction (Section 22.4) the a-bromination of a carboxylic acid by treatment with bromine and phosphorus tribromide. 

Carbonyl compounds are in a rapid equilibrium with their cnols, a process called keto-enol tautomerism. Although enol tautomers arc normally present to only a small extent at equilibrium and can t usually be isolated in pure form, they nevertheless contain a highly nucleophilic double bond and react with electrophiles. P or example, aldehydes and ketones are rapidly halogenated at the a position by reaction with CI2 Br2, or I2 in acetic acid solution. Alpha bromination of carboxylic acids can be similarly accomplished by the Hell-Volhard-Zelinskii (HVZ) reaction, in which an acid is treated with Bt2 and PBrThe a--halogenated inoducts can then undergo base-induced E2 elimination to yield a,/3-un.salurated carbonyl compounds. 

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