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Mechanism aromatic bromination

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. [Pg.100]

Click Mechanisms in Motion to view Electrophilic Aromatic Bromination. [Pg.687]

The mechanism via bromine atoms is supported by molecular bromine formation in the interaction of with Br in the absence of a hydrocarbon (Bf2 is apparently formed by bromine atom recombination). This mechanism is also consistent with the fact that bromide ions, while catalyzing the oxidation in the case of alkylaromatic compounds, are not particularly effective in the case of simple alkanes. This corresponds to the difference of bromine atom reactivity with respect to alkylaromatic and aliphatic hydrocarbons. The bond energy in the H-Br molecule (85 kcal mole ) is practically equal to the energy of the C-H bond in the n.-position to the aromatic ring, so that the reaction... [Pg.383]

Indole reacts readily with electrophilic brominating agents such as Al-bromo-succinimide [36] and pyridinium bromide perbromide [37], In hydroxylic solvents oxindoles are formed by hydrolytic capture of the 3-bromoindolenine intermediate [46]. Recently, indole was among a number of reactive aromatics brominated using NBS under UV irradiation. A good yield of 3-bromoindole was reported, but the mechanism under these conditions is not clear [47]. [Pg.57]

The Lewis acids typically used are aluminum chloride (AICI3) and iron chloride (FeCIa) for chlorination, and iron bromide (FeBts) for bromination. The purpose of the Lewis acid is to make the halogen a stronger electrophile. A mechanism for electrophilic aromatic bromination is shown here. [Pg.673]

As previously discussed, the key to HALS-flame retardant incompatibility is acid generation by the flame retardant, which in turn deactivates the HALS. The mechanism for bromine radical generation by flame retardants is quite structure dependent. Aliphatic brominated flame retardants are primarily decomposed thermally, which may occur during the extrusion process. Aromatic brominated flame retardants are relatively stable through the processing step but may generate bromine radicals during UV exposure. [Pg.367]

The halogen carriers or aromatic halogenation catalysts are usually all electrophilic reagents (ferric and aluminium haUdes, etc.) and their function appears to be to increase the electrophilic activity of the halogen. Thus the mechanism for the bromination of benzene in the presence of iron can be repre-sfflited by the following scheme ... [Pg.533]

Complexation of bromine with iron(III) bromide makes bromine more elec trophilic and it attacks benzene to give a cyclohexadienyl intermediate as shown m step 1 of the mechanism (Figure 12 6) In step 2 as m nitration and sulfonation loss of a proton from the cyclohexadienyl cation is rapid and gives the product of electrophilic aromatic substitution... [Pg.480]

A second difference between alkene addition and aromatic substitution occurs after the carbocation intermediate has formed. Instead of adding Br- to give an addition product, the carbocation intermediate loses H+ from the bromine-bearing carbon to give a substitution product. Note that this loss of H+ is similar to what occurs in the second step of an El reaction (Section 11.10). The net effect of reaction of Br2 with benzene is the substitution of H+ by Br+ by the overall mechanism shown in Figure 16.2. [Pg.549]

There are many other kinds of electrophilic aromatic substitutions besides bromination, and all are thought to occur by the same general mechanism. Let s look at some of these other reactions briefly. [Pg.550]

Since chlorine is always in more than a hundred-fold excess compared to bromine the reaction is occurring by pseudo monomolecular kinetics. The reaction occurs via nucleophilic aromatic substitution by an addition-elimination mechanism, the so-called SjsfAr mechanism (ref. 24). [Pg.378]

There are two other mechanistic possibilities, halogen atom abstraction (HAA) and halonium ion abstraction (EL), represented in Schemes 4.4 and 4.5, respectively, so as to display the stereochemistry of the reaction. Both reactions are expected to be faster than outer-sphere electron transfer, owing to stabilizing interactions in the transition state. They are also anticipated to both exhibit antiperiplanar preference, owing to partial delocalization over the C—C—Br framework of the unpaired electron in the HAA case or the electron pair in the EL case. Both mechanisms are compatible with the fact that the activation entropies are about the same as with outer-sphere electron donors (here, aromatic anion radicals). The bromine atom indeed bears three electron pairs located in two orthogonal 4p orbitals, perpendicular to the C—Br bond and in one s orbital. Bonded interactions in the transition... [Pg.258]

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]

Bromination of benzene follows the same general mechanism of the electrophilic aromatic substitution. The bromine molecule reacts with FeBr3 by donating a pair of its electrons to it, which creates a more polar Br—Br bond. [Pg.258]

Reaction 1 has been postulated both in oxidations of alkanes in the vapor phase (29) and in the anti-Markovnikov addition of hydrogen bromide to olefins in the liquid phase (14). Reaction 2 involves the established mechanism for free-radical bromination of aromatic side chains (2). Reaction 4 as part of the propagation step, established in earlier work without bromine radicals (26), was not invoked by Ravens, because of the absence of [RCH3] in the rate equation. Equations 4 to 6, in which Reaction 6 was rate-determining, were replaced by Ravens by the reaction of peroxy radical with Co2+ ... [Pg.399]


See other pages where Mechanism aromatic bromination is mentioned: [Pg.673]    [Pg.81]    [Pg.321]    [Pg.322]    [Pg.218]    [Pg.157]    [Pg.165]    [Pg.552]    [Pg.117]    [Pg.374]    [Pg.2]    [Pg.494]    [Pg.258]    [Pg.242]    [Pg.118]    [Pg.645]    [Pg.576]    [Pg.288]    [Pg.304]    [Pg.576]    [Pg.393]    [Pg.220]    [Pg.321]    [Pg.322]    [Pg.1046]   
See also in sourсe #XX -- [ Pg.548 ]

See also in sourсe #XX -- [ Pg.548 ]

See also in sourсe #XX -- [ Pg.326 ]




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Aromatic bromination

Aromatic brominations

Aromatics brominated

Mechanism aromatic

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