Hypohalite reagents

The oxidation of a methyl ketone to a carboxylic acid can be effected by the use of the haloform reaction. This involves treatment of the methyl ketone with an alkaline hypohalite reagent. A trihalomethyl ketone is initially formed which then undergoes hydrolysis under the basic conditions used. 

Halogenations are strongly catalyzed by mercuric acetate or trifluoroacetate. These conditions generate acyl hypohalites, which are the active halogenating agents. The trifluoroacetyl hypohalites are very reactive reagents. Even nitrobenzene, for example, is readily brominated by trifluoroacetyl hypobromite.19... 

The hypohalite route to 1-chloro-, 1-bromo-, or 1-iodoalkynes is both general and convenient. The purity of the reagents does not appear to be critical. 

The most widely used, and often most convenient reagents for such one-pot reactions are sodium hypochlorite (45) or hypobromite (16). These reactions are performed in the presence of an organic base (generally triethylamine) that normally enhances the yield of cycloaddition products (45). This method was employed for many intermolecular reactions (71) and also seems especially suited for intramolecular ones (72-77) as well as for the solid-phase synthesis (78) of 2-isoxazolines. Hypohalite can also be replaced by sodium broruite in combination with a catalytic amount of tri-u-butyltin chloride (79). In a related method, O-tributylstannyl oximes were treated with tert-butyl hypochlorite to produce nitrile oxides that were trapped with aUcenes or alkynes to afford the corresponding isoxazolines or isoxazoles in moderate to good yield (80). 

A-Haloamides and imides are a most versatile group of halogenating reagents. They are normally prepared in an aqueous solution by reaction of the parent amide/imide with the corresponding hypohalite. An improved method for the synthesis of A-bromo amides/imides has recently been published, in which the parent compounds are oxy-brominated in aqueous solution by a mixture of HBr/NaBr03 or NaBr/NaBr03/H2S04 (equation 95)700. 

Benzidine acetate test (DANGER THE REAGENT IS CARCINOGENIC) A neutral or weakly acetic acid solution of a peroxodisulphate converts benzidine into a blue oxidation product. Perborates, percarbonates, and hydrogen peroxide do not react. Chromates, hexacyanoferrate(III) ions, permanganates, and hypohalites react similarly to peroxodisulphates. 

In connection with analysis, we shall encounter two reagents used to oxidize alcohols of special kinds (a) hypohalite (Sec. 16.11), and (b) periodic acid (Sec. 16.12). 

Alkoxy radicals for ring expansion can be generated from alcohols by oxidative methods such as hypohalite thermolysis/photolysis [19a] and lead tetraacetate oxidation [19b], or peroxide reduction [19c]. The recent development of the hyper-valent organoiodine reagent (diacetoxyiodo)benzene (DIB) provides another way for efficient generation of alkoxy radicals (Scheme 11) [19d]. Additional oxidative methods to prepare cyclopropyloxy radicals include reaction of tertiary cyclopropanols or their silyl ether derivatives with various reagents such as manganese(III) tris(pyridine-2-carboxylate) [Mn(pic)3] [20a], Fe(III) salts [20b], and vanadyl ace-tylacetate [20c] (Scheme 12). 

Disulphides are obtained by oxidation of the corresponding thiols, by a variety of reagents, e.g. oxygen, hydrogen peroxide, iodine, bromine, hypohalites, ferric chloride, nitrous oxide, sulphonyl chloride diethyl azocarboxylate (1) N-bromosuccinimide , tetranitromethane, peroxy-... 

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