Halogens and Hypohalites

The oxidation with halogens and hypohalites is a complicated reaction, as it depends strongly on the conditions of temperature, acidity, and concentration of the reacting species. The halogens show considerable differences in the positions of the various equilibria and the speed with which the equilibria are attained (see Table I). In acidic solution, the equilibrium between free halogen and hypohalous acid [Eq. (7. )] lies far to the left, and the concentration of hypohalous acid is very low. When alkali is added to the system, the concentration of hypohalite ion increases, according to Eq. (14). 

The alkaline-metal ion oxidations of reducing carbohydrates are useful as analytical methods, when carefully standardized, to detect and quantitatively determine reducing sugars. They, however, produce a number of oxidized carbohydrate products, including the onic acids, a-keto onic acids, and other degradation products. They, therefore, are not useful for the preparation of the onic acids. For this purpose, halogens and hypohalites are used (see Chapter 4). 

Addition of Hypohalous Acids and Hypohalites (Addition of Halogen, Oxygen)... 

Hence, the concentrations of free halogen, halic acid, and hypohalite vary greatly with the acidity. For example, at pH 1, the total chlorine present... 

The Hofmann rearrangement is the conversion of primary amides into primary amines by the action of sodium hypohalite (usually generated in situ from halogen and sodium hydroxide). The most important feature of the rearrangement is that the amine formed has... 

Neither the Viehe nor the Arens routes to the ethynyl ether are plausible for this system. The l-phenyl-l-methoxy-2-haloalkenes of equation (259), for example, may be recovered intact when treated in MeOH with 4m NaOMe at 155 °C . Though these conditions are presumably suitable for the generation of or C vinyl anions, no onium process (equation 253) seems to have occurred. Further, it seems improbable that phenylacetylide could be a precursor of the ethynylether (Arens mechanism), since this ion abstracts protons from protic solvents (/r lO M s at 25 °C in water) and halogen from hypohalite (OX") [k(Cl) = 2-3 x IQ- m" s" at 25 °C in water] . Thus, the possibility that there is an Arens ion-molecule intermediate, which can survive long enough in methanol to rearrange and form the alkynyl ether... 

The Hofmann reaction involves the conversion of carboxylic primary amides to primary amines or their derivatives. Generally, alkaline hypohalites or a combination of halogens and alkaline hydroxides are used in aqueous solutions. The rearrangement generally occurs by heating. The intermediate isocyanates are easily hydrolyzed to amines under the reaction conditions, as shown in Scheme 3. When sodium methoxide in methanol is used in place of aqueous alkaline hydroxide, the corresponding methylurethanes are formed and may be hydrolyzed to yield amines. 

Fluorination of ice yields milligram amounts of hypofluorous acid, HOF, which is unstable at RT . Solutions of the other hypohalous acids and hypohalites are formed via disproportionative hydrolysis of the halogens ... 

MEK evaporates readily into the atmosphere and is subject to rapid photochemical decomposition. It reacts to form a haloform that is more toxic than the original compound in water containing free halogens or hypohalites. MEK does not accumulate in any environmental compartment, and it is rapidly metabolized by microbes and mammals. There is no evidence of bioaccumulation. 

Halogen cations afford hypohalogen acids or X2 + H20, or 7V-halo compounds. The HO- is obtained from water, which must be present in sufficient amount.261 The corresponding halohydrin ethers are formed when the reaction is carried out in an alcohol. Halogens and the silver salts of carboxylic acids in CC14 give acyl hypohalites RCOOX, which are added as X+ and RCOO- to C=C bonds at low temperatures.262-265... 

Halogenations are also strongly catalyzed by certain other metal ions. A well-studied case is catalysis by mercuric ion. In solutions of halogen and mercuric carboxylate salts, the dominant halogenating agents is the acyl hypohalite. The... 

Oxidation with Hypohalites in Alkaline Solutions. In alkaline solution the halogens exist as hypohalous acid and hypohalite ions. The oxidation is likely to be more drastic than for the free halogens. Thus, whereas free iodine will not act as an oxidant, hypoiodite is a powerful oxidizing agent. Hypobromite and hypochlorite particularly are likely to produce oxidation of primary and secondary alcoholic groups and cause cleavage of carbon-carbon bonds. As noted above, the processes are complicated by the tendency of hypohalite to be converted to halate ions. 

The halogen oxoacids and oxoanions are produced by reactions of the halogens and their oxides with water. Most of the oxoacids are stable only in solution. Table 14.3 shows ball-and-stick models of the acids in which each atom has its lowest formal charge note the formulas, which emphasize that H is bonded to O. The hypohalites (XO ), halites (XO2 ), and halates (XOj ) are oxidizing agents formed by aqueous disproportionation reactions [see the Group 7A(17) Family Portrait, reaction 2]. You may have heated solid alkali chlorates in the laboratory to form small amounts of O2 ... 

The latter is an exttemely reactive species. Trifluoroacetate is a good leaving group and facilitates cleavage of the O—Br bond. The acyl hypohalites are also the active halogenating species in solutions of the hypohalous acids in carboxylic acids, where they exist in equilibrium. 

ForCb, Bt2 and h, K. c is 4.2 x 10- 7.2 x 10 and 2.0 x 10 mol 1 respectively, thereby favouring the free halogens, whereas Kaik is 7.5 X lO, 2 X lO and 30mol I respectively, indicating a tendency to disproportionation which is overwhelming for CI2 but progressively less pronounced for Br2 and I2. In actuality the situation is somewhat more complicated because of the tendency of the hypohalite ions themselves to disproportionate further to produce the corresponding halite ions ... 

In addition to the applications indicated on p. 858. hypohalous acids are useful halogenating agents for Ixjth aromatic and aliphatic compounds. HOBr and HOI are usually generated in silii. The ease of aromatic halogenation increa.ses in the sequence OCl < OBr < Ol and is facilitated by salts of Pb or Ag. Another well-known reaction of hypohalites is their cleavage of methyl ketones to form carboxylates and haloform ... 

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