Rate-determining step bromination

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... 

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. 

Finally, the brominations of mesitylene, 1,2,4,5-tetramethyl- and pentamethyl-benzene in chloroform (which is more polar than carbon tetrachloride) are first-order in bromine and iodine monobromide318, so that this is entirely consistent with the pattern developed above, i.e. the more polar the solvent and the more reactive the compound, the fewer the number of molecules of iodine monobromide that are involved in the rate-determining step. Measurements of rates between 25 and 42 °C revealed no significant trend owing to the variability of the rate coefficients determined at any temperature, but even so it is clear that there is no appreciable activation energy for these compounds, and there may have been temperature inversion for some of them. 

Studies of the relative rates of the zinc chloride-catalysed bromination of alkyl-and halogeno-benzenes in nitromethane at 25 °C have lead to the suggestion that the rate-determining step of the reaction is formation of Ji-complex, since low substrate selectivity was found to be coupled with high (i.e. normal) positional selectivity323. Under some conditions (column 1 in Table 75) the low selectivity... 

The rate-determining step was, therefore considered to be reaction of bromine with peroxyacetic acid to give a species (suggested as bromine acetate)which subsequently and rapidly, brominates. Formation of bromine acetate was believed to take place according to the reaction scheme represented by equilibrium (158) (which is analogous to the mercuric oxide oxidation of bromine) followed by either equilibrium (159), (160) or (161), viz. 

Systematic studies of the selectivity of electrophilic bromine addition to ethylenic bonds are almost inexistent whereas the selectivity of electrophilic bromination of aromatic compounds has been extensively investigated (ref. 1). This surprising difference arises probably from particular features of their reaction mechanisms. Aromatic substitution exhibits only regioselectivity, which is determined by the bromine attack itself, i.e. the selectivity- and rate-determining steps are identical. 

Several trends emerge in these data (1) The reductive elimination of bromine is 6-13kJmol more facile than reductive elimination of chlorine in similar structures, which is consistent with weaker chalcogen-bromine bonds relative to chalcogen-chlorine bonds.(2) The reductive elimination of chlorine is accelerated by the presence of a chloride counterion as opposed to a less nucleophilic counterion such as hexafluorophosphate. (3) The rate of reductive elimination is accelerated by the presence of a more polar solvent (acetonitrile) relative to tetrachloroethane, which is consistent with development of charge in the rate-determining step. These observations suggest mechanisms for oxidative... 

Other 6-substituted dihydrodiazepinium salts may also be brominated. Kinetic measurements show that 6-methyl derivatives undergo addition of bromine at position 6 in a fast reaction between bromine molecules and dihydrodiazepinium cations [75JCS(P2)325]. The bromination of both 6-bromo and 6-methyl derivatives can be accommodated within a single reaction scheme, but the rate-determining steps are different for the two types of compounds. For 6-methyl derivatives the initial bromination is rate-determining, whereas for the 6-bromo derivatives the subsequent hydrolysis is rate-determining [75JCS(P2)325]. 

The hydrolysis of BrONOz does not generate active bromine directly, but rather HOBr, which must then photolyze to OH + Br. Calculate the lifetime of HOBr with respect to photolysis at a solar zenith angle of 86° and an altitude of 25 km. Assume the earth-sun distance correction factor is 1.0. Is the hydrolysis of BrONOz or the photolysis of HOBr the rate-determining step ... 

The kinetic order in bromine, which is often higher than unity, indicates the involvement of a second molecule of bromine in the rate-determining step assisting ionization. 

Each act of proton abstraction from the a carbon converts a chiral molecule to an achiral enol or enolate ion. The sp3-hybridized carbon that is the chirality center in the starting ketone becomes, s/t2-hybiidized in the enol or enolate. Careful kinetic studies have established that the rate of loss of optical activity of sec-butyl phenyl ketone is equal to its rate of hydrogen-deuterium exchange, its rate of bromination, and its rate of iodina-tion. In each case, the rate-determining step is conversion of the starting ketone to the enol or enolate anion. 

In experiments of major importance, first published in 1950, Melander found that in the nitration and bromination of a number of benzene derivatives the tritium isotope effect (kHlkT) is not 10-20 as is to be expected if carbon-hydrogen bond breaking occurs in the rate-determining step, but rather is less than 1.3. The direct displacement mechanism was thus ruled out, and the two-step mechanism of Equation 7.70 with the first step rate-determining was implicated.157... 

Use of deuterated substrates gives hAd = 6.5. This is a primary kinetic deuterium isotope effect, indicating diat proton removal is an essential component of die rate-determining step. The lack of rate dependence on bromine requires diat bromine is added to die molecule after die rate-determining step. A mechanism consistent widi diese facts has proton removal and enolate formation rate determining. 

For chlorination, the formation of the (7-complex would be expected to be the rate-determining step since the aromatic chlorination of other substrates does not appear to give rise to deuterium isotope effects (Baciocchi et al., 1960 de la Mare and Lomas, 1967). The effect of the experimental conditions on the rate-determining step in the aqueous bromination of aromatic amines has been investigated in detail by Dubois and his co-workers (Dubois et al., 1968a, b, c Dubois and Uzan, 1968). This work suggests that, for tertiary aromatic amines, the proton loss is fast for para-bromination but partly or wholly rate-determining for ortho-bromination. There is however some... 

It is well known that in many brominations and protonations of cyclohexenols (91) axial entry is favored (Eliel et al., 1965). This is attributed to the parallel alignment of the v orbitals on the three centers. The overlap preference is well illustrated in the oxidation of allyl vs. saturated alcohols. Normally, axial alcohols are oxidized more rapidly by chromic acid than equatorial alcohols. In the absence of large strain factors, equatorial allyl alcohols are oxidized faster than axial alcohols by chromic acid hydrogen is abstracted in the rate-determining step. The contribution of a-j8 ketonic resonance lowers the activation energy,... 

If the reaction between the enol and the electrophile proceeds extremely fast, the enol tautomer of a carbonyl or carboxyl compound might be consumed completely. The generation of enol becomes the rate-determining step. This situation occurs with the enol titration of ace-toacetic ester, (Figure 12.4). In this process, bromine is added to an equilibrium mixture of the ketone form (B) and the enol form (iso-B) of an acetoacetic ester. Bromine functionalizes the enol form via the intermediacy of the carboxonium ion E to form the bromoacetic ester D. The trick of conducting the enol titration is to capture the enol portion of a known amount of acetoacetic ester by adding exactly the equivalent amount of bromine. From the values for... 

The pattern you saw for epoxidation with peroxy-acids (more substituted alkenes react faster) is followed by bromination reactions too. The bromonium ion is a reactive intermediate, so the rate-determining step of the brominations is the bromination reaction itself. The chart shows the effect on the rate of reaction with bromine in methanol of increasing the number of alkyl substituents from none (ethylene) to four. Each additional alkene substituent produces an enormous increase in rate. The degree of branching (Me versus n-Bu versus t-Bu) within the substituents has a much smaller, negative effect (probably of steric origin) as does the geometry (E versus Z) and substitution pattern (1.1- S... 

The second step is the rate-determining step, and the presence of a bromine atom at the a position slows this step down still further if a proton can be lost from a different a position—one without a Br atom—it will be lost. The transition state for proton removal illustrates why bromine slows this step down. The part of the structure close to the bromine atom is positively charged. 

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