Mechanism bimolecular nucleophilic substitution

Substitution nucleophilic bimolecular (Sn2) mechanism (Sec tions 4 12 and 8 3) Concerted mechanism for nucleophilic substitution in which the nucleophile attacks carbon from the side opposite the bond to the leaving group and assists the departure of the leaving group... 

To make this more specific. Table 5-3 gives examples of several reaction types that fit the RIP pattern. Consider nucleophilic substitution on saturated carbon. The concerted mechanism is the one-step bimolecular 5, 2 process ... 

It is quite reasonable to expect the bimolecular two-stage mechanism Sj Ar ) to predominate in most aromatic nucleophilic substitutions of activated substrates. However, only in rare instances is there adequate evidence to rule out the simultaneous occurrence or predominance of other mechanisms. The true significance of the alternative mechanisms in azines needs to be determined by trapping the intermediates or by applying modem separation and characterization methods to the identification of at least the major portion of the products, especially in kinetic studies. 

The synchronous bimolecular mechanism for aromatic nucleophilic substitution involves unfavorable geometry (bonds made and broken are both in the plane of the ring and backside attack is not possible) and unfavorable energetics (one high-energy step is required... 

In most cases the alkoxide or phenoxide 1 reacts with the alkyl halide 2 by a bimolecular nucleophilic substitution mechanism ... 

This intermediate is similar to those encountered in the neighboring-group mechanism of nucleophilic substitution (see p. 404). The attack of W on an intermediate like 2 is an Sn2 step. Whether the intermediate is 1 or 2, the mechanism is called AdE2 (electrophilic addition, bimolecular). 

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 ... 

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. 

The basic classification of nucleophilic substitutions is founded on the consideration that when a new metal complex is formed through the breaking of a coordination bond with the first ligand (or water) and the formation of a new coordination bond with the second ligand, the rupture and formation of the two bonds can occur to a greater or lesser extent in a synchronons manner. When the mpture and the formation of the bonds occur in a synchronous way, the mechanism is called substitution nucleophilic bimolecular (in symbols Sn2). On the other extreme, when the rupture of the first bond precedes the formation of the new one, the mechanism is called substitution nucleophilic unimolecular (in symbols SnI). Mechanisms Sn2 and SnI are only limiting cases, and an entire range of intermediate situations exists. 

Similar qualitative relationships between reaction mechanism and the stability of the putative reactive intermediates have been observed for a variety of organic reactions, including alkene-forming elimination reactions, and nucleophilic substitution at vinylic" and at carbonyl carbon. The nomenclature for reaction mechanisms has evolved through the years and we will adopt the International Union of Pure and Applied Chemistry (lUPAC) nomenclature and refer to stepwise substitution (SnI) as Dn + An (Scheme 2.1 A) and concerted bimolecular substitution (Sn2) as AnDn (Scheme 2.IB), except when we want to emphasize that the distinction in reaction mechanism is based solely upon the experimentally determined kinetic order of the reaction with respect to the nucleophile. 

Students of reaction mechanism will recognize intuitively that the difference between the narrow and broad borderline regions observed for nucleophilic substitution of azide ion at secondary and tertiary carbon (Fig. 2.2) is due to the greater steric hindrance to bimolecular nucleophilic substitution at the tertiary carbon. This leads to a large difference in the effects of an a-Me group on (s ) for the stepwise solvolysis and s ) for concerted bimolecular nucleophilic... 

The yield of the nucleophilic substitution product from the stepwise preassociation mechanism k[ = k. Scheme 2.4) is small, because of the low concentration of the preassociation complex (Xas 0.7 M for the reaction of X-2-Y). Formally, the stepwise preassociation reaction is kinetically bimolecular, because both the nucleophile and the substrate are present in the rate-determining step ( j). In fact, these reactions are borderline between S l and Sn2 because the kinetic order with respect to the nucleophile cannot be rigorously determined. A small rate increase may be due to either formation of nucleophile adduct by bimolecular nucleophilic substitution or a positive specific salt effect, whUe a formally bhnole-cular reaction may appear unimolecular due to an offsetting negative specific salt effect on the reaction rate. 

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. 

The mechanism involves nucleophilic addition to a Z-substituted olefin followed by an intramolecular bimolecular nucleophilic substitution. Several side reactions also occur. Discuss the chemistry involved in this reaction, pointing out substituent effects at each stage. 

The HDO and isomerization reactions were previously described as bimolecular nucleophilic substitutions with allylic migrations-the so-called SN2 mechanism (7). The first common step is the fixation of the hydride on the carbon sp of the substrate. The loss of the hydroxyl group of the alcohols could not be a simple dehydration -a preliminar elimination reaction- as the 3-butene-l-ol leads to neither isomerization nor hydrodehydroxyl at ion (6). The results observed with vinylic ethers confirm that only allylic oxygenated compounds are able to undergo easily isomerization and HDO reactions. Moreover, we can note that furan tetrahydro and furan do not react at all even at high temperature (200 C). 

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,... 

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,... 

The usually considered monomolecular mechanism of substitution implies that one-electron reduction activates a substrate sufficiently so that it could dissociate with no further assistance from a nucleophile. The next steps of the reaction consist of transformations of the resultant radical. However, in substrates having sp3 carbon as a reaction center, the influence of the leaving group has been fixed (Russell Mudryk 1982a, 1982b). This led to the formulation of the SRN2 bimolecular mechanism of radical-nucleophilic substitution. In this mechanism, the initial products of single-electron transfer are combined to form the... 

Now we get to the meaning of 2 in SN2. Remember from the last chapter that nucleophilicity is a measure of kinetics (how fast something happens). Since this is a nucleophilic substitution reaction, then we care about how fast the reaction is happening. In other words, what is the rate of the reaction This mechanism has only one step, and in that step, two things need to find each other the nucleophile and the electrophile. So it makes sense that the rate of the reaction will be dependent on how much electrophile is around and how much nucleophile is around. In other words, the rate of the reaction is dependent on the concentrations of two compounds. Therefore, we call the reaction bimolecular and we put a 2 in the name of the reaction. 

Experimental data from nucleophilic substitution reactions on substrates that have optical activity (the ability to rotate plane-polarized light) shows that two general mechanisms exist for these types of reactions. The first type is called an S 2 mechanism. This mechanism follows second-order kinetics (the reaction rate depends on the concentrations of two reactants), and its intermediate contains both the substrate and the nucleophile and is therefore bimolecular. The terminology S 2 stands for substitution nucleophilic bimolecular. ... 

The Sn2 reaction involves the attack of a nucleophile from the side opposite the leaving group and proceeds with exclusive inversion of configuration in a concerted manner. In contrast to the popular bimolecular nucleophilic substitution at the aliphatic carbon atom, the SN2 reaction at the vinylic carbon atom has been considered to be a high-energy pathway. Textbooks of organic chemistry reject this mechanism on steric grounds [175]. 

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... 

This inversion is called the Walden Inversion and the mechanism called SN2 mechanism. The SN stands for substitution nucleophilic. The 2 signifies that the rate of reaction is second order or bimolecular and depends on both the concentration of the nucleophile and the concentration of the alkyl halide. The SN2 mechanism is possible for the nucleophilic substitutions of primary and secondary alkyl halides, but is difficult for tertiary alkyl halides. We can draw a general mechanism (Fig. F) to account for a range of alkyl halides and charged nucleophiles. 

Reactions involving four electrons and three centres can include the formation of a chemical bond at the expenses of another bond which is consequently broken. A large variety of reactions can be explained by such a mechanism, by way of example attention here will be focused on bimolecular nucleophilic substitutions (Sn2) and proton transfers. Typically a four electron - three centre unit AXB, in which the central atom X could be a hydrogen or a carbon atom, is mainly described by the resonance of the following three classical VB structures... 

This rate law is consistent with mechanism (3), in which the bond to the leaving group (chloride) is broken and the bond to the nucleophile (hydroxide) is formed simultaneously, in the same step. A reaction that occurs in one step is termed a concerted reaction. Because two species (hydroxide ion and chloroethane) are involved in this step, the step is said to be bimolecular. This reaction is therefore described as a bimolecular nucleophilic substitution reaction, or an SN2 reaction. 

---