In the halogenation

Enolization is the rate-determining step in the halogenation of normal ketones. Where alternate directions for enolization exist, the preferred direction (and hence the position of kinetic bromination) depends on the substituents and stereochemistry. Furthermore, the orientation of the bromine introduced depends on stereochemical and stereoelectronic factors. 

Initiation step (Section 4.17) A process which causes a reaction, usually a free-radical reaction, to begin but which by itself is not the principal source of products. The initiation step in the halogenation of an alkane is the dissociation of a halogen molecule to two halogen atoms. 

The rate equation is first-order in acetone, first-order in hydroxide, but it is independent of (i.e., zero order in) the halogen X2. Moreover, the rate is the same whether X2 is chlorine, bromine, or iodine. These results can only mean that the transition state of the rds contains the elements of acetone and hydroxide, but not of the halogen, which must enter the product in a fast reaction following the rds. Scheme VI satisfies these kinetic requirements. 

Bismuth, excess entropy of solution of noble metals in liquid bismuth, 133 Block polymers, 181 Bond energies in the halogens, 61 Boron fluoride as initiator in polymerization, 156... 

Regioselectivity in the halogenation of unsymmetrical ketones can be attained by treatment of the appropriate enol borinate of the ketone with NBS or NCS. The desired halo ketone is formed in high yield. Another method for achieving the... 

Caccamese, S., R. Azzolina, R. M. Toscano and K. L. Rinehart, Jr. 1981. Variations in the halogenated metabolites of Laurencia obtusa from eastern Sicily. Biochem. Syst. Ecol. 9 241-246. 

San-Martin, A. and Rovirosa, J. 1986. Variations in the halogenated monoterpene metabolites of Plocamium cartilagineum of the Chilean coast. Biochem. Syst. Ecol. 14 459-461. 

In addition, there is interest in the halogenation of a wide range of anthropogenic contaminants, some of which may occur in raw water before treatment. 

Another characteristic of many radical reactions is that, once initiated, they often proceed with great rapidity owing to the establishment of fast chain reactions of low energy requirement, e.g. in the halogenation of alkanes (3, cf. p. 323) ... 

Multiple substitutions almost always occur in the halogenation of alkanes. 

In this particular case the chemical shifts in the halogenated compounds are referred to X = CH3 rather than X = H. 

Similar trends were observed in the halogenation of 1,3,5-trimethoxybenzene with diorganoselenium(IV) dihalides as shown in Fig. 18. Di-4-chlorophenyltellur-ium(IV) dibromide (36) gave less than 5% bromination of 1,3,5-trimethoxybenzene after 48 h at reflux in chloroform. Bromination of 1,3,5-trimethoxybenzene with... 

The formation of halogenosulfonium salts is considered to be the first step in the -halogenation of sulfoxides. The mechanism and stereochemistry of a-halogenation of sulfoxides are discussed in detail by Montanari in his recent review paper (9). 

The second step involves nucleophilic attack of the Br" ion on the carbocation. In this mechanism, unlike in the halogenation reaction, the Br" ion can attack from either side of the carbocation. 

Selenium burns with a blue flame that produces selenium dioxide (SeO ). Selenium will react with most metals as well as with nonmetals, including the elements in the halogen group... 

Iodine is the least reactive of the elements in the halogen group 17. Most people associate iodine with the dark-brown color of the tincture of iodine used as an antiseptic for minor skin abrasions and cuts. A tincture is a 50% solution of iodine in alcohol. Although it is still used, iodine is no longer the antibiotic of choice for small skin wounds. Since iodine is a poison that kills bacteria, iodine tablets are often used by campers and others to purify water that is taken from outdoor streams. 

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