Electrophilic reactions bimolecular substitution

In the case of bimolecular aliphatic electrophilic substitution reactions, there are three principal pathways, namely SE2 (front), SE2 (back), and the internal electrophilic substitution, SEi (otherwise known as SF2 or SE2 (cyclic)). Inversion would occur with SE2 (back), while retention would occur with the other two. Experiments to distinguish between these latter two have indicated that the situation may be even more complicated, in that an alternative pathway, which is labelled either SE2 (co-ord), or SEC, may operate. 

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. 

In Section 4.5 we discussed reactions in which electrophilic substitution of a metal ion takes place by a bimolecular pathway. The unimolecular substitution is less common, although there are some examples in cases where the carbanion is well stabilized.120 For our purposes here the most important SE1 reactions are those in which the leaving group is a proton or a neutral carbon molecule. 

Ingold3 has used the term SE2 to describe these bimolecular substitutions which proceed via open transition states (as shown in reaction (4)), but Reutov4 uses the symbol SE2 to describe all bimolecular electrophilic substitutions, including those in which cyclic transition states are formed as well as those in which the transition state is open. More recently, Abraham et at.6 have suggested that bimolecular electrophilic substitutions in which an open transition state is formed could more explicitly be denoted by the term SE2(open). 

The nomenclature used in describing bimolecular electrophilic substitutions involving cyclic transition states reflects, in part, the above-mentioned difficulty. Ingold3 has adopted the nomenclature of Winstein et al.1 and refers to such substitutions as SEi, but to the present author this is not a particularly appropriate choice since it does not indicate the bimolecular nature of the substitution. Dessy et al.8 have used the term SF2 to describe a mechanism, such as that in reaction (5), in which a four-centred transition state is formed, but not only is such a term too restricted, it also provides no indication that the mechanism is one of electrophilic substitution. The view of Reutov4 is that the cyclic, synchronous mechanism is very close to the open mechanism and that both can be described as SE2 mechanisms. Dessy and Paulik9 used the term nucleophilic assisted mechanisms to describe these cyclic, synchronous mechanisms and Reutov4,10 has recently referred to them in terms of internal nucleophilic catalysis , internal nucleophilic assistance , and nucleophilic promotion . Abraham, et al,6 have attempted to reconcile these various descriptions and have denoted such mechanisms as SE2(cyclic). 

However, Jensen et al.40,43, have correctly pointed out that the fourth-order coefficient (k4 in equation (18)) or the second-order coefficient (k°2 in equation (28)) are actually complex coefficients and include K, the equilibrium constant for reaction (24) or (26). In terms of mechanism B, k4 = K(24) xk(25) and in terms of mechanism C, k4 = Ki26)xkl2V. Thus substituent effects may well refer to the equilibrium (24) or (26) rather than to the actual electrophilic substitution, reaction (25) or (27). In this connection it is worth recalling that in the bimolecular one-alkyl exchange (13) the sequence of / -substituents in the benzyl group is p-Cl < H < p-Pr1, but in the anion-catalysed exchange (13), which takes place via a pre-rate-determining equilibrium, the sequence is p-C 1 > H > p-Pr (see Table 4, p. 45) it seems to the author that the substituent effects shown in Table 9 may also be explained as effects on the equilibrium constants K(24) or K(26). 

In Scheme 4.1 the mechanisms of typical monomolecular (SnI) and bimolecular (Sn2) nucleophilic substitutions at a neutral electrophile with an anionic nucleophile are sketched. SnI reactions usually occur when the electrophile is sterically... 

The mechanism of the reaction was shown by Eaborn and Taylor to be (60JCS3301) an acid-catalyzed version of the SE2 mechanism (A-SE2), which applies to most electrophilic substitutions (Scheme 2.1). This involves a bimolecular reaction between an acid (HA) and the aromatic to give a Wheland intermediate, which then loses a hydrogen ion to give... 

To drive the reaction over to the sulfonic acid, an excess of sulfuric acid is generally employed which also assists the forward reaction by removing the water formed. Sulfonation is a bimolecular electrophilic substitution (Se2) reaction and is therefore facilitated by the presence of electron-donating substituents on the aromatic nucleus and is made more difficult by the introduction of electron withdrawing groups (see chapter 7, p. 98). 

Sulfonation is a bimolecular electrophilic substitution reaction (SE2) and the electrophile is sulfur trioxide.3a Sulfur trioxide is a powerful electrophile because of the electron-withdrawing effect of the three double-bonded oxygen atoms. Consequently, oleum (fuming sulfuric acid), which contains approximately 10% of excess sulfur trioxide, is a much more powerful sulfonating agent than concentrated sulfuric acid. Sulfur trioxide is a sufficiently powerful electrophile to attack benzene (23) directly. The mechanism of the sulfonation of benzene by hot concentrated sulfuric acid to give benzenesulfonic acid (24) is shown in Scheme 15.4a... 

Tetradifon is a valuable acaricide for the control of phytophagous mites on a wide range of crops the last step in the synthesis is an example of the Friedel-Crafts reaction, and can be likened to the reaction between t-butylbenzene (62) and a sulfonyl chloride (51) in the presence of aluminium trichloride catalyst to give the sulfone (62) (see Chapter 10, p. 196) (Scheme 37). This reaction is a bimolecular electrophilic substitution reaction(SE2) and... 

The mechanisms that occur in aliphatic electrophilic substitution reactions are less well defined than those that occur in aliphatic nucleophilic substitution and aromatic electrophilic reactions. There is still, however, the usual division between unimolecular and bimolecular pathways the former consisting of only the SE1 mechanism, while the latter consists of the SE2 (front), SE2 (back) and the SEi mechanism. 

The reaction of alkylcobalamins with metal electrophiles, such as Hg(II), has been studied extensively since MeBjj may be responsible for formation of highly toxic meth-ylmercury(II) compounds under environmental conditions ". The reaction mechanism has been studied for both alkylcobaloximes and alkylcobalamins " . Reaction of alkylcobaloximes with Hg(II) salts can be described as a bimolecular electrophilic substitution (Se2), which takes place with inversion of configuration . 

Notes-. Most used reactions are SEAr (iododemetallation) and SNAr (halogen exchange, copper assisted). SeI, unimolecular electrophilic substitution Se2, bimolecular electrophilic substitution SeAp aromatic electrophilic substitution SeI, intramolecular electrophilic substitution S l, unimolecular nucleophilic substitution 3 2, bimolecular nucleophilic substitution S Ar, aromatic nucleophilic substitution. 

It would be unreasonable to expect a nucleophilic species not to interact with both an electrophilic alpha carbon and beta hydrogen at some stage during the reaction. However, whether this interaction is representative of the rate-determining step is a matter of conjecture. Most probably this dual interaction occurs early in the reaction profile and is followed by partitioning to give the two different transition states commonly accepted for bimolecular elimination and substitution. The closer the dual interaction and the transition states are on the reaction profile, then the more closely the elimination rate will respond to carbon nucleophilicity. 

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