Bromine rates

Amines. Although the chlorination of aqueous aLkylamines is analogous to that of ammonia, mono- and diaLkylarnines generally chlorinate faster because of their higher basicities (13). Bromination rates are significantly faster than chlorination rates, and in some cases, eg, (CH2)2NH... 

Some bromination rate eonstants are summarized below. Compare the correlation of the rate data with o and substituent constants. What is the value of p What is the mechanistic significance of these results ... 

The reaction rates for phenoxide ions are thus similar to those observed for dialkylanilines (and also enolate ions) and seem to represent an upper limit for brominating rate in aqueous solution. Consequently, the reactions have an almost zero activation energy and there is an apparent lack of deactivation by the nitro group. That bromination by BrJ occurs in this reaction is not surprising, since the high reactivity of the phenoxide ion means that it will not discriminate very much between electrophiles of differing reactivity. 

This olefin was chosen because its slow bromination rate allows to make accurate measurements of transient CTC s with a conventional spectrophotometer. Curve d is a difference spectrum between a solution of Bi2 plus a hundred fold excess of olefin (curve c) and those of the single reagents (curves a and b), and represents the tail of the expected CT band. From the linearity of the plots of the difference absorbance agains the olefin concentration (Fig. 2) it was possible to evaluate a limiting value of Kf < 0.1 M l. On the other hand, a AH = - 0.9 kcal moTl was obtained from a plot (Fig. 3) of the in of the products KfScx. obtained from the first plot, against 1/T, assuming ecj to be temperature independent (ref. 3). 

Table 4 Structural Effects on 1 1 CTC Formation Constants (Kf) Between Br2 and Olefins and the Respective Bromination Rates.

Table 5 shows the rate ratios between ethylenes differing by an increase by two in number of alkyl substituents. It can be observed that in solvents as different as methanol, ethanol, and acetic acid, the rate ratio is always around 10, that is of the same order of magnitude of the increase in Kf. This indicates that substituent effects are not much more influential on the kinetic constants that on Kf. A possible rationalization of the lower accelerating effects by alkyl substituents on the bromination rate, relative to what could be expected for an AdgCl mechanism on... 

Methods for obtaining reliable bromination rate constants 211... 

It is noteworthy that in protic solvents most of the bromination rate constants used in mechanistic studies are k, the rate constant for free bromine addition only, that is, for a pathway from which any contribution of bromide is excluded since the involvement of this ion is taken into account in the kBrj- term. 

With these techniques (Fig. 2), bromination rate constants in acetic acid, water, methanol, ethanol and more generally in any solvent in which a bromide is soluble to at least 0.2 m are obtained with a precision of about 2% when the method is easily applied and 5% when it is used close to its limit. 

Since it is now established that CTCs are involved in the bromination pathway, a question about the meaning of the experimental bromination rate constant arises. These constants are not those of an elementary step but are the products of the CTC formation constants KCTC and of the rate constants k-, for CTC ionization into the -intermediates (8). 

In the nineteen sixties, there was some confusion about the value of p, the reaction constant for polar effects of alkyl groups on bromination rates, as calculated according to Taft s equation, log (k/k0) = p a. For 22 alkenes substituted by one, two or three linear alkyl groups, there is a satisfactory relation (16) between their reactivities, log k, in methanol and the sum of their... 

The p -value for bromination established in methanol is also valid in a variety of other protic solvents. Linear correlations between the bromination rates of unbranched alkenes in methanol and those in a 70-30 methanol-water mixture (M70) [(22) Barbier and Dubois, 1968], in pure water [(23) Bienvenue-Goetz and Dubois, 1968] and in acetic acid [(24) Ruasse and Zhang, 1984) are observed. An approximately linear relationship (25) between... 

Fig. 9 Comparison of polar and steric effects of alkyl groups on bromination rates of linear ( ), branched (O) and adamantyl (A) alkenes in acetic acid and in methanol (Ruasse and Zhang, 1984 Ruasse et al., 1990). Polar effects are identical in both solvents [full line, eq. (24)], but steric effects differ. Deviations of branched alkenes are attributed to steric inhibition of nucleophilic solvation by methanol.

While p for the polar effects of alkyl groups does not vary with the solvent, pn for the polar effects of aromatic substituents is solvent-dependent. For example, there is a fairly linear log/log relationship between the bromination rates of styrenes in acetic acid and methanol (36) with a slope higher than unity, to be compared with 0.99 in (24). The p+-value for styrenes... 

Fig. 11 Non-additivity of the X- and Y-substituent effects on the bromination rates of 1,1-diarylethylenes (Hegarty et al., 1972). px for the variation of Y when X is fixed is linearly related to <7, according to eq. (37).

Bromination rates of aliphatic enol ethers have been included in the interactive treatment of alkenes GRIC=CR R, with G being a conjugated group most of them fit the multiparameter equation (41) satisfactorily. A more detailed analysis of reactivity-selectivity effects in the reaction of 1-ethoxyethylene [22] and its a- and / -methyl analogues [23] and [24] has been carried out,... 

According to (57), the main driving force for the reaction in non-protic media is the formation of a tribromide ion from bromine and the developing bromide. Kinetic (Ruasse et al., 1986) and thermodynamic (Bienvenue-Goetz et al., 1980) data on equilibrium (58) are therefore relevant to the effect of non-protic solvents on bromination rates. 

The recently determined kinetic data for the bromination of bicyclopropylidene (1) and spirocyclopropanated bicyclopropylidenes 55, 56 in methanol at 25 °C disclose that the addition of Br2 onto the double bonds in 1,55,56 proceeds essentially with the same rate as the bromination of corresponding oligomethyl-ated ethylenes. The bromination rate increases with an increasing number of spiroannelated three-membered rings, and the rate of bromination correlates with the TT-ionization energies of the molecules (Table 5) [134]. 

Kinetic studies on alkyl- and phenyl-isoxazoles in 85% acetic acid at 150°C have shown that isoxazoles are 100-1000 times more reactive than benzene. The relative 4-bromination rates for 5-phenyl- 3,4-diphenyl- ... 

Erlenmeyer was first to consider ends as hypothetical primary intermediates in a paper published in 1880 on the dehydration of glycols.1 Ketones are inert towards electrophilic reagents, in contrast to their highly reactive end tautomers. However, the equilibrium concentrations of simple ends are generally quite low. That of 2-propenol, for example, amounts to only a few parts per billion in aqueous solutions of acetone. Nevertheless, many important reactions of ketones proceed via the more reactive ends, and enolization is then generally rate-determining. Such a mechanism was put forth in 1905 by Lapworth,2 who showed that the bromination rate of acetone in aqueous acid was independent of bromine concentration and concluded that the reaction is initiated by acid-catalyzed enolization, followed by fast trapping of the end by bromine (Scheme 1). This was the first time that a mechanistic hypothesis was put forth on the basis of an observed rate law. More recent work... 

Most of the rate comparisons in the halogenation of aromatic amines refer to bromination rate coefficients for para-substitution are collected in Table 10. Further results for o/7/io-substitution are provided in the cited references. Some of the early calculation based on (39) and (40) may be in error, because it was not then realized that the appropriate acidity function in (40) depends on the structure of the substrate (cf. Bell and Ramsden., 1958 Bell and Ninkov, 1966). The appropriate acidity function was used for the results listed in Table 10 but it is still advisable for rate comparisons to be based on experiments carried out under the same conditions. 

From this we obtain the bromination rate of silver ... 

Since the rate of enolisation generally determines the rate of bromination of a ketone [i2g], it should be possible to correlate bromination rates for steroid ketones with their rates of enolisation. These in turn should be related to the thermodynamic stabilities of the respective olefinic bonds, insofar as transition states for enolisation under acidic conditions resemble the structures of the enols (p. 154). The very limited kinetic data available [133,134] confirm this relationship for three 3a-cholestanones whose reactivity falls in the order C(3) > C(e> > C(7). This would reasonably be predicted from consideration of strain associated with the A -, A -, and A -double bonds, coupled with steric hindrance to the transition... 

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