Bromine molecule

The chain propagation step consists of a reaction of allylic radical 3 with a bromine molecule to give the allylic bromide 2 and a bromine radical. The intermediate allylic radical 3 is stabilized by delocalization of the unpaired electron due to resonance (see below). A similar stabilizing effect due to resonance is also possible for benzylic radicals a benzylic bromination of appropriately substituted aromatic substrates is therefore possible, and proceeds in good yields. 

The same ideas may be applied to the other processes of Fig. 1. The work required to dissociate a diatomic molecule into two electricallt/ neutral atoms may he quite small the dissociation energy of the bromine molecule Br2 in a vacuum, for example, is only 1.915 electron-volts. On the other hand, the work to dissociate a molecule into two atomic ions in a vacuum cannot be as small as this, since work must be done to set up the full electrostatic field of the positive ion, and the full electrostatic field of the negative ion and this must amount to at least a few electron-volts.1 In addition, the non-electrostatic forces may make a small or large contribution. 

FIGURE 18.11 As a bromine molecule approaches a double bond in an alkene, the atom closer to the ethene molecule acquires a partial positive charge (the blue region). The computation that produced this image was carried out for the point at which the bromine molecule is so close to the double bond that a carbon-bromine bond is starting to form. 

The bromination of benzene illustrates the difference between addition to alkenes and substitution of arenes. First, to achieve the bromination of benzene it is necessary to use a catalyst, such as iron(III) bromide. The catalyst acts as a Lewis acid, binding to the bromine molecule (a Lewis base) and ensuring that the outer bromine atom has a pronounced partial positive charge ... 

The iron(IIl) bromide is released in this step and is free to activate another bromine molecule. 

FIGURE 18.13 The catalyst FeBr, acts b> forming a complex with a bromine molecule. As a result, the bromine atom not directlv attached to the iron atom acquires a partial positive charge (the blue region). This partial charge enhances the ability of the bromine molecule to act as an electrophile. 

In a subsequent study (S9), isotherms of bromine on pyrolytic graphite showed the presence of several phases C4 Br (n = 2 to 5). X-ray studies confirmed these to be stages 2 to 5, respectively. At intermediate concentrations, X-ray patterns showed mixtures of higher and lower stages. The density and configuration of intercalated bromine molecules were believed to be the same in all stages. Other structural types... 

It is generally supported that the bromination with NBS proceeded by a radical (ref. 11) or an ionic mechanism via bromine molecule. For instance, the former was suggested in benzylic and allylic bromination with NBS for Whol-Ziegler reaction (ref. 12). Calo et al. (ref. 5) accounted NBS brominated phenol by the latter mechanism. 

In the above section we describe the convenient preparation of 2-allyl-6-bromophenol without any bromine adduct. It seems that these results are not able to be explained by the mechanisms via bromine molecule, which are described later. 

It may be asked why, if Br2 is the reacting species, it does not add to the double bond, either by an ionic or by a free-radical mechanism (see 15-37). Apparently, the concentration is too low. In bromination of a double bond, only one atom of an attacking bromine molecule becomes attached to the substrate, whether the addition is electrophilic or free radical ... 

Our senses tell us that molecules escape from a liquid. The red gas phase above the liquid bromine in Figure 11-4 shows that bromine molecules are in both phases. The evaporation of a rain puddle in the sunshine shows that water molecules escape from the liquid into the gas phase. The smell of gasoline around an open tank informs our noses that gasoline molecules escape from the liquid phase into the gas phase. 

The fraction of molecules with enough kinetic energy to escape a liquid depends on the strength of intermolecular forces and temperature, (a) At 300 K, more bromine molecules can escape than water molecules, (b) More bromine molecules can escape at 320 K than at 300 K. 

In a unimolecular reaction, a molecule fragments into two pieces or rearranges to a different isomer, hi either case, a chemical bond breaks. For example, in the fragmentation of bromine molecules, breaking a ffbond gives a pair of bromine atoms Bf2 2 Br Recall that this unimolecular process is the first step of the reaction between molecular hydrogen and molecular bromine to give HBr. 

Energy profiles for two unimolecular processes (a) the unimolecular decomposition of a bromine molecule (b) the unimolecular Isomerization of ds-2-butene. 

Furan was dimethoxylated to give 2,5-dihydro-2,5-dimethoxyfuran, using electrogenerated bromine molecules generated from bromide salts in electrolyte solutions [71]. This reaction was characterized in classical electrochemical reactors such as pump cells, packed bipolar cells and solid polymer electrolyte cells. In the last type of reactor, no bromide salt or electrolyte was used rather, the furan was oxidized directly at the anode. H owever, high consumption of the order of 5-9 kWh kg (at 8-20 V cell voltage) was needed to reach a current efficiency of 75%. 

It seems likely that benzene forms a n complex (12) with, for example, Br2 (cf. p. 131), and that the Lewis acid then interacts with this. The catalyst probably polarises Br—Br, assists in the formation of a a bond between the bromine molecule s now electrophilic end and a ring carbon atom, and finally helps to remove the incipient bromide ion so as to form a [Pg.138]

It is found in practice with (5), and with other simple acyclic alkenes, that the addition is almost completely stereoselective, i.e. 100% ANTI addition. This result also is incompatible with a one-step pathway, as the atoms in a bromine molecule are too close to each other to be able to add, simultaneously, ANTI. 

These observations are explainable by a pathway in which one end of a bromine molecule becomes positively polarised through electron repulsion by the n electrons of the alkene, thereby forming a n complex with it (8 cf. Br2 + benzene, p. 131). This then breaks down to form a cyclic bromonium ion (9)—an alternative canonical form of the carbocation (10). Addition is completed through nucleophilic attack by the residual Br (or added Ye) on either of the original double bond carbon atoms, from the side opposite to the large bromonium ion Br , to yield the meso dibromide (6) ... 

Enough mutual polarisation can apparently result, in (8), for (9) to form, but polarisation of the bromine molecule may be greatly increased by the addition of Lewis acids, e.g. AlBr3 (cf. bromination of benzene, p. 138), with consequent rise in the rate of reaction. Formation of (9) usually appears to be the rate-limiting step of the reaction. 

In certain highly energetic collisions with any molecule M in the system, a bromine molecule may be dissociated in a homolytic split of the bond joining two bromine atoms. 

A bromine molecule becomes polarized as it approaches the alkene. The polarized bromine molecule transfers a positive bromine atom (with six electrons in its valence shell) to the alkene resulting in the formation of a bromonium ion. 

The bromine molecule becomes polarized as the k electrons of the alkene approaches the bromine molecule. 

There are no polar solvent molecules to solvate (and thus stabilize) the bromide ion formed in the first step, the bromide ion uses a bromine molecule as a substitute. => In a nonpolar solvent the rate equation is second order with respect to bromine. 

The most significant difference between brominations in protic and non-protic solvents concerns the kinetic law. Whereas in protic media the reaction is first-order in bromine, in halogenated media it is second-order (Bellucci et ai, 1980). CTC ionization is electrophilically assisted by hydrogen bonding by a protic solvent to the leaving bromide and leads to a bromonium-bromide ion pair. In non-protic media, assistance to the bromination step is provided by a second bromine molecule, leading to a bromonium-tribromide ion pair. In other words, in protic media bromination is solvent-assisted (56) while in halogenated media it is bromine-catalysed (57). 

In halogenated solvents, catalysis by a second bromine molecule, which assists the Br—Br bond heterolysis, is the main driving force. The role of the solvent is electrostatic, but the absence of an extensive Kirkwood relationship suggests that there is some other kind of contribution (Bellucci et al, 1985b). 

As the mobile ji electrons of the alkene approach the bromine molecule, the electrons of the bromine-bromine bond are drifted in the direction of that bromine which is more distant from the alkene..Thus the bromine molecule becomes polarized and a partial... 

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