Reaction halogenation

One of the most important reactions in arene synthesis is the halogenation of arenes. Conventionally, these reactions are performed directly by bromine or chlorine [7]. However, on the laboratory scale chlorine is not easily manageable and is a toxic gas. Therefore, it is not often used in academic research. For iodinations normally a strong oxidizing agent is required. In halogenations, often unintentional side-chain 

Heme-dependent haloperoxidases generate HOX as reactive species from H2O2 and X, which represents an X+ equivalent capable of undergoing electrophilic addition at electron-rich centers [270,271]. Aprototype biocatalyst of this group is the chloroperoxidase from Caldariomyces Jumago [272]. In many natural systems, such enzymes are responsible for the halogenation of electron-rich aromatic cores. 

Considering the tremendous progress made in recent years to understand and utilize enzymatic halogenations in stereoselective synthesis, this area will continue to receive significant attention and interesting new developments can be expected in the years to come. 

Another reaction which can be brought about by PCI5 is the conversion of aldehydes and ketones into the corresponding geminal dichlorides and this is illustrated in Protocol 7 by the conversion of terephthalaldehyde 36 into the tetrachloride 37. Unusually for a reaction using PCI5, no HC1 is evolved here and the product is sufficiently unreactive to be isolated using an aqueous work-up. It should be noted that chlorination of aromatic aldehydes in this way is the method of choice for synthesis of benzylidene chlorides since the products obtained from the alternative chlorination of toluenes are always contaminated by the benzyl chlorides and benzotrichlorides. 

Synthesis of a, a, a, a -tetrachloro-p-xylene [1,4-bis(dichloromethyl) benzene] (R. A. Aitken and A. 0. Oyewale, unpublished data) 

To a single-necked, round-bottomed flask (250 mL) add terephthalaldehyde 36 (5.0 g, 37 mmol), dichloromethane (100 mL) and a magnetic stirrer bar. Stir until a clear solution is obtained. 

Using a powder funnel, cautiously add phosphorus pentachloride (16.0 g, 77 mmol) in portions, over a period of 10 min. Do not use a metal spatula for this since it will be corroded—if necessary use a glass rod. 

Place a stopper on the flask and stir at room temperature for 2 h. 

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Simpson W C and Yarmoff J A 1996 Fundamental studies of halogen reactions with lll-V semiconductor surfaces Ann. Rev. Phys. Chem. 47 527-54... 

These halogenation reactions all take place in the 5-position (408. 409. 430) even when there is a phenyl or a 2-pyridyl (382) substituent on the exocyclic nitrogen. Crystalline perbromides have been isolated (166. 320. 

The 4,5-dihalogenothiazoles are obtained by cyclization-halogenation reactions as show in scheme 12 (3). 2-Acetamido-4,5-diiodothiazole has been obtained by Hurd and Wehrmeister (80). The triiodothiazole can be prepared by iodination by molecular iodine of the mercuric complex of 2-iodothiazole following the Travagli method (81). 

Halogenation reactions usually involve pyrroles with electronegative substituents. Mixtures are usually obtained and polysubstitution products, ie, tetrahalopyrroles, predominate. The monohalopyrroles are difficult to prepare and are not very stable in air or light. 

The halogenation reaction conditions can be chosen to direct attack to the methyl group (high temperature or light to form free-radicals) or the aromatic ring (dark, cold conditions with FeX present to form electrophilic conditions). 

Halogen donors are chemicals that release active chlorine or bromine when dissolved in water. After release, the halogen reaction is similar to that of chlorine or bromide from other sources. SoHd halogen donors commonly used in cooling water systems include l-bromo-3-chloro-5,5-dimethyIhydantoin, l,3-dichloro-5,5-dimethyIhydantoin, and sodium dichloroisocyanurate. 

Aromatic compounds may be chlorinated with chlorine in the presence of a catalyst such as iron, ferric chloride, or other Lewis acids. The halogenation reaction involves electrophilic displacement of the aromatic hydrogen by halogen. Introduction of a second chlorine atom into the monochloro aromatic stmcture leads to ortho and para substitution. The presence of a Lewis acid favors polarization of the chlorine molecule, thereby increasing its electrophilic character. Because the polarization does not lead to complete ionization, the reaction should be represented as shown in equation 26. 

Halogen Reactions. Hydrolysis of chlorotoluenes to cresols has been effected by aqueous sodium hydroxide. Both displacement and benzyne formation are involved (27,28). o-Chlorotoluene reacts with sodium in Hquid ammonia to afford a mixture of 67% of o-toluidine [95-53-4] and 33% of yW-toluidine [108-44-1], C H CIN, as shown in equation 3 (29). 

Halobutyls. Chloro- and bromobutyls are commercially the most important butyl mbber derivatives. The halogenation reaction is carried out in hydrocarbon solution using elemental chlorine or bromine (equimolar ratio with enchained isoprene). The halogenation is fast, and proceeds mainly by an ionic mechanism. The stmctures that may form include the following ... 

The changes in the sU engths of molecular bonding in the sequence of hydrogen-halogen reactions is... 

These data suggest that both die chain reactions and die biinolecular reactions occur simultaneously in all systems but that the biinolecular reaction, which dominates the formation of HI, occurs only to a minor extent in the formation of HCl and HBr. To analyse the difference between these alternate mechanisms of hydrogen-halogen reactions, we should first consider die relative energies of each of the chain reaction steps for each system. The results for for... 

OL Halogenation (Sections 18.2 and 18.3) Halogens react with aldehydes and ketones by substitution an a hydrogen is replaced by a halogen. Reaction occurs by electrophilic attack of the halogen on the carbon-carbon double bond of the enol form of the aldehyde or ketone. An acid catalyst increases the rate of enolization, which is the ratedetermining step. 

The chemical reactions of XY can be conveniently classified as (a) halogenation reactions, (b) donor-acceptor interactions and (c) use as solvent systems. Reactions frequently parallel those of the parent halogens but with subtle and revealing differences. CIF is an effective fluorinating agent (p. 820) and will react with many metals and non-metals either at room temperature or above, converting them to fluorides and liberating chlorine, e.g. ... 

The presence of an TV-oxide group activates the 1,2,4-triazine ring toward electrophilic attack, for instance, in halogenation reactions. Thus, 3-methoxy- and 3-amino(alkylamino)-1,2,4-triazine 1-oxides 16 react easily with chlorine or bromine to form the corresponding 6-halo-1,2,4-triazine 1-oxides 17 (77JOC3498, 78JOC2514). 

The reaction capability of PS is weak, but the reaction capability can be improved by anchoring the functional group to the aliphatic chain or aromatic ring of PS using chemical or conversion reactions. Aliphatic chain reactions are halogenation reactions, oxidation reactions, or unsaturated acids to bonded aliphatic chain of PS (in the presence of a radical catalysis). 

When the halogenation reaction is carried out on a cycloalkene, such as cyclopentene, only the trews stereoisomer of the dihalide addition product is formed rather than the mixture of cis and trans isomers that might have been expected if a planar carbocation intermediate were involved. We say that the reaction occurs with anti stereochemistry, meaning that the two bromine atoms come from opposite faces of the double bond—one from the top face and one from the bottom face. 

This allylic bromination with NBS is analogous to the alkane halogenation reaction discussed in the previous section and occurs by a radical chain reaction pathway. As in alkane halogenation, Br- radical abstracts an allylic hydrogen atom of the alkene, thereby forming an allylic radical plus HBr. This allylic radical then reacts with Br2 to yield the product and a Br- radical, which cycles back... 

Interactive to use a web-based palette to predict products from a variety of halogenation reactions of carbonyls and carboxylic acids. 

Table 19-111. the energy required for some halogen reactions... 

The addition of hydrogen chloride to give chloroethane is an example of a hydro-halogenation reaction ... 

These reactions proceed at lower temperature (250-750°C) than those based on the methyl-radical mechanism reviewed above. The halogen reaction mechanism is still controversial and the optimum precursor species are yet to be determined.P9] To proceed, the reactions must be highly favored thermodynamically. This is achieved when the reaction products are solid carbon and stable gaseous fluorides or chlorides (HF, HCl, SFg). 

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