Transition state bimolecular nucleophilic substitution

As can be seen from the data presented, the high energies of complex formation decrease sharply the endothermicity of the retro-Wittig type decomposition and, moreover, fundamentally change the reaction mechanism. As has been shown for betaines (")X-E14Me2-CH2-E15( + )Me3 (X = S, Se E14 = Si, Ge E14 = P, As), the reaction occurs as bimolecular nucleophilic substitution at the E14 atom. For silicon betaines, the transition states TS-b-pyr with pentacoordinate silicon and nearby them no deep local minima corresponding to the C-b complexes can be localized in the reaction coordinate. 

There is minimal steric hindrance in the transition state for coupled concerted bimolecular nucleophilic substitution at primary carbon [D 0, Fig. 2.4(11)] to... 

We can use the overall reaction order to distinguish between the two possible mechanisms, A and B. Experimentally, the rate of formation of methanol is found to be proportional to the concentrations both of chloromethane and of hydroxide ion. Therefore the reaction rate is second order overall and is expressed correctly by Equation 8-2. This means that the mechanism of the reaction is the single-step process B. Such reactions generally are classified as bimolecular nucleophilic substitutions, often designated SN2, S for substitution, N for nucleophilic, and 2 for bimolecular, because there are two reactant molecules in the transition state. To summarize For an SN2 reaction,... 

Bimolecular nucleophilic substitution at tetracoordinate phosphorus (Eq. (8)) may proceed by either direct (Sjq2) substitution or by an addition-elimination mechanism In the former, 16 represents a transition state, while in the... 

Bimolecular nucleophilic substitution reactions (Sn2) involve pentacoordinate carbon in their transition state. Whereas pentacoordinate CHs -like intermediates of Se2 reactions represent only an eight-electron system around the carbocationic center involving 3c-2c bonding (octet rule is obeyed), an Sn2 reaction intermediate would represents a 10-electron system involving three-center, four electron (3c-4e) bonding (a lO-C-5 species). This, however, is an unstable situation since carbon cannot accommodate 10 electrons in its valence shell. A simple picture of typical molecular orbitals in 3c-4c bonding is shown in Figure 6.9. 

A new type of benzo[fo]furan was constructed by a bimolecular nucleophilic substitution reaction at the sp carbon atom, which was envisaged to proceed via a five-coordinate carbon compound as a transition state <05OL2739>. 

The Marcus equation was first formulated to model the dependence of rate constants for electron transfer on the reaction driving force [47-49]. Marcus assumed in his treatment that the energy of the transition state for electron transfer can be calculated from the position of the intersection of parabolas that describe the reactant and product states (Fig. 1.2A). This equation may be generalized to proton transfer (Fig. 1.2A) [46, 50, 51], carbocation-nucleophile addition [52], bimolecular nucleophilic substitution [53, 54] and other reactions [55-57] by assuming that their reaction coordinate profiles may also be constructed from the intersection of... 

Thermal organic reactions are often classified in terms of the molecular and electronic structure of their transition state or reactive intermediate (which is often taken as a model of the transition state). Thus, for instance, one has the Sn2 transition state for concerted bimolecular nucleophilic substitution reactions one has the E2 and Ei transition states in elimination reactions, etc. Given the transient nature of the transition states, the use of quantum chemical methodologies is essential for the determination of their detailed geometrical and electronic structure. Furthermore, the computation of the associated transition vectors provides information on the reactive mode... 

The C-X bond of alkyl halides and sulfonate esters is polarized such that the carbon has a positive dipole. Halides and sulfonate anions are good leaving groups. Nucleophiles attack primary and secondary alkyl halides, displacing the leaving group in what is known as aliphatic, bimolecular nucleophilic substitution, the Sn2 reaction. The 8 2 reaction follows second-order kinetics, has a transition state rather than an intermediate, and proceeds via back-side attack of the nucleophile on the halide and inversion of configuration. 

PROBLEM 5.59 Observe the reaction titled Bimolecular nucleophilic substitution. Notice that a bromide is being displaced from a carbon atom, in this case C(2) of 2-bromo-propane. We can imagine a closely related reaction in which displacement takes place in bromocyclopropane. All we have (mentally) done is to connect the two end carbons of 2-bromo-propane with a carbon-carbon bond. Would the reaction with bromocyclopropane be faster or slower (it can t be the same ) as the reaction with 2-bromopropane Hint Think about the transition state for this reaction—what is the h)d)ridization of carbon in the transition state ... 

The effects of solvents on reaction rates have been studied most extensively on unimolecular solvolysis reactions (S l) and on bimolecular nucleophilic substitution reactions (Sj 2). Absolute rate theory specifies that the activated complex in the transition state is at equilibrium with the reactants and is formed on provision of the activation (Gibbs) energy, AG. In other words, the energy barrier that the reaction must pass to proceed has to be overcome. The specific rate constant is given by ... 

It is generally agreed that both hydrolysis and condensation occur by adder base-catalyzed bimolecular nucleophilic substitution reactions involving, e.g., Sf Z-Si, S 2 -Si, or S 2 -Si transition states or intermediates. The acid-catalyzed mechanisms are preceded by rapid protonation of the OR or OH substituents bonded to Si, whereas under basic conditions hydroxyl or silanolate anions attack Si directly. Statistical and steric effects are probably most important in influencing the kinetics however. Inductive effects are certainly evident in the hydrolysis of organoalkoxysilanes. 

Hughes and Ingold interpreted second order kinetic behavior to mean that the rate determining step is bimolecular that is that both hydroxide ion and methyl bromide are involved at the transition state The symbol given to the detailed description of the mech anism that they developed is 8 2 standing for substitution nucleophilic bimolecular... 

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). 

Nucleophilic attack by iV-methylaniline is favoured by electron-withdrawing groups on the amide and acyloxyl side chains. A series of / ara-substituted Af-acetoxy-Af-butoxy-benzamides (138) (Table 6) gave a weak but positive Hammett correlation with a constants (p = 0.13, r = 0.86) °. The analogous reactions of pyridine with para-substituted phenacyl halides in methanol afforded a similar Hammett correlation a, p = 0.25) . The bimolecular rate constants for the limited series of Ai-benzoyloxy-A-benzyloxybenzamides (139) in Table 6 correlated strongly with Hammett a constants (p = 1.7, r = 0.97) °. Stabilization of developing carboxylate character supported the computed charge redistribution in the transition state ... 

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... 

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