Replacement of mercury by halogen

Bromobenzene. -The replacement of hydrogen by the halogens Cl and Br, in the nucleus of aromatic hydrocarbons, is assisted by the presence of a halogen carriei, the action of which lias been referred to in the Note on the piepaiations of chlor- and bioin-acetic acids, p. 252. Iodine, iion, iron and alupiinium chlorides and bromides, the aluminium-mercury... 

In the case of halogen-substituted hydrocarbons the principal method used is the mercuration of the sulphinic acid, and not the hydrocarbon itself, whilst the method acts equally well with nitrobenzene. The nitrotoluenes when mercurated by mercuric oxide in the presence of sodium hydroxide yield compounds containing mercury in the side chain, but with mercuric acetate at 140° C., in the absence of a solvent, the mercury enters the ring. If nitrotoluene sulphinic acids are used as starting-points, the acid group is replaced b mercury by prolonged boiling with 50 per cent, aqueous alcoholic mercuric chloride. 

The different reactivities of the two substituents on C-l allow selective replacement and conversion of them. Alkyl monothio-acetals [proposed as intermediates in mercury(II)-catalyzed demer-captalation reactions—see Section IV,l,b] have been prepared from a-bromothioethers by the combined action of an alcohol and silver(I) carbonate the introduction of S-nucleophiles is discussed in Section 11,6. Reduction of 81 by lithium aluminum hydride effects hydrogen-olysis of the carbon-halogen bond, whereas the action of Raney nickel on the derived S-ethyl O-methyl monothioacetal specifically cleaves the carbon-sulfur bond to afford the pentaacetate of 1-0-methyl-D-galactitol.327... 

The procedure to replace mercury in organic compounds by lithium was developed already in 1917 by Schlenk and Holtz and it is still the method of choice for the preparation of halide-free organolithium compounds. Wittig and Bickelhaupt were the first who used a mercury-lithium exchange reaction for the synthesis of a dilithiocompound not available by halogen-lithium exchange, o-dilithiobenzene 75. 

The dihalides, RgShHaLg, may be obtained by heating mercury diphenyl with antimony trichloride at a high temperature in an autoclave, or as by-products in the preparation of triarylstibmes by the Fittig reaction. Halogens also add on directly to triarylstibines, gi nng the dihalides, and in the case of chlorine, the following chlorides may replace the free element in the preparation copper, iron, thallium, phosphorus or arsenic chloride. 

Anthraquinone can be sulfonated, nitrated, or halogenated. Sulfonation is of the greatest technical importance because the sulfonic acid group can be readily replaced by an amino or chloro group. Sulfonation with 20—25% oleum at a temperature of 130—135°C produces predominandy anthraquinone-2-sulfonic acid [84-48-0]. By the use of a stronger oleum, disulfonic acids are produced. The second sulfonic acid substituent never enters the same ring a mixture of 2,6- and 2,7-disulfonic acids is formed (Wayne-Armstrong rule). In order to sulfonate in the 1-, 1,5-, or 1,8-positions, mercury or one of its salts must be used as a catalyst. 

The halogen in the acylglycosyl halide is reactive and may be readily displaced, for example, by an alkoxy group on reaction with an alcohol under anhydrous conditions in the presence of a silver or mercury(n) salt. In this case the products are glycosides which are the mixed cyclic acetals related to the cyclic hemiacetal forms of the monosaccharides. In the case of the D-glucose derivative shown below (and of other 1,2-cis acylglycosyl halides) the replace-... 

When the mercuric acetate in the preceding preparation is replaced by 16 grams of mercuric acetate in 50 c.c. of 3 per cent, acetic acid, colourless needles ciyrstaliise out in a short time, and these are filtered off and washed with water. The diacetoxy derivative is stable to ammonium sulphide, but behaves towards halogen salts in a similar way to the mono mercury compound. 

There is a lot of information on the mercuration of heterocycles in the old literature, but it is seldom used nowadays due to the major disadvantages of toxicity and associated waste management it can be, however, a very useful reaction. The advantages of mercuration are that it can be carried out in hydroxylic and acidic solvents and in the presence of air, and that mercury in the product is easily replaced by ipso-substitution with other electrophiles, such as halogens, and gives boronic acids by reaction with borane. 

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