Substitution, electrophilic unimolecular,

The Sgl mechanism substitution electrophilic unimolecular) is rare, being found only in certain cases in which carbon is the leaving atom (see 11-37,11-38) or when a very strong base is present (see 11-1,11-11, and 11-42). It consists of two steps with an intermediate carbanion. The lUPAC designation is Dg + Ag. 

In analogy to the traditional terms SW1 and Sw2, which refer to the extreme aliphatic substitution mechanisms, workers in the field of electrophilic substitution refer to S 1 (for substitution-electrophilic-unimolecular) and SE2 (substitution-electrophilic-bimolecular) mechanisms. Equations 4.40 and 4.41... 

The nomenclature used to describe this mechanism is not a subject of controversy, and all workers3-6 have used the symbol SE1 that is substitution, electrophilic, unimolecular. The unimolecular process referred to is the elementary reaction (1). 

This mechanism is known as SnI (substitution, nucleophilic, unimolecular). Because carbocations are such reactive electrophiles, even poor nucleophiles such as alcohols and water, react with them. 

It is convenient to categorize reactions with concise descriptive labels. For substitution reactions we often use the notation SxM, in which the letter S indicates a substitution reaction. The subscript x indicates something of the mechanism, such as N for nucleophilic or E for electrophilic. M usually indicates the molecularity of the reaction, the nature of the reacting species, or additional information. The most familiar terms for substitution reactions are SnI (for substitution nucleophilic unimolecular ), as shown in equation 8.4,... 

For aliphatic electrophilic substitution, we can distinguish at least four possible major mechanisms, which we call Sgl, 8 2 (Iront), Se2 (back), and Sgi. The Sgl is unimolecular the other three are bimolecular. 

We may define a unimolecular electrophilic substitution, SE1, by Equations 5.22 and 5.23. The electrophilic substitutions have not been as thoroughly studied as... 

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. 

Nucleophilic catalysis in the unimolecular mechanism is straightforward, since there is but one reactant that can bring the nucleophile into the transition state. If, however, a bimolecular substitution is found to be subject to nucleophilic catalysis and if it is concluded that, say, one molecule of the nucleophile, B, is involved in the transition state, then two main possibilities exist. The nucleophile may be brought into the transition state either by the organometallic substrate as in process (22) or by the electrophile as in process (23), viz. 

Comparison of results from the gas-phase proton-induced unimolecular isomerization of (R)- -d -3-(p-fluorophcnyl )bulanc (11) with the positional selectivity of the corresponding gas-phase bimolecular arene alkylation confirms the presence of non-covalent j-type intermediates and their important role in determining the intramolecular selectivity of gas-phase electrophilic aromatic substitutions.20... 

Electrophilic aromatic substitution also occurs intramolecularly to generate polymers with indanyl end groups [Eq, (93)] entropy strongly favors this unimolecular reaction. 

Benzoic acid and most mono-substituted benzoic acids are stable with respect to decarboxylation in aqueous solution, even at a temperature of 100 °C. However, decarboxylation may occur with a measurable rate if either strong electron-withdrawing or strong electron-releasing substituents are present in the aromatic acid. The decarboxylation rate of 2,4,6-trinitrobenzoic acid is increased by addition of base to the aqueous solution, and it attains a maximum value when the substrate is completely transformed to the anion [236]. A carbon-13 isotope effect of ft, 2/ft, 3 = 1.036 (50 °C) has been observed [237]. There is no D20 solvent isotope effect [238]. These findings indicate that the mechanism of decarboxylation of 2,4,6-trinitrobenzoic acid is a unimolecular electrophilic substitution (SE1), viz. 

For aliphatic electrophihc substitution, we can distinguish at least four possible major mechanisms, which we call Sgl, Se2 (front), Se2 (back), and Sei. The Sgl is unimolecular the other three are bimolecular. It is noted that the term SeAt has been proposed to represent electrophilic aromatic substitution, so that the term Se2 refers exclusively to electrophihc substitutions where a steric course is possible. To describe the steric course of an aliphatic substitution reaction, the suffixes ret and inv were proposed, referring to retention and inversion of conhguration, respectively. 

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. 

In the unimolecular mechanism, i.e. the SE1 mechanism, the electrophilic substitution takes place in two steps that parallel those that occur in the SN1 mechanism. Accordingly, write down the two steps for a general electrophilic substitution reaction between RX and E+, and indicate which is the slow step. 

HVZ MPV SE1 SE1(N) Hell-V olhard-Zelinskii. Meerwein-Ponndorf-Verley reduction. Electrophilic substitution unimolecular. SEI pathway in which a nucleophile (which may be the solvent) assists in the removal of the electrofuge. 

Ad 2 (Addition, Electrophilic, Bimolecular), A -i- An and Hetero Ad Z, p.t. + AdN El (Elimination, Unimolecular), Dn + Dg and Lone-Pair-Assisted El, Ep + p.t. Se2Ar Electrophilic Aromatic Substitution, Ag + Dg ElcB (Elimination, Unimolecular, Conjugate Base), p.t.-i-Ep AdN2 (Addition, Nucleophilic, Bimolecular), AdN + P f ... 

Carbocations, however formed, are very electrophilic. They react readily with nucleophiles, as shown in reaction (5.18). These reactions are important as steps in electrophilic addition to double bonds and unimolecular nucleophilic substitution (SnI) reactions. 

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