Electrophilic addition reactions bimolecular

In addition to the transfer reactions already discussed, propagating carbe-nium ions also react with nucleophilic hydride and methide anions. This reaction may be bimolecular, or it may occur by an intramolecular hydride shift to form a more stable carbenium ion. The activation energy of hydride transfer is usually higher than that of propagation, and therefore occurs only at elevated temperatures. Nevertheless, hydride transfer is the dominant reaction when a-methylstyrene is initiated by triphenylcarbenium ions. That is, steric hindrance prevents initiation by direct electrophilic addition of the carbenium ion to a-methylstyrene. Instead, it occurs by hydride transfer from monomer to yield triphenyl methane and the primary carbenium ion of a-methylstyrene [cf., Eq. (40)]. 

It was clear in section 2.5 that the group connected to a carbonyl unit played a significant role in the course of nucleophilic acyl addition reactions. Bimolecular processes in general are dependent on the nature of the substrate (the molecule containing the electrophilic center). For reactions of primary alkyl halides, bimole-... 

For reasons that will become apparent later in this chapter (Section 5.26), reactions of this type never take place by a single concerted process. The groups X and Y add in two separate steps, either as followed by Y (electrophilic addition) or as X" followed by Y" (nucleophilic addition) or as X- followed by Y (radical addition). Here we will consider electrophilic additions in which the rate-determining step is a bimolecular reaction between the olefin and XY to form an intermediate cation and Y ,... 

Thus, three types of reactions were divided into reactions of nucleophilic and electrophilic substitution, addition, and elimination. The kinetic study showed that some reactions involving two reactants occur as bimolecular reactions, whereas others occur as monomolecular reactions with the rate proportional to the substrate concentration and independent of the agent concentration. Thus, all variety of heterolytic reactions involving two reactants is divided into the following 10 groups ... 

The rate of a bimolecular nucleophilic or electrophilic addition, substitution, elimination, or addition-elimination reaction may be increased by several orders in the presence of an electrophilic catalyst (E+), such as metal ion, if E+ is capable of forming a complex with both reactants (say electrophile Y - Sub - X and nucleophile W - H) in an appropriate molecular geometry, which allows the reaction to proceed intramolecularly as shown in Equation 2.35. 

Chlorination and chloramination of a widely used antibacterial additive, triclo-san, which is used in many household personal care products, results in the formation of chloroform, 5,6-dichloro-2-(2,4-dichlorophenoxy)phenol, 4,5-dichloro-2-(2,4-dichlorophenoxy)phenol, 4,5,6-trichloro-2-(2,4-dichlorophenoxy)phenol, 2, 4-dichlorophenol, and 2,4,6-trichlorophenol [119]. The reaction of triclosan with monochloramine is slow, however, compared to chlorine [120]. The chlorophenox-yphenols are formed via bimolecular electrophilic substitution of triclosan. 

Table 11 presents one more result important for the chemistry of epoxy compounds, namely within the experimental error the rate constant of the free ion is the same for all counterions. This means that such strong nucleophilic particles as carbanions (and evidently alkoxy anions) are capable of opening the epoxy ring without additional electrophilic activation. This result explains the apparently contradictory results that, depending on the reaction conditions, either tri-140 144,166-I71) or bimolecular kinetics 175-I79> is observed. The bimolecular kinetics also can be explained in terms of the trimolecular mechanism, since proton-donor additives play a dual role. 

When reaction (16) (X = Br) is carried out30 in the presence of small quantities (0.25-0.50 vol. %) of various aliphatic alcohols, aliphatic ethers, or water, the kinetic form becomes that of the second-order overall. Furthermore, the reaction rate does not depend on the illimination. It was suggested30,32 that in the presence of the above additives, reaction (16) (X = Br) is an electrophilic bimolecular substitution with a complex of the type Br-Br ORR as the electrophile. 

The concept of selectivity is most commonly encountered (and most useful in mechanistic investigations, see Chapter 2) when a reactant or a reactive intermediate has alternative bimolecular routes it is then also very useful in yield optimisation in chemical process development [12]. The reaction in Scheme 4.3 involves an electrophilic intermediate (X) which is captured by nucleophilic reagent C (which could be solvent). If another nucleophilic species (D) is added to the reaction mixture, the additional product D—X is formed in competition with C—X. If Ap is known (e.g. if D is known to react with electrophiles at the diffusion limit), then values for [D], [C] and the product ratio [C—X]/[D—X] allow determination of kc, i.e. quantitative information about the reactivity of X with C, and information about the selectivity of X in reactions with nucleophiles. 

A serious limitation of VNS is connected with its mechanism, namely, conversion of intermediate ct adducts into the VNS products via bimolecular base-induced p-elimination. To cause the reaction, it is therefore necessary that these adducts be produced in a reasonable concentration. Indeed, low nucleophilic carbanions, such as dimethyl chloromalonate, do not react with moderately electrophilic nitrobenzene because of unfavorable equilibrium of the addition step, but react nicely with more electrophilic nitrothiazoles (Scheme 8) [34]. 

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