Substitution, nucleophilic, bimolecular reactions

The other major type of nucleophilic substitution reaction is bimolecular, and is given the label SN2. In this case, the rate of the reaction is observed to be... 

Equation 4 can be classified as S, , ie, substitution nucleophilic bimolecular (221). The rate of the reaction is influenced by several parameters basicity of the amine, steric effects, reactivity of the alkylating agent, and solvent polarity. The reaction is often carried out in a polar solvent, eg, isopropanol, which may increase the rate of reaction and make handling of the product easier. 

The preceding Sections illustrate several experimental features of heteroaromatic substitutions. It is now intended to comment on some of these features which are most significant in terms of reaction mechanism. As stated in the Introduction, a possible mechanism of nucleophilic bimolecular aromatic substitution reactions is that represented by Eq. (14), where an intermediate of some stability... 

Sfj2 reaction (Section 11.2) A bimolecular nucleophilic substitution reaction. 

Sn2 stands for substitution nucleophilic bimolecular. The lUPAC designation (p. 384) is AnDn- In this mechanism there is backside attack The nucleophile approaches the substrate from a position 180° away from the leaving group. The reaction is a one-step process with no intermediate (see, however, pp. 392-393 and 400). The C—Y bond is formed as the C—X bond is broken ... 

Gas-phase SN2 nucleophilic substitution reactions are particularly interesting because they have attributes of both bimolecular and unimolecular reactions.1 As discovered from experimental studies by Brauman and coworkers2 and electronic structure theory calculations,3 potential energy surfaces for gas-phase SN2 reactions of the type,... 

The mechanism of these bimolecular nucleophilic substitution reactions is shown in Scheme 11.3 for the reaction between a primary amine and the intermediate dichlorotriazine. A corresponding scheme can be drawn for reaction of a secondary amine, an alcohol or any other nucleophile in any of the replacement steps. It follows from this mechanism that the rate of reaction depends on ... 

FIGURE 2.10 Differentiation of SN1 (substitution nucleophilic unimolecular, first order) and SN2 (substitution nucleophilic bimolecular, second order) reactions. 

We have examined the competing isomerization and solvolysis reactions of 1-4-(methylphenyl)ethyl pentafluorobenzoate with two goals in mind (1) We wanted to use the increased sensitivity of modern analytical methods to extend oxygen-18 scrambling studies to mostly aqueous solutions, where we have obtained extensive data for nucleophilic substitution reactions of 1-phenylethyl derivatives. (2) We were interested in comparing the first-order rate constant for internal return of a carbocation-carboxylate anion pair with the corresponding second-order rate constant for the bimolecular combination of the same carbocation with a carboxylate anion, in order to examine the effect of aqueous solvation of free carboxylate anions on their reactivity toward addition to carbocations. 

A reaction described as Sn2, abbreviation for substitution, nucleophilic (bimolecular), is a one-step process, and no intermediate is formed. This reaction involves the so-called backside attack of a nucleophile Y on an electrophilic center RX, such that the reaction center the carbon or other atom attacked by the nucleophile) undergoes inversion of stereochemical configuration. In the transition-state nucleophile and exiphile (leaving group) reside at the reaction center. Aside from stereochemical issues, other evidence can be used to identify Sn2 reactions. First, because both nucleophile and substrate are involved in the rate-determining step, the reaction is second order overall rate = k[RX][Y]. Moreover, one can use kinetic isotope effects to distinguish SnI and Sn2 cases (See Kinetic Isotope Effects). 

The inversion of stereochemical configuration at a chiral center in a nucleophilic substitution reaction The uncatalyzed reaction is bimolecular i.e., it is an Sn2 reaction (in fact the Walden inversion was noted before the criteria for Sn2 reactions were presented). See Sm"/ and 5 2 Reactions Nucleophilic Substitution Reactions... 

Nucleophilic substitution of azide ion at (4-Me)-l-Cl is zero order in the concentration of azide ion [NJ] but, there is a strong bimolecular substitution reaction of azide ion with (4-Me)-l-S(Me)2 This change in the kinetic order for the reaction of azide ion shows that the pentavalent transition state... 

Nucleophilic Substitution at Benzyl Derivatives. The sharp break from a stepwise to a concerted mechanism that is observed for nucleophilic substitution of azide ion at X-l-Y (Figs. 2.2 and 2.5) is blurred for nucleophilic substitution at the primary 4-methoxybenzyl derivatives (4-MeO,H)-3-Y. For example, the secondary substrate (4-MeO)-l-Cl reacts exclusively by a stepwise mechanism through the liberated carbocation intermediate (4-MeO)-T, which shows a moderately large selectivity toward azide ion ( az/ s = 100 in 50 50 (v/v) water/ trifluoroethanol). The removal of an a-Me group from (4-MeO)-l-Cl to give (4-MeO,H)-3-Cl increases the barrier to ionization of the substrate in the stepwise reaction relative to that for the concerted bimolecular substitution of azide ion. The result is that both of these mechanisms are observed concurrently for nucleophilic substitution of azide ion at (4-MeO,H)-3-Cl in water/acetone solvents. These concurrent stepwise and concerted nucleophilic substitution reactions of azide ion with (4-MeO,H)-3-Cl show that there is no sharp borderline between mechanisms for substitution at primary benzylic carbon, but instead a region of overlap where both mechanisms are observed. 

Figure C shows an extreme case of the dependence of a substitution reaction rate on the nature of the incoming group. This happens to be the hydrolysis of the trisacetylacetonate complex of silicon (IV), cationic species, which Kirchner studied first—the rate of racemization or rate of dissociation. We studied the base-catalyzed rate of dissociation and showed that a large number of anions and nucleophilic groups, in general, would catalyze in the dissociation process. We found that the reaction rates were actually for a second-order process, so these units are liters per mole per second. But the reaction rate did vary over an enormous range—in this case, about a factor of 109—and this is typical of the sort of variation in rates of reaction (that you can get) for processes that seem to be Sn2 bimolecular displacement processes.

If a reaction occurs by this first mechanism, it is commonly termed an SN2 reaction (i.e., substitution, nucleophilic, bimolecular). It represents an example of a simple elementary bimolecular reaction, as we discussed in Section 12.3, and it therefore follows a second-order kinetic rate law ... 

A number of methods for cleaving phosphorus esters proceed by nucleophilic attack on the alkyl group of the ester, rather than attack at the phosphorus. As expected for a bimolecular nucleophilic substitution reaction, these reactions are fastest for benzyl and methyl groups, and the rate drops rapidly as the steric hindrance around the alkyl group increases. Thus,... 

WALDEN INVERSION. Inversion of configuration of a chiral center m bimolecular nucleophilic substitution reactions. See also Rearrangement (Organic Chemistry),... 

The changes in the geometric parameters of the CISiC/jO coordination centre are typical of a bimolecular nucleophilic substitution reaction at a tetrahedral atom and consistent with the results obtained using the Burgi-Dunitz structure correlation method with the experimental results of Macharashvili and coworkers26. 

Short-lived organic radicals, electron spin resonance studies of, 5, 53 Small-ring hydrocarbons, gas-phase pyrolysis of, 4, 147 Solid state, tautomerism in the, 32, 129 Solid-state chemistry, topochemical phenomena in, 15, 63 Solids, organic, electrical conduction in, 16, 159 Solutions, reactions in, entropies of activation and mechanisms, 1, 1 Solvation and protonation in strong aqueous acids, 13, 83 Solvent effects, reaction coordinates, and reorganization energies on nucleophilic substitution reactions in aqueous solution, 38, 161 Solvent, protic and dipolar aprotic, rates of bimolecular substitution-reactions in,... 

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