Halogenation rates

Halogen derivatives are hardly inflammable. Their behaviour depends on the halogen rate that is present in the molecule and its nature (brominated derivatives are less inflammable than chlorinated) as well as the temperature to which they are exposed. So some of them that used to be used as extinguishing... 

One of the first indications of the applicability of a linear free-energy treatment to aromatic substitution was the observation of a linear relationship between the equilibrium basicity and halogenation rates of the polymethylbenzenes (Fig. 5). Condon (1948) was able to calculate the rates of halogenation of the polymethylbenzenes with reasonable precision on the basis of a linear free-energy treatment. 

Comparison of the overall rate constants (when ionisation occurs along two competitive paths) or of the rate constants (when there is only one enolisation site) with that of a parent unsubstituted methyl ketone, e.g. acetone or acetophenone, shows that an alkyl group usually decreases ketone reactivity under conditions of base catalysis. This is in agreement with a small electron-repelling inductive effect which makes the carbanion ion less stable (e.g. the halogenation rate constant decreases by a factor of 6.5 on going from acetophenone to propiophenone when the reaction is catalysed by acetate ion [acetic acid-water 75 25 at 25°CI (Evans and Gordon, 1938). However, the factor is very small and could be explained by steric effects as well. 

Metal clusters are also susceptible to fragmentation through electrophilic attack (e.g. by halogens). Rates can be very rapid indeed but are very dependent on the nature of any substituents. 

Halogen Rate Relative to Benzene % ortho %meta %para... 

Much of tills chapter concerns ET reactions in solution. However, gas phase ET processes are well known too. See figure C3.2.1. The Tiarjioon mechanism by which halogens oxidize alkali metals is fundamentally an electron transfer reaction [2]. One might guess, from tliis simple reaction, some of tlie stmctural parameters tliat control ET rates relative electron affinities of reactants, reactant separation distance, bond lengtli changes upon oxidation/reduction, vibrational frequencies, etc. 

A point in case is provided by the bromination of various monosubstituted benzene derivatives it was realized that substituents with atoms carrying free electron pairs bonded directly to the benzene ring (OH, NH2, etc) gave 0- and p-substituted benzene derivatives. Furthermore, in all cases except of the halogen atoms the reaction rates were higher than with unsubstituted benzene. On the other hand, substituents with double bonds in conjugation with the benzene ring (NO2, CHO, etc.) decreased reaction rates and provided m-substituted benzene derivatives. 

In the absence of catalysts, toluene when treated with chlorine (or bromine) at the boiling point, preferably with exposure to sunlight or other bright light source, undergoes halogenation in the side chain. The entrance of the first chlorine atom, for example, proceeds at a much faster rate than the entrance of the second chlorine atom so that in practice the major portion of the toluene is converted into benzyl chloride before appreciable chlorination of benzyl chloride occurs ... 

Both halogens of the dihalogenothiazoles can be replaced by nucleophiles. At any rate, the halogen in position 2 is always more reactive than those in positions 4 or 5, as previously discussed. Analogously,the halogen can be selectively removed only from position 2 by reduction with metals (Table V-5). 

The rate of elimination depends on the halogen the reactivity of alkyl halides increasing with decreasing strength of the carbon-halogen bond... 

What are the implications of the effects of the various halide leaving groups Because the halogen with the weakest bond to carbon reacts fastest Ingold concluded that the carbon-halogen bond breaks m the rate determining step The weaker the carbon-halogen bond the easier it breaks... 

The alkyl halide m this case 2 bromo 2 methylbutane ionizes to a carbocation and a halide anion by a heterolytic cleavage of the carbon-halogen bond Like the dissoci ation of an aUcyloxonmm ion to a carbocation this step is rate determining Because the rate determining step is ummolecular—it involves only the alkyl halide and not the base—It is a type of El mechanism... 

Because the carbon-halogen bond breaks m the slow step the rate of the reaction depends on the leaving group Alkyl iodides have the weakest carbon-halogen bond and are the most reactive alkyl fluorides have the strongest carbon-halogen bond and are the least reactive... 

Table 6 3 shows that the effect of substituents on the rate of addition of bromine to alkenes is substantial and consistent with a rate determining step m which electrons flow from the alkene to the halogen Alkyl groups on the carbon-carbon double bond release electrons stabilize the transition state for bromonium ion formation and increase the reaction rate... 

Unlike elimination and nucleophilic substitution reactions foimation of oigano lithium compounds does not require that the halogen be bonded to sp hybndized carbon Compounds such as vinyl halides and aiyl halides m which the halogen is bonded to sp hybndized carbon react m the same way as alkyl halides but at somewhat slowei rates... 

As m the acid catalyzed halogenation of aldehydes and ketones the reaction rate is mde pendent of the concentration of the halogen chlorination brommation and lodmation all occur at the same rate Formation of the enolate is rate determining and once formed the enolate ion reacts rapidly with the halogen... 

The strength of their carbon-halogen bonds causes aryl halides to react very slowly in reactions in which carbon-halogen bond cleavage is rate determining as m nude ophilic substitution for example Later m this chapter we will see examples of such reactions that do take place at reasonable rates but proceed by mechanisms distinctly dif ferent from the classical S l and 8 2 pathways... 

Either mechanism can be used to describe how antimony—halogen systems operate in both the condensed and vapor phases. In the condensed phase a chat that is formed during the reaction of the polymer, antimony trioxide, and the halogen reduces the rate of decomposition of the polymer therefore, less fuel is available for the flame (16). 

In aqueous solution, OF2 oxidizes HCl, HBr, and HI (and thek salts), Hberating the free halogens. Oxygen difluoride reacts slowly with water and a dilute aqueous base to form oxygen and fluorine. The rate of this hydrolysis reaction has been determined (23). 

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