Halogenation thermodynamics

The physical properties of the halogen fluorides are given in Table 1. Calculated thermodynamic properties can be found in Reference 24. 

A commonly used alternative to the direct bromination of ketones is the halogenation of enol acetates. This can be carried out under basic conditions if necessary. Sodium acetate, pyridine or an epoxide is usually added to buffer the reaction mixture. The direction of enolization is again dependent upon considerations of thermodynamic and kinetic control therefore, the proportion of enol acetates formed can vary markedly with the reaction conditions. Furthermore, halogenation via enol acetates does not necessarily give the same products as direct halogenation of ketones 3. 23... 

Anhydrous HX are versatile and vigorous reagents for the halogenation of metals, non-metals, hydrides, oxides and many other classes of compound, though reactions that are thermodynamically permissible do not always occur in the absence of catalysts, thermal initiation or photolytic encouragement, because... 

These values should be compared with those for the other halogens (in 1 M acid) (p. 854). Noteworthy features are that At is the only halogen with an oxidation state between 0 and V that is thermodynamically stable towards disproportionation, and that the smooth trends in the values of E°( X2/X ) and (HOX/jXi) continue to At. 

Halogenation of steroid 3-ketones can lead to complicated mixtures by virtue of the fact that the kinetic enol leads to 3 halo products, whereas the thermodynamic product is that halogenated at the 4 position. Carefully controlled reaction of the 5a-androstanolone with chlorine thus leads to the 2a-chloro derivative (29). Reaction of that intermediate with O(p-nitrophenyl)-hydroxylamine affords the androgenic agent ni stremine acetate (30). ... 

For complexes like PtL2X2 (X = halogen L = NH3, PR3, etc.) where cis-and trans-isomers exist, the trans-isomer is usually thermodynamically more stable. The c/s-isomer may be formed first in a reaction and, in the case of platinum, may be relatively inert to substitution. (Thermodynamic data are relatively scarce trans-Pt(NH3)2Cl2 is some 13kJmol-1 more stable than the cis-isomer.)... 

The equilibrium concentrations of many disubstituted benzenes (containing alkyl and halogen substituents) show that the meta isomer is in nearly all cases the most thermodynamically stable. It is not obvious why this should be so. Shine182 had discussed this problem in terms of the relative sizes of the standard enthalpy and entropy changes between any pair of isomers. 

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). 

An irreversible reaction of the intermediate of a redox reaction will greatly facilitate redox catalysis by thermodynamic control. A good example is the reduction of the carbon halogen bond where the irreversible reaction is the cleavage of the carbon halogen bond associated, or concerted, with the first electron transfer -pEe... 

It is also important to note that trans-alkenes are often more stable than cis-alkenes due to diminished steric hindrance (p. 190), but this is not always the case. It is known, for example, that c -l,2-difluoroethene is thermodynamically more stable than tra j-l,2-difluoroethene. This appears to be due to delocalization of halogen lone-pair electrons and an antiperiplanar effect between vicinal antiperiplanar bonds. 

This reaction is a principal method of forming IIIB-transition-metal cr bonds. The formation of thermodynamically favored alkali-metal halides or related salts and acids HX enhances the easy formation of those bonds. A second possible interaction between anionic metal bases and group-IIIB halides is a simple acid-base relationship without elimination of halide anions. However examples of this are rare, and they have been described often for group-IIIB compounds without halogen ligands ( 6.5.3.2). 

Electronic Spectroscopy and Thermodynamics of Donor/Acceptor Interactions with Halogen Derivatives... 

Halogen oxide radicals such as CIO and BrO are important reactive intermediates in the catalytic cycles of ozone destruction in the middle and upper stratosphere. The first absorption band CIO(/l211 <— X2 I) starts from 318 nm and has a series of vibronic bands that converge to a broad continuum at wavelengths shorter than 264nm (Fig. 8).98-101 In this continuum region four dissociation pathways are thermodynamically possible,33... 

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