Reaction mechanisms nucleophilic substitution reactions

These observations are easily explained by another simple reaction mechanism, nucleophilic substitution of an alkoxide on silicon (12). In this case, the basic alkoxide oxygens tend to repel the nucleophile, OH, and the bulkier alkyl groups tend to crowd it. Therefore, more highly hydrolyzed silicons are more prone to attack. Because this mechanism would have a pentacoordinated silicon atom in the activated complex, hydrolysis of a polymer would be more sterically hindered than hydrolysis of a monomer. Reesterification would be much more difficult in alkaline solution than in acidic solution, because silanols are more acidic than the hydroxyl protons of alcohols and would be deprotonated and negatively charged at a pH lower than the point at which the nucleophile concentration becomes significant (ii). Thus, although hydrolysis in alkaline solution is slow, it still tends to be complete and irreversible, if extensive polymerization does not occur first. 

The ideas of reaction mechanisms of substitution reactions came primarily from organic chemistry. The first notion was that the initial reaction step is the addition of the reactant to the substrate, yielding a new molecular compound from which the group to be replaced is eliminated. This idea was put forward in 1909 but the relevant publications appeared in 1911. The same year Le Bel suggested that the entry of the nucleophilic reagent and the departure of the leaving group were two independent events in a simultaneous process. 

Because carbocations are key intermediates in many nucleophilic substitution reactions, it is important to develop a grasp of their structural properties and the effect substituents have on stability. The critical step in the ionization mechanism of nucleophilic substitution is the generation of the tricoordinate carbocation intermediate. For this mechanism to operate, it is essential that this species not be prohibitively high in energy. Carbocations are inherently high-energy species. The ionization of r-butyl chloride is endothermic by 153kcal/mol in the gas phase. ... 

In addition to steric effects, there are other important substituent effects which determine both the rate and mechanism of nucleophilic substitution reactions. It was... 

One after the other, step through the sequence of structures corresponding to the three nucleophile substitution reactions shown above (reaction 1, reaction 2, reaction 3). Decide whether loss of Br occurs with or without the assistance of RO /ROH. The nucleophile-assisted and unassisted mechanisms are called Sn2 and SnI mechanisms respectively. Label each reaction as Sn2 or SnI as appropriate. 

C-Methylation products, o-nitrotoluene and p-nitrotoluene, were obtained when nitrobenzene was treated with dimethylsulfoxonium methylide (I)." The ratio for the ortho and para-methylation products was about 10-15 1 for the aromatic nucleophilic substitution reaction. The reaction appeared to proceed via the single-electron transfer (SET) mechanism according to ESR studies. 

Arynes are intermediates in certain reactions of aromatic compounds, especially in some nucleophilic substitution reactions. They are generated by abstraction of atoms or atomic groups from adjacent positions in the nucleus and react as strong electrophiles and as dienophiles in fast addition reactions. An example of a reaction occurring via an aryne is the amination of o-chlorotoluene (1) with potassium amide in liquid ammonia. According to the mechanism given, the intermediate 3-methylbenzyne (2) is first formed and subsequent addition of ammonia to the triple bond yields o-amino-toluene (3) and m-aminotoluene (4). It was found that partial rearrangement of the ortho to the meta isomer actually occurs. 

The substitution of fairly labile nucleophilic groups (halogen, methoxy group) in the 3- and 5-positions, and the noncatalytic substitution of the diazonium group at C-4, are to be considered as nucleophilic substitution reactions. As in the benzenoid series, these reactions are believed to proceed in the former case by the Sx2 mechanism and in the latter by the SnI mechanism. 

An a-halosulfone 1 reacts with a base by deprotonation at the a -position to give a carbanionic species 3. An intramolecular nucleophilic substitution reaction, with the halogen substituent taking the part of the leaving group, then leads to formation of an intermediate episulfone 4 and the halide anion. This mechanism is supported by the fact that the episulfone 4 could be isolated. Subsequent extrusion of sulfur dioxide from 4 yields the alkene 2 ... 

From this and nearly a dozen other series of similar reactions, workers concluded that the nucleophilic substitution reaction of a primary or secondary alkyl halide or tosylate always proceeds with inversion of configuration. (Tertiary alkyl halides and tosylates, as we ll see shortly, give different stereochemical results and react by a different mechanism.)... 

A mechanism that accounts for both the inversion of configuration and the second-order kinetics that are observed with nucleophilic substitution reactions was suggested in 1937 by E. D. Hughes and Christopher Ingold, who formulated what they called the SN2 reaction—short for substitution, nucleophilic, birnolecu-lar. (Birnolecular means that two molecules, nucleophile and alkyl halide, take part in the step whose kinetics are measured.)... 

Acidic ether cleavages are typical nucleophilic substitution reactions, either SN1 or Sn2 depending on the structure of the substrate. Ethers with only primary and secondary alkyl groups react by an S 2 mechanism, in which or Br attacks the protonated ether at the less hindered site. This usually results in a selective cleavage into a single alcohol and a single alkyl halide. For example, ethyl isopropyl ether yields exclusively isopropyl alcohol and iodoethane on cleavage by HI because nucleophilic attack by iodide ion occurs at the less hindered primary site rather than at the more hindered secondary site. 

Application of the principle of microscopic reversibility can be used to eliminate a mechanism suggested at one time for the nucleophilic substitution reactions of square-planar platinum(II) complexes. For the sake of specificity, we take PtCl - as a typical... 

Several distinct mechanisms are possible for aliphatic nucleophilic substitution reactions, depending on the substrate, nucleophile, leaving group, and reaction conditions. In all of them, however, the attacking reagent carries the electron pair with it, so that the similarities are greater than the differences. Mechanisms that occur at a saturated carbon atom are considered first. By far the most common are the SnI and Sn2 mechanisms. 

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 entities. The reaction is said to be second order, and we signify this by placing a 2 in the name of the reaction. 

The mechanistic aspects of nucleophilic substitution reactions were treated in detail in Chapter 4 of Part A. That mechanistic understanding has contributed to the development of nucleophilic substitution reactions as important synthetic processes. Owing to its stereospecificity and avoidance of carbocation intermediates, the Sw2 mechanism is advantageous from a synthetic point of view. In this section we discuss... 

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 ... 

According to this mechanism, there is a first-order dependence on both the concentration of [ A B] and B, and the reaction is called an SN2 process (substitution, nucleophilic, second-order). Although many nucleophilic substitution reactions follow one of these simple rate laws, many others do not. More complex rate laws such as... 

Molecular transport junctions differ from traditional chemical kinetics in that they are fundamentally electronic rather than nuclear - in chemical kinetics one talks about nucleophilic substitution reactions, isomerization processes, catalytic insertions, crystal forming, lattice changes - nearly always these are describing nuclear motion (although the electronic behavior underlies it). In general the areas of both electron transfer and electron transport focus directly on the charge motion arising from electrons, and are therefore intrinsically quantum mechanical. 

The reverse emulsion stabilized by sodium dodecylsulfate (SDS, R0S03 Na+) retards the autoxidation of dodecane [24] and ethylbenzene [21,26,27]. The basis for this influence lies in the catalytic decomposition of hydroperoxides via the heterolytic mechanism. The decay of hydroperoxides under the action of SDS reverse micelles produces olefins with a yield of 24% (T=413 K, 0.02mol L 1 SDS, dodecane, [ROOH]0 = 0.08 mol L 1) [27], The thermal decay gives olefins in negligible amounts. The decay of hydroperoxides apparently occurs in the ionic layer of a micelle. Probably, it proceeds via the reaction of nucleophilic substitution in the polar layer of a micelle. 

Classical Michaelis-Arbuzov or Michaelis-Becker approaches toward formation of C-P bonds involving aromatic carbon sites are (understandably) not generally feasible. Nucleophilic substitution reactions on aromatic carbon proceed only under particular circumstances relating to the nature of attendant additional substituents, and then often with mechanisms quite different from those observed in... 

So the tertiary halide reacts by a different mechanism, which we call SnI- It s still a nucleophilic substitution reaction (hence the S and the N ) but this time it is a unimolecular reaction, hence the 1 . The rate-determining step during reaction is the slow unimolecular dissociation of the alkyl halide to form a bromide ion and a carbocation that is planar around the reacting carbon. 

Figure 3.14 Mechanism of nucleophilic substitution reactions of lignin during sulfite pulping.

Many theories have been put forward to explain the mechanism of inversion. According to the accepted Hugles, Ingold theory aliphatic nucleophilic substitution reactions occur eigher by SN2 or SN1 mechanism. In the SN2 mechanism the backside attack reduces electrostatic repulsion in the transition state to a minimum when the leaving meleophile leaves the asymmetric carbon, naturally an inversion of configuration occurs at the central carbon atom. 

The first evidence that an elimination-addition mechanism could be important in nucleophilic substitution reactions of alkanesulfonyl derivatives was provided by the observation (Truce et al., 1964 Truce and Campbell, 1966 King and Durst, 1964, 1965) that when alkanesulfonyl chlorides RCH2S02C1 were treated in the presence of an alcohol R OD with a tertiary amine (usually Et3N) the product was a sulfonate ester RCHDS020R with exactly one atom of deuterium on the carbon alpha to the sulfonyl group. Had the ester been formed by a base-catalysed direct substitution reaction of R OD with the sulfonyl chloride there would have been no deuterium at the er-position. Had the deuterium been incorporated by a separate exchange reaction, either of the sulfonyl chloride before its reaction to form the ester, or of the ester subsequent to its formation, then the amount of deuterium incorporated would not have been uniformly one atom of D per molecule. The observed results are only consistent with the elimination-addition mechanism involving a sulfene intermediate shown in (201). Subsequent kinetic studies... 

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