Halogenation Haloform reaction

This IS called the haloform reaction because the trihalomethane produced is chloroform (CHCI3) bromoform (CHBrj) or iodoform (CHI3) depending on the halogen used... 

The mechanism of the haloform reaction begins with a halogenation via the eno late The electron attracting effect of an a halogen increases the acidity of the protons on the carbon to which it is bonded making each subsequent halogenation at that car bon faster than the preceding one... 

Methyl ketones 1, as well as acetaldehyde, are cleaved into a carboxylate anion 2 and a trihalomethane 3 (a haloform) by the Haloform reaction The respective halogen can be chlorine, bromine or iodine. 

Haloform reaction (Section 22.6) The reaction of a methyl ketone with halogen and base to yield a haloform (CHX3) and a carboxylic acid. 

Haloalkane,. see Alkyl halide Haloform reaction, 854-855 Halogen, inductive effect of, 562 resonance effect of, 563 Halogenation, aldehydes and, 846-848... 

In the haloform reaction, methyl ketones (and the only methyl aldehyde, acetaldehyde) are cleaved with halogen and a base. The halogen can be bromine, chlorine, or iodine. What takes place is actually a combination of two reactions. The first is an example of 12-4, in which, under the basic conditions employed, the methyl group is trihalogenated. Then the resulting trihalo ketone is attacked by hydroxide ion ... 

There is also the haloform reaction (effect of a haloform on a ketone in a basic medium), already described with halogen derivatives on p.272 (the danger is more related to the interaction of the polyhalogen derivative with the base, according to the author). A large number of accidents involved the ketone as much as butanone. The accident below illustrates the danger of this reaction ... 

Any compound that oxidizes to a methyl ketone also gives a haloform reaction, because halogens are also oxidizing agents. For example, the compound shown in Figure 11-8 reacts. 

Figure 11-9 illustrates the mechanism for the haloform reaction. The mechanism involves a repeated series of base attacks (removal of an a-hydrogen) followed by the reaction with the halogen until all three a-hydrogen atoms are replaced. Then the base attacks the carbonyl carbon to induce the loss of a carbanion Q.CXf. The highly reactive carbanion quickly attacks and removes the hydrogen from the carboxylic acid group. 

The haloform reaction is a useful method of preparing a carboxylic acid (carboxylate ion) with one less carbon. It is one of the very few cases where carbanion loss occurs. It s only possible because the three halogen atoms are capable of stabilizing the negative charge. 

In general, ketones don t undergo oxidation however, methyl ketones undergo a haloform reaction. In a haloform reaction, the oxidation converts the methyl group to a haloform molecule (usually iodoform (CHI3)), which leaves the Ccirbon backbone one carbon atom shorter. The oxidant in a haloform reaction is sodium hypohalite (NaOX), which forms by the reaction of sodium hydroxide (NaOH) with a halogen (X, where X = Cl, Br, or 1). 

Haloform reaction, 237, 296 Halogenation alkanes, 300, 323 alkenes, 179,186, 313 benzene, 138,316 ketones, 295 Hammett equation, 362 additional parameters, 374, 388, 395 derivation of, 362 deviations from, 375 empirical nature of, 395 implications of, 394 reaction pathway, and, 375 solvent effects and, 388 spectroscopic correlations, 392 standard reaction for, 362, 395 steric effects and, 361, 383 thermodynamic implications of, 394 Hammett plots, 359 change in rate-limiting step and, 383 change in reaction pathway and, 378... 

Haloform reaction, 297 Halogenated aromatic hydrocarbons, 533 reactions and characterisation of, 542 ... 

This sequence is called the haloform reaction because it results in the production of chloroform, bromoform, or iodoform, depending upon the halogen used. The haloform reaction is a useful method for identification of methyl ketones, particularly when iodine is used, because iodoform is a highly insoluble, bright-yellow solid. The reaction also is very effective for the synthesis of carboxylic... 

Because the haloform reaction is fast, in some cases it can be used to prepare unsaturated acids from unsaturated ketones without serious complications caused by addition of halogen to the double bond ... 

When a base is used with a methy] ketone, the alpha carbon will become completely halogenated. This trihalo product reacts further with the base to produce a carboxylic acid and a haloform (chloroform, CHCI j bromoform, CHBr, or iodoform, CHIj). This is called the Haloform Reaction. 

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