Transition states in bromination

Fig. 6.2. Enthalpy differences of starting alkenes and transition states in bromination.

An interpretation of activation parameters has led to the conclusion that the bromination transition state resembles a three-membered ring, even in the case of alkenes that eventually react via open carbocation intermediates. It was foimd that for cis trans pairs of alkenes tiie difference in enthalpy at the transition state for bromination was greater than the enthalpy difference for the isomeric alkenes, as shown in Fig. 6.2. This... 

The enhanced selectivity of alkane bromination over chlorination can be explained by turning once again to the Hammond postulate. In comparing the abstractions of an alkane hydrogen by Cl- and Br- radicals, reaction with Br- is less exergonic. As a result, the transition state for bromination resembles the alkyl radical more closely than does the transition state for chlorination, and the stability of that radical is therefore more important for bromination than for chlorination. 

The nature of the transition state in bromodesilylation is problematical, since the reaction appears to take place in the non-polar solvents benzene and carbon tetrachloride with inversion of configuration at silicon, and, therefore, cannot proceed through a 4-centre intermediate (LVII) as this would lead to retention of configuration746,747. The results are, however, consistent with a six-centre transition state (LVIII), which could follow from the high kinetic order in bromine... 

Fig. 4 Symmetrical charge distribution in bromonium-ion like transition states in alkene bromination (data from Bienvenue-Goetz and Dubois, 1978). The effect of the R-substituent depends neither on the number nor on the position of methyl groups on the double bond.

Bromination is less stereoselective, and the reactions of cis- and trans-olefins tend to be stereoconvergent. The stereospecific formation of the mixed bromoadducts in protic media, such as methanol or acetic acid, could be interpreted in the light of the recent finding (Ruasse et al., 1991) that these solvents assist the formation of the ionic intermediate nucleophilically. If a solvent molecule is close to the cationic part of the transition state in the rate-limiting step, the intermediate can be trapped by this solvent molecule in a necessarily trans mode with respect to the first bromine, before the two components of the ion-pair diffuse away from each other (15). This would... 

Evidence for the polar character of the transition state is that electron-withdrawing groups in the para position of toluene (which would destabilize a positive charge) decrease the rate of hydrogen abstraction by bromine while electron-donating groups increase it,10 However, as we might expect, substituents have a smaller effect here (p -1,4) than they do in reactions where a completely ionic intermediate is involved, e.g., the SnI mechanism (see p. 344). Other evidence for polar transition states in radical abstraction reactions is mentioned on p. 685. For abstraction by radicals such as methyl or phenyl, polar effects are... 

Table 9.12 compares partial rate factors for substitution by phenyl radical with those for electrophilic bromination. Selectivity is clearly much lower for the radical substitution furthermore, for attacking phenyl radical, nearly all positions in the substituted benzenes are more reactive than in benzene itself, a finding that reflects the tendency for most substituents to stabilize a radical, and thus to lower transition state energy for formation of the cyclohexadienyl intermediate, when compared with hydrogen. The strong polar effects, which cause the familiar pattern of activation and deactivation in the electrophilic substitutions, are absent. One factor that presumably contributes to the low selectivity in radical attack is an early transition state in the addition step, which is exothermic by roughly 20 kcal mole-1.178... 

Comparison of transition states for bromination and chlorination. In the endothermic bromination, the transition state resembles the products (the free radical and HBr). In the exothermic chlorination, the free radical has just begun to form in the transition state, so the transition state resembles the reactants. 

Figure 4-12 compares the transition states for bromination and chlorination. In the product-like transition state for bromination, the C—H bond is nearly broken and the carbon atom has a great deal of radical character. The energy of this transition state reflects most of the energy difference of the radical products. In the reactant-like transition state for chlorination, the C—H bond is just beginning to break, and the carbon atom has little radical character. This transition state reflects only a small part (about a third) of the energy difference of the radical products. Therefore, chlorination is less selective. 

The stereoselectivity of bromination of 4-f-butylcyclohexanone enamines has been studied and the ratio of axial. equatorial bromine incorporation shown to vary with the amine moiety (pyrrolidine 51 49 piperidine 66 34 morpholine 74 26 Af-methylaniline 52 48, di-isobutylamine 52 48)237. This variation in the axial equatorial selectivity has been rationalized in terms of the nature of the transition state. C-Bromination of the pyrrolidine, Af-methylaniline and di-isobutylamine enamines must occur via an early reactant-like transition state thus resulting in low stereoselectivity (Scheme 99). The... 

Jensen and Rickbom (2) discussed the possibility of transition states in which the electrophilic species was partially bound to the o- orbital of carbon-mercury bonds. Thus the fourth "center of the four-center bromination mechanism was pictured as the carbon-mercury bond itself, or more specifically, the carbon sps orbital (2). 

A value of kH/kD = 1.4 was obtained [114] for the rate of proton transfer compared with deuteron transfer from chloroform to hydroxide ion and this result is similar to the values determined earlier for several haloforms [164, 166]. In the most recent work [171(b)] a value kH /kD = 1.11 0.05 was determined for chloroform. These values are close to those observed for reaction of cyanocarbon acids (though a different base catalyst is involved) and in Sect. 4.3 it was argued that isotope effects as low as these are expected for a transition state in which proton transfer is almost complete. The isotope effect for proton transfer from chloroform was measured using a new and useful method [114]. It can be shown that the ratio of initial rates of uptake of tritium for the first ten per cent of reaction from tritiated water into CHC13 and CDC13 is identical to the primary isotope effect for proton loss (feH /fcD). The procedure can be used for measuring isotope effects on proton transfer from carbon acids to hydroxide ion or buffer catalysts and is more convenient than other methods. Other methods which have been used, for example, involve the comparison of rates of detritiation and dedeuteration or the comparison of rates of bromination for isotopically different acids (RCH and RCD) [113]. 

This large solvent effect points to a polarized transition state. In accordance with the fact that the stereochemistry is retained in the cyclopropane formation, it is assumed that the N(2)-C(3) bond is not completely broken before bonding between C(3) and C(5) has started, and a transition state like 90 is proposedThe effect of having a bromine... 

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