Bromine molecule, bonding

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. 

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 ... 

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. 

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]

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) ... 

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. 

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... 

The dipole moment of the adsorbed water molecules is estimated to be = 0.22 D (unit of D = 3.36 x 10 ° C m) from the slope of the observed curves shown in Fig. 5-25. Since this dipole moment is nearly one tenth of the dipole moment of gaseous water molecules (m = 1.84 D), the dipole of the adsorbed water molecules on the silver surface is suggested to be aligned almost parallel to the metal surface by forming hydrogen-bonded two-dimensional clusters of water molecules. On the other hand, bromine molecules are in the state of dissociative adsorption on the silver surface, producing adsorbed bromine atoms which receive electrons... 

The bromine atom then adds to the alkene, generating a new carbon radical. In the case of propene, as shown, the bromine atom bonds to the terminal carbon atom. In this way, the more stable secondary radical is generated. This is preferred to the primary radical generated if the central carbon were attacked. The new secondary radical then abstracts hydrogen from a further molecule of HBr, giving another bromine atom that can continue the chain reaction. 

The triphenylphosphine complex 42 b reacts with bromine to give a platinum(IV) species 42f, assigned the cyclic structure with four platinum-carbon c-bonds. The reaction sequence here must begin with the attack of a bromine molecule on an uncoordinated olefin, as otherwise it is hard to see why two metal-carbon bonds are formed, and not one metal-carbon and one metal-bromine. 

Mechanism. The reaction is analogous to the addition of bromine molecules to an alkene. The electrophilic mercury of mercuric acetate adds to the double bond, and forms a cyclic mercurinium ion intermediate rather than a planer carbocation. In the next step, water attacks the most substituted carbon of the mercurinium ion to yield the addition product. The hydroxymercurial compound is reduced in situ using NaBH4 to give alcohol. The removal of Hg(OAc) in the second step is called demer-curation. Therefore, the reaction is also known as oxymercuration-demercuration. 

Mechanism. When Br2 approaches to the double bond it becomes polarized. The positive part of the bromine molecule is attacked by the electron rich... 

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. 

A bromine atom, Br, contains seven electrons in its valence shell. A bromine molecule contains two bromine atoms bonded by a single bond. A bromide ion is a bromine atom that has gained one electron in its valence shell it has a complete octet and a negative charge. 

H, Me, r-Bu, or Ph or R = H and R = Me, r-Bu, or Ph), was performed. Two possible reactions were investigated (a) the reactions suitable for the gas-phase interactions, which start from a 1 1 Br2-alkyne r-complex and do not enter into a 2 1 Br2-alkyne jt-complex and (b) the processes passing through a 2 1 Br2-alkyne 7r-complex, which appear more realistic for the reactions in solutions. The structures of the reactants and (g) the final products and also the possible stable intermediates have been optimized and the transition states for the predicted process have been found. Both trans- and cw-dibromoalkenes may ensue without the formation of ionic intermediates from a n-complex of two bromine molecules with the alkyne (solution reactions). The geometry around the double bond formed in dibromoalkenes strongly depends on the nature of the substituents at the triple bond. The cluster model was used for the prediction of the solvent influence on the value of the activation barrier for the bromination of the but-2-yne.35... 

When the bromine molecule approaches end-on to the alkene double bond and an electrophilic centre is included (Following fig.). Since the alkene double bond is electron rich, it repels the electrons in the bromine molecule and this results in a polarisation of the Br-Br bond in such a way that the nearer bromine becomes electron deficient (electrophilic). In this way, when an electrophilic centre has been generated, the mechanism is the same as before. 

O The electrophile is the bromine molecule. One Br leaves as a negative ion. As this Br leaves, the other Br becomes electrophilic and attracts the electrons of the alkene pi bond. If a carbocatlon were formed at this stage, it would look like this. 

Fig. 40.—Energy contour-map of system for the addition of a bromine molecule to the carbon-carbon double bond. The north and south axis refers to the distance between Br-Br and C-C. The east-west axis refers to the distance between bromine atoms. Contour lines are projections of energy plotted at right angles to the plane.

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