Halogen-carriers

If cold benzene is treated with bromine in the absence of sunlight, very little reaction occurs if, however, a halogen carrier, such as iron, iodine, pyridine, etc., is also present, a rapid reaction by substitution occurs, forming first... 

Bromination of fatty acids in the a-position can be effected quite readily in the presence of phosphorus trichloride, red phosphorus or pyridine as catalysts or halogen carriers with acetic acid, the addition of acetic anhydride (to ensure the absence of water) improves the yield and facilitates the bromination. Examples are —... 

The halogen carriers or aromatic halogenation catalysts are usually all electrophilic reagents (ferric and aluminium haUdes, etc.) and their function appears to be to increase the electrophilic activity of the halogen. Thus the mechanism for the bromination of benzene in the presence of iron can be repre-sfflited by the following scheme ... 

Other halogen carriers may be used, e.g., 1-2 g. of iron filings, or 1 g. of aluminium amalgam. The bromine must then be added slowly from a dropping funnel to the benzene warmed on a water bath the apparatu.s shown in Fig. II, 13, 9 is suitable and a trap for the hydrogen bromide must, however, be inserted into the top of the condenser. After all the bromine has been introduced, the mixture is heated on a water bath until no red vapours are visible above the liquid. The Subsequent procedure is as above. 

The pyridine acts as halogen carrier, probably by forming" the additive compound CjH-NBiu, which gives up its bromine to the benzene. 

The action of chlorine on the aliphatic acids takes place in presence of sunlight, also on the addition of small quantities of the halogen-carriers, iodine, sulphur, and red phosphorus. By the action ofiodine, I Cl is formed, which decomposes moie readily than the molecule of chlorine, and hydriodic acid is libciatcd,... 

If chlorine and bromine are allowed to act upon an aromatic hydrocarbon like toluene, which has a side-chain, substitution may occur in the nucleus or the side-chain, according to the conditions. Generally speaking, in the cold and in presence of a halogen carrier, nuclear substitution occurs, Irut at a high temperatuie the halogen passes into the side-chain (see Piep. 

Halogen-Ubertrkger, m. halogen carrier. -Ur, n. (ous) halide. C/. Chlortir. -verbindung, /. halogen compound. 

The anion FeBr4e assists in the removal of a proton from the a complex (13). The classical halogen carrier iron filings does, of course, act only after it has been converted into the Lewis acid, FeX3. 

Substitution occurs exclusively in the ring when typical halogen-carriers such as iron filings or iodine are used. From toluene the o-and p-derivatives are produced together. 

Halogen carriers, 533 Halogen compounds, reaction with alcoholic silver nitrate, 1059 reaction with sodium iodide in acetone, 1059, 1060 ... 

Fifty grams of sulfur monochloride are placed in the distilling flask of the set-up outlined in the procedure for that substance (No. 81). About O.lg of iron powder is added to the liquid as a halogen carrier and bone-dry chlorine is led in steadily (3-4 bubbles/sec) for one-half hour. The dark liquid that is produced is allowed to stand one hour and then... 

The alnminium-mercnry couple (prepared by immersing aluminium foil in mercuric clilorido solution) is a halogen carrier of some value, and is used to facilitate substitution in the n.romatic series of compounds, by chlorine and bromine. 

Benzene undergoes a substitution reaction with chlorine only in the presence of halogen carriers. An addition reaction takes place in the absence of a carrier, leading to the formation of benzene hexa-chloride. Iron is the only halogen carrier used extensively in industry, a suitable form being fine iron powder. 

H add, see l-Amino-8-naphthol-3,6-disulfom c add Halogenation, general, 65 Halogen atoms active, 85, 103 introduction, 9 Halogen carriers, 66 Hansa yellow, 133, 143, 266 Helian ine, 275 Helio Bordeaux BL, 268 Heliogen blue B, 338 Hemp, 368... 

Hydrochloric acid is without action on gold, but addition of a small quantity of ferric chloride causes the gold to dissolve, the ferric salt presumably acting as a chlorine-carrier 2 in the presence of hydrochloric acid and oxygen. Salts of iron are frequently used in organic chemical processes as halogen-carriers.3... 

Halogen Substitution Products.—When, however, chlorine acts on benzene in diffused light, substitution products are formed directly. This takes place more readily in the presence of halogen carriers, e,g, ferric chloride, FeCU aluminium bromide, AlBrs antimony chloride. 

The halogenation is usually performed in the presence of catalysts. A great variety of so-called halogen-carriers can be used. Iron, phosphorus or its halides, and sulfur and its halides are among the most common. 

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