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Bromination isotope effects

The data presented in Fig. 5.7 demonstrate the enormous increase in counterion quadrupole relaxation rate which accompanies micelle formation. Temperature dependence studies [319-321 324 326], as well as studies of the bromine isotope effect [303 319-321 324 326], show that the counterions exchange between different binding environments at a rapid rate compared to that of relaxation. To rationalize the concentration dependence of counterion quadrupole relaxation in micellar solutions it has been assumed that only two binding sites for the counterions have to be considered, i,e, the counterions are either free or attached to the micelles. It is further assumed that the ratio of counterions to surfactant ions in the micelles is independent of concentration and that the pseudo-phase separation model of micelle formation applies. This model [313] treats micelle formation analogously to a phase separation, with the c.m.c. corresponding to the saturation concentration of the molecule-disperse amphiphile. With these assumptions it may be shown [322] that for concentrations below the... [Pg.148]

The distribution of a-bromoketones formed in the reaction of acetylcyclopentane with bromine was studied as a function of deuterium substitution. On the basis of the data given below, calculate the primaiy kinetic isotope effect for enolization of... [Pg.448]

Bromination has been shown not to exhibit a primary kinetic isotope effect in the case of benzene, bromobenzene, toluene, or methoxybenzene. There are several examples of substrates which do show significant isotope effects, including substituted anisoles, JV,iV-dimethylanilines, and 1,3,5-trialkylbenzenes. The observation of isotope effects in highly substituted systems seems to be the result of steric factors that can operate in two ways. There may be resistance to the bromine taking up a position coplanar with adjacent substituents in the aromatization step. This would favor return of the ff-complex to reactants. In addition, the steric bulk of several substituents may hinder solvent or other base from assisting in the proton removal. Either factor would allow deprotonation to become rate-controlling. [Pg.578]

Christen and Zollinger303 have made an extensive study of kinetic isotope effects in bromination of the disodium salt of 2-naphthol-6,8-disulphonic acid with hypobromous acid and with bromine in aqueous buffers at 20 °C. Both bro-minating agents give the same rate (within 20 %) and the reactions are first-order... [Pg.124]

A kinetic isotope effect, kH/kD = 1.4, has been observed in the bromination of 3-bromo-l,2,4,5-tetramethylbenzene and its 6-deuterated isomer by bromine in nitromethane at 30 °C, and this has been attributed to steric hindrance to the electrophile causing kLx to become significant relative to k 2 (see p. 8)268. A more extensive subsequent investigation304 of the isotope effects obtained for reaction in acetic acid and in nitromethane (in parentheses) revealed the following values mesitylene, 1.1 pentamethylbenzene 1.2 3-methoxy-1,2,4,5-tetramethyl-benzene 1.5 5-t-butyl-1,2,3-trimethylbenzene 1.6 (2.7) 3-bromo-1,2,4,5-tetra-methylbenzene 1.4 and for 1,3,5-tri-f-butylbenzene in acetic acid-dioxan, with silver ion catalyst, kH/kD = 3.6. All of these isotope effects are obtained with hindered compounds, and the larger the steric hindrance, the greater the isotope... [Pg.125]

Variation of the isotope effect with bromide ion concentration has also been observed for the bromination of 4-methoxybenzenesulphonic acid and its ortho dideuterated derivative at 0 °C, the value of kH/kD changing from 1.0 with no Br to 1.31 at 2.0 M Br" 308. [Pg.126]

The bromination of phenol in acetic acid, containing lithium bromide and perchlorate at a constant total concentration of 0.2 M, gave kinetic isotope effects... [Pg.126]

The use of secondary deuterium kinetic isotope effects in mechanistic studies of olefin bromination... [Pg.117]

Apart from a few studies (ref. 7), the use of deuterium kinetic isotope effects (kie s) appears to have had limited use in mechanistic studies of electrophilic bromination of olefins. Secondary alpha D-kie s have been reported for two cases, trans-stilbene fi and p-substituted a-d-styrenes 2, these giving relatively small inverse kie s of... [Pg.117]

The first step, as we have already seen (12-3), actually consists of two steps. The second step is very similar to the first step in electrophilic addition to double bonds (p. 970). There is a great deal of evidence for this mechanism (1) the rate is first order in substrate (2) bromine does not appear in the rate expression at all, ° a fact consistent with a rate-determining first step (3) the reaction rate is the same for bromination, chlorination, and iodination under the same conditions (4) the reaction shows an isotope effect and (5) the rate of the step 2-step 3 sequence has been independently measured (by starting with the enol) and found to be very fast. With basic catalysts the mechanism may be the same as that given above (since bases also catalyze formation of the enol), or the reaction may go directly through the enolate ion without formation of the enol ... [Pg.776]

However, a number of examples have been found where addition of bromine is not stereospecifically anti. For example, the addition of Bf2 to cis- and trans-l-phenylpropenes in CCI4 was nonstereospecific." Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific.Likewise in the case of triple bonds, stereoselective anti addition was found in bromination of 3-hexyne, but both cis and trans products were obtained in bromination of phenylacetylene. These results indicate that a bromonium ion is not formed where the open cation can be stabilized in other ways (e.g., addition of Br+ to 1 -phenylpropene gives the ion PhC HCHBrCH3, which is a relatively stable benzylic cation) and that there is probably a spectrum of mechanisms between complete bromonium ion (2, no rotation) formation and completely open-cation (1, free rotation) formation, with partially bridged bromonium ions (3, restricted rotation) in between. We have previously seen cases (e.g., p. 415) where cations require more stabilization from outside sources as they become intrinsically less stable themselves. Further evidence for the open cation mechanism where aryl stabilization is present was reported in an isotope effect study of addition of Br2 to ArCH=CHCHAr (Ar = p-nitrophenyl, Ar = p-tolyl). The C isotope effect for one of the double bond carbons (the one closer to the NO2 group) was considerably larger than for the other one. ... [Pg.973]

The lack of steric effects in oxidations of hydrocarbons by Cr(VI) renders D and E unacceptable. The activated complex of scheme C is non-linear and hence does not comply with the magnitude of the observed isotope effect. Two pieces of evidence are quoted which indicate A to be the more probable of the remaining two. Firstly, the p constant of —1.17 is more in agreement with that obtained for bromine atom abstraction from toluenes (—1.369 to —1.806) than those found for solvolyses involving electron-deficient carbon ( — 2.57 to —4.67) . Secondly, the correlation between the relative rates of oxidation of the series... [Pg.295]

Kinetic isotope effects have not been observed for chlorination, and only rarely for bromination, i.e. the reactions normally follow pathway [2a] like nitration. In iodination, which only takes place with iodine itself on activated species, kinetic isotope effects are the rule. This presumably arises because the reaction is readily reversible (unlike other halogenations), loss of I occurring more often from the a complex (14) than loss of H, i.e. k, > k2 ... [Pg.139]

Kinetic solvent isotope effects 268 The TBr scale for bromination 270... [Pg.207]

It was shown quite early (Gamier et al., 1971) and confirmed later (Ruasse and Motallebi, 1988), from solvent isotope effects (kMeoH/ mcod) on the rates of 1-pentene bromination and from a linear relationship (53) between these... [Pg.267]

Kinetic solvent isotope effect as a measure of electrophilic assistance to bromide ion departure limiting values rate data in ethanol, methanol and their aqueous mixtures using Bentley s TBr scale its decrease corresponds to the involvement of nucleophilic assistance. R = (/caqhtOII//cAcoH)r as a measure of nucleophilic solvent assistance. Model for a limiting bromination mechanism. Ruasse et al. (1991). /Ruasse and Zhang (1984). 9Argile and Ruasse (to be published). Modro et al. (1979). [Pg.268]

The electrophile E+ attacks the unhindered side of the still unsubstituted second aromatic ring. A proton (deuteron) is transferred from this ring to the second, originally substituted ring, from which it leaves the molecule. Thus, the electrophile enters, and the proton (deuteron) leaves the [2.2]paracyclophane system by the least hindered paths. Some migration of deuterium could be detected in the bromination of 4-methyl[2.2]paracyclophane (79). The proposed mechanism is supported by the kinetic isotope effects ( h/ d) found for bromination of p-protio and p-deuterio-4-methyl[2.2]paracyclophanes in various solvents these isotope effects demonstrate that proton loss from the a complex is the slowest step. [Pg.104]

The deuterium kinetic isotope effect (DKIE) for the electrophilic bromination of ethylene-/z4 and ethylene- 4 in methanol and dichloroethane at 25 °C has been... [Pg.421]

Watkins and Pound (104) performed an experiment which illustrates the effect of strain on the magnetic resonance of bromine and iodine nuclei in cubic KBr and KI. These crystals were subjected to plastic deformation (a 22% change in dimension occurred along the 100 axis for the KI crystal) and it was found that the intensity of the resonance line in the original undeformed crystals decreased to 0.4 of its original value for the bromine isotopes (Br , Br , I = and to 0.3 of its original value for the I (I = resonance. [Pg.61]

B3LYP/6-31G computations. Thus, 3 mol of 1-pentene was cyclopropanated to 89-93% conversion with CCI2 (generated by the phase-transfer-catalyzed reaction of CHCI3 and NaOH). The unreacted 1-pentene was isolated and brominated, and the resulting dibromide was analyzed by and nuclear magentic resonance (NMR) spectroscopy. The C and H isotopic abundances were compared to those in dibromide derived from a sample of the initial 1-pentene, leading to the isotope effects shown for a typical experiment in structure 18 (Fig. 7.10). [Pg.290]

A review of methods of synthesis of aromatic iodo compounds has appeared offering considerable information of potential value to research chemists wishing to prepare iodoheterocycles (84RCR343). Iodination differs from chlorination and bromination in that a much less reactive electrophile (and a much larger one) is involved. The second step of the reaction is usually at least partially rate-determining. Isotope effects are noted in the iodination of indole [68AC(R) 1435], and the transition state resembles the Wheland intermediate more than in chlorination and bromination. [Pg.298]

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... [Pg.385]


See other pages where Bromination isotope effects is mentioned: [Pg.516]    [Pg.552]    [Pg.516]    [Pg.552]    [Pg.115]    [Pg.87]    [Pg.123]    [Pg.124]    [Pg.125]    [Pg.126]    [Pg.134]    [Pg.4]    [Pg.298]    [Pg.899]    [Pg.295]    [Pg.283]    [Pg.232]    [Pg.295]    [Pg.422]    [Pg.422]    [Pg.423]    [Pg.199]    [Pg.362]    [Pg.682]    [Pg.365]    [Pg.390]   
See also in sourсe #XX -- [ Pg.393 ]

See also in sourсe #XX -- [ Pg.393 ]

See also in sourсe #XX -- [ Pg.393 ]

See also in sourсe #XX -- [ Pg.97 , Pg.393 ]




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