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Remarkably, ketone halogenation also occurs in biological systems, particularly in marine algae, where dibromoacetaldehyde, bromoacetone, 1, l,l -tribromoacetone, and other related compounds have been found. The halogenation is a typical α-substitution reaction that proceeds by acid catalyzed formation of an enol intermediate. [1]: 846
The position alpha to the carbonyl group (C=O) in a ketone is easily halogenated. This is due to its ability to form an enolate (C=C−O −) in basic solution, or an enol (C=C−OH) in acidic solution. An example of alpha halogenation is the mono-bromination of acetone ((CH 3) 2 C=O), carried out under either acidic or basic conditions, to ...
They are often prepared by reaction of the acid or the ester with halogen: RCH 2 CO 2 R' + Cl 2 → RCHClCO 2 R' + HCl. A related method is the Hell-Volhard-Zelinsky halogenation. Amino acids are susceptible to diazotization in the presence of chloride, a process that affords chiral 2-chloro carboxylic acids and esters. [1]
An example of the Hell–Volhard–Zelinsky reaction can be seen in the preparation of alanine from propionic acid.In the first step, a combination of bromine and phosphorus tribromide is used in the Hell–Volhard–Zelinsky reaction to prepare 2-bromopropionic acid, [3] which in the second step is converted to a racemic mixture of the amino acid product by ammonolysis.
When the α(alpha) position has been exhaustively halogenated, the molecule reacts with hydroxide, with − CX 3 being the leaving group stabilized by three electron-withdrawing groups. In the third step the − CX 3 anion abstracts a proton from either the solvent or the carboxylic acid formed in the previous step, and forms the haloform.
In the Hunsdiecker reaction, carboxylic acids are converted to organic halide, whose carbon chain is shortened by one carbon atom with respect to the carbon chain of the particular carboxylic acid. The carboxylic acid is first converted to its silver salt, which is then oxidized with halogen: R−COO − Ag + + Br 2 → R−Br + CO 2 + Ag + Br −
Steric considerations suppress this tendency in α,β-unsaturated carboxylic acids, which instead polymerize (see below). [12]: 468 Mercuric oxide and bromine convert 3-chlorocyclobutanecarboxylic acid to 1-bromo-3-chlorocyclobutane. This is known as Cristol-Firth modification.
The general structure is RR′C(X)C(=O)R where R is an alkyl or aryl residue and X any one of the halogens. The preferred conformation of a halo ketone is that of a cisoid with the halogen and carbonyl sharing the same plane as the steric hindrance with the carbonyl alkyl group is generally larger.