Concerted reaction bimolecular nucleophilic

Reactions and reactivity of nucleophiles with thiolsulfonates 137 Nucleophilic substitutions of sulfenyl derivatives general considerations 139 Bimolecular substitution at sulfenyl sulfur stepwise or concerted 140 Reversibility in reactions of nucleophiles with cyclic thiolsulfonates 145 Other reactions of thiolsulfonates 147... 

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 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]. 

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. 

For the recombination of a-phenylethyl cations with strong nucleophiles, Richard and Jencks (1984a,b,c) obtained good Y-T correlations with the same r value of 1.15 as observed for the solvolysis (Fig. 35). The p value of -2.7 for the bimolecular substitution reaction of azide ion with 1-phenylethyl derivatives is significantly more positive than the value of p = -5.7 for the solvolysis reaction. This shows that there is a smaller development of positive charge in the transition state for the reaction of azide ion than for solvolysis. It is consistent with a coupled concerted reaction with a transition state in which positive charge development at the benzylic carbon is neutralized by bonding to azide ion. 

Phosphorylated y-picoline was found to react with a series of anionic oxygen nucleophiles in a bimolecular concerted reaction, with a dependence on the pK of the nucleophile that is described by 31ogk/3pK = B uc = 0.25. Figure 1 shows that the bimolecular rate constant for the reaction of this compound with water falls on the correlation line that is defined by the rate constants for reaction of the anionic nucleophiles there is no positive deviation... 

The alternative mechanism, the Sn2 reaction, is a concerted reaction in which the nucleophile approaches from the side of the R groups as the other group (Cl in the example) leaves. In this case the configuration of the molecule is inverted. If the original molecule is optically active, the product has the opposite activity, an effect known as Walden Inversion. The notations SnI and Sn2 refer to the kinetics of the reactions. In the SnI mechanism, the slow step is the first one, which is unimolecular (and first order in CR3CI). In the Sn2 reaction, the process is bimolecular (and second order overall). 

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... 

There are a number of bimolecular nucleophilic elimination reactions like that of equation (5.188) that take place in a single, concerted step. Since the transition state for such an 2 reaction is odd, being isoconjugate with the pentadienate anion (CH2 CH—CH CH—CH2) , while the reactants and products are even, -type groups at the central carbon atoms can exert mesomeric effects in the transition state that are different from those in the reactants or products. Such eliminations are therefore anti-BEP reactions and are classed as EO. ... 

The imidazole-catalysed hydrolysis of polar substituted 2,4-dinitrophenyl acetates (21 X = Cl, OMe) has been investigated at different temperatures. The observed rates correspond to the bimolecular nucleophilic addition of the imidazole at the carboxylic carbon atom followed by a very fast hydrolysis of the (V-acetylimidazole in water. The influence of polar substituents in the acid moiety of the ester molecule on the hydrolysis reaction can be described by an electrostatic dipole-dipole interaction in the same way as the neutral hydrolysis of polar substituted ethyl acetates. By the use of both quantum and classical dynamics, a study of the neutral hydrolysis of 4-methoxyphenyl dichloroacetate (22) in water concluded that the rate-determining step is a proton transfer concerted with formation of a C-O bond. ... 

Bimolecular nucleophilic substitution is a one-step transformation The nucleophile attacks the haloalkane, with simultaneous expulsion of the leaving group. Bond making takes place at the same time as bond breaking. Because the two events occur in concert, we caU this process a concerted reaction. 

The kinetics of the reaction of nucleophiles with primary (and most secondary) haloalkanes are second order, indicative of a bimolecular mechanism. This process is called bimolecular nucleophilic substitution (Sn2 reaction). It is a concerted reaction, one in which bonds are simultaneously broken and formed. Curved arrows are typically used to depict the flow of electrons as the reaction proceeds. 

Partial but not complete loss of optical activity m S l reactions probably results from the carbocation not being completely free when it is attacked by the nucleophile Ionization of the alkyl halide gives a carbocation-hahde ion pair as depicted m Figure 8 8 The halide ion shields one side of the carbocation and the nucleophile captures the carbocation faster from the opposite side More product of inverted configuration is formed than product of retained configuration In spite of the observation that the products of S l reactions are only partially racemic the fact that these reactions are not stereospecific is more consistent with a carbocation intermediate than a concerted bimolecular mechanism... 

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 ... 

The abbreviation E2 conveys the information elim-ination-bimolecular . The reaction is a concerted process in which a nucleophile removes an electrophile at the same time as a leaving group departs. It is bimolecular, since kinetic data indicate that two species are involved in the rate-determining step ... 

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 rate law is consistent with a concerted or one-step mechanism. Because two species, ethoxide ion and tort-butyl bromide, react in this step, the reaction is described as a bimolecular elimination or an E2 reaction. This reaction is quite common when alkyl halides are treated with strong bases. Because many nucleophiles are also quite basic, the E2 reaction often competes with the SN2 reaction. 

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