The Hell-Volhard-Zelinsky halogenation reaction halogen

Halogen

The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...

ates carboxylic acid

Carboxylic acid

Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...

s at the α carbon

Carbon

Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

. The reaction is named after three chemists, the German chemists Carl Magnus von Hell

Carl Magnus von Hell

Carl Magnus von Hell was the German chemist who discovered, together with Jacob Volhard and the Russian chemist Nikolay Zelinsky, the Hell-Volhard-Zelinsky halogenation reaction.-Life:...

 (1849–1926) and Jacob Volhard

Jacob Volhard

Jacob Volhard was the German chemist who discovered, together with his student Hugo Erdmann, the Volhard-Erdmann cyclization reaction. He was also responsible for the improvement of the Hell-Volhard-Zelinsky halogenation....

 (1834–1910) and the Russian chemist Nikolay Zelinsky (1861–1953).

Scheme

Unlike other halogenation reactions, this reaction takes place in the absence of a halogen carrier. The reaction is initiated by addition of a catalytic amount of PBr₃, after which one molar equivalent of Br₂

Bromine

Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...

 is added.

PBr₃ replaces the carboxylic OH

Hydroxyl

A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...

 with a bromide, resulting in a carboxylic acid bromide. The acyl bromide

Acyl halide

An acyl halide is a chemical compound derived from an oxoacid by replacing a hydroxyl group with a halide group....

 can then tautomer

Tautomer

Tautomers are isomers of organic compounds that readily interconvert by a chemical reaction called tautomerization. This reaction commonly results in the formal migration of a hydrogen atom or proton, accompanied by a switch of a single bond and adjacent double bond...

ize to an enol

Enol

Enols are alkenes with a hydroxyl group affixed to one of the carbon atoms composing the double bond. Alkenes with a hydroxyl group on both sides of the double bond are called enediols. Deprotonated anions of enols are called enolates...

, which will readily react with the Br₂ to brominate a second time at the α

Alpha carbon

The alpha carbon in organic chemistry refers to the first carbon that attaches to a functional group . By extension, the second carbon is the beta carbon, and so on....

 position.

In neutral to slightly acid

Acid

An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

ic aqueous

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 solution, hydrolysis

Hydrolysis

Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...

 of the α-bromo acyl bromide occurs spontaneously, yielding the α-bromo carboxylic acid in an example of a nucleophilic acyl substitution

Nucleophilic acyl substitution

Nucleophilic acyl substitution describes the substitution reaction involving nucleophiles and acyl compounds. Acyl compounds are carboxylic acid derivatives including esters, amides and acid halides...

. If an aqueous solution is desirable, a full molar equivalent of PBr₃ must be used as the catalytic chain is disrupted.

If little nucleophilic

Nucleophile

A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...

 solvent is present, reaction of the α-bromo acyl bromide with the carboxylic acid yields the α-bromo carboxylic acid product and regenerates the acyl bromide intermediate. In practice a molar equivalent of PBr₃ is often used anyway to overcome the slow reaction kinetics

Chemical kinetics

Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition...

.

The mechanism for the exchange between an alkanoyl bromide and a carboxylic acid is at below.
The α-bromoalkanoyl bromide has a strongly electrophilic carbonyl carbon because of the electron-withdrawing effects of the two bromides. The carboxylic acid will act as a nucleophile that the carbonyl oxygen of the carboxylic acid has a partial negative charge.