Other sources of positive halogen

Chlorinating Agents Sodium hypochlorite solution N-Chlorosuccinimide 

Formation of chlorohydrins from olefins. Chlorination with solvent participation and cycliz-ation. 

Substitute for bromine when increased selectivity or mild reaction conditions are required 

Both 1,2 and 1,4 addition occur in the chlorination and bromination of butadiene. Both reactions give mixtures containing slightly more of the 1,2-addition product than the 1,4-addition product.  

CHAPTER 4 ELECTROPHILIC ADDITIONS TO CARBON-CARBON MULTIPLE BONDS 

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Steroidal alcohols have been oxidised with a variety of other reagents. Sources of positive" halogen, such as N-bromoamides [30-32], isocyanuric chloride [33], and tert-butyl hypochlorite [34], readily convert many alcohols into ketones. Axial alc idls are again the most probably for reasons similar to those discussed above [31]. This reactivity difference has been exploited, for example, in the selective oxidation of 5a-cholestane-3j5,5a,6jS"triol (5) to give the 3 S,3a"dihydroxy-6-ketone (6) [35]. Equatorial alcohols display differences in rates of oxidation the more... 

The elemental halogens are not the only source of electrophilic halogen atoms, and, for some synthetic purposes, other positive halogen compound may be preferable... 

The amide N-H may also be halogenated, oxidized and nitrosated. A -Bromosuccinimide (NBS), like a number of other iV-halo compounds, readily undergoes a radical fission to give a bromine radical. This provides a useful reagent for radical bromination at, for example, allylic or benzylic positions. In the presence of acid, NBS is also a mild source of the halonium ion, which is used for the addition of hypobromous acid (Scheme 3.74) to alkenes or for the bromination of reactive aromatic rings. 

Jorge Ayala determined the rate constants for thermal electron attachment to aliphatic halides and the halogen molecules to confirm values measured by other techniques. The electron affinities of the halogen molecules had been determined by endothermic charge transfer experiments [57-59]. In the case of the halogen molecules, the ECD results lead to the rate constant for thermal electron attachment rather than the electron affinity of the molecule. Two-dimensional Morse potentials for the anions were constructed based on these data. Freeman and Ayala searched for a nonradioactive source for the ECD. In 1975 the data on the electron affinities of atoms were summarized and correlations examined between these values and the position of the atoms in the Periodic Table [60]. A large number of the atomic electron affinities were measured by photoelectron spectroscopy [61]. A similar compilation of the electronegativities of elements was carried out. In this case some of the values were obtained from the work functions of salts [62], These results will be updated in Chapter 8. 

The reactivity of bromine with cyclohexene is explained in terms of the polarizability of the bromine. Indeed, diatomic halogens such as chlorine (Cl-Cl), bromine (Br-Br), and iodine (I-I) are highly polarizable. In the absence of another molecule, there is no difference in electronegativity between the atoms in Br-Br and Cl-Cl, or I-I. When the halogen is in close proximity to an electron-rich atom or group, however, the halogen atom closest to the electron source becomes 8+ and the other becomes 6-. This means that the atoms are polarized via an induced dipole. In the case of cyclohexene, the electron source is the 7t-bond and if it donates electrons to the positive halogen, the C=C unit reacts as a Lewis base. 

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