Reaction rate nucleophilic substitution reactions

In an attempt to combine the two approaches of accelerating organic reactions, a nucleophilic substitution reaction has been carried out in a microemulsion in the presence of a phase transfer agent [51]. The aim of the work was to investigate if a combination of the two approaches would give a reaction rate higher than that obtained in either the microemulsion approach or in a two-phase system using phase transfer catalysis. 

The results obtained in the work on isotopic exchange reactions and nucleophilic substitution reactions, in general, support this conclusion. To illustrate, the rate constants for the isotopic exchange reaction studio by Kla-bunde and Burton (22) are linearly related to the Hammett a constants. Fig. 

Unlike elimination and nucleophilic substitution reactions foimation of oigano lithium compounds does not require that the halogen be bonded to sp hybndized carbon Compounds such as vinyl halides and aiyl halides m which the halogen is bonded to sp hybndized carbon react m the same way as alkyl halides but at somewhat slowei rates... 

The term nucleophilicity refers to the effect of a Lewis base on the rate of a nucleophilic substitution reaction and may be contrasted with basicity, which is defined in terms of the position of an equilibrium reaction with a proton or some other acid. Nucleophilicity is used to describe trends in the kinetic aspects of substitution reactions. The relative nucleophilicity of a given species may be different toward various reactants, and it has not been possible to devise an absolute scale of nucleophilicity. We need to gain some impression of the structural features that govern nucleophilicity and to understand the relationship between nucleophilicity and basicity. ... 

Examples of effects of reactant stmcture on the rate of nucleophilic substitution reactions have appeared in the preceding sections of this chapter. The general trends of reactivity of primaiy, secondary, and tertiaiy systems and the special reactivity of allylic and benzylic systems have been discussed in other contexts. This section will emphasize the role that steric effects can pl in nucleophilic substitution reactions. 

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

Section 23.4 Aiyl halides aie less reactive than alkyl halides in reactions in which C—X bond breaking is rate-detennining, especially in nucleophilic substitution reactions. 

A difficulty that occasionally arises when carrying out nucleophilic substitution reactions is that the reactants do not mix. For a reaction to take place the reacting molecules must collide. In nucleophilic substitutions the substrate is usually insoluble in water and other polar solvents, while the nucleophile is often an anion, which is soluble in water but not in the substrate or other organic solvents. Consequently, when the two reactants are brought together, their concentrations in the same phase are too low for convenient reaction rates. One way to overcome this difficulty is to use a solvent that will dissolve both species. As we saw on page 450, a dipolar aprotic solvent may serve this purpose. Another way, which is used very often, is phase-transfer catalysis ... 

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. 

Additional experimental, theoretical, and computational work is needed to acquire a complete understanding of the microscopic dynamics of gas-phase SN2 nucleophilic substitution reactions. Experimental measurements of the SN2 reaction rate versus excitation of specific vibrational modes of RY (equation 1) are needed, as are experimental studies of the dissociation and isomerization rates of the X--RY complex versus specific excitations of the complex s intermolecular and intramolecular modes. Experimental studies that probe the molecular dynamics of the [X-. r - Y]- central barrier region would also be extremely useful. 

Finally it should be mentioned that a number of nucleophilic substitution reactions of unactivated halides can be made to proceed in bipolar non-protic solvents such as dimethyl sulphoxide (DMSO), Me2S —Oe. No hydrogen-bonded solvent envelope, as in for example MeOH, then needs to be stripped from Ye before it can function as a nucleophile AG is thus much lower and the reaction correspondingly faster. Rate differences of as much as 109 have been observed on changing the solvent from MeOH to Me2SO. Chlorobenzene will thus react readily under these conditions with Me3COe ... 

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 ... 

Nucleophilic substitution reactions in the selenophene series have attracted some interest. Debromination of bromonitro compounds [(50, X = S, Se) and (53, X = S, Se)] with sodium thiophenoxide and sodium selenophen-oxide72 was studied. Selenophene compounds were four times more reactive than the thiophene derivatives. The position of attack, a or /), had very little influence on the rate ratio. The kinetics of the side-chain nucleophilic reactions of selenophene derivatives, shown in Scheme 4, has been reported.7 3... 

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... 

Nucleophilic substitution reactions have rates that vary enormously. For example, the reaction... 

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. 

The interfacial mechanism probably competes to some extent with the extraction mechanism, particularly with the less lipophilic catalysts. The dependence of the rate of many nucleophilic substitution reactions on the stirring rate up to 250-300 rpm and the independence of the reaction rate at higher stirring rates has been taken as evidence for a change over from a predominant interfacial mechanism to an extraction process. The interfacial mechanism is also particularly relevant to base-initiated 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. 

The barrier that the reaction must overcome in order to proceed is determined by the difference in the solvation of the activated complex and the reactants. The activated complex itself is generally considered to be a transitory moiety, which cannot be isolated for its solvation properties to be studied, but in rare cases it is a reactive intermediate of a finite lifetime. The solvation properties of the activated complex must generally be inferred from its postulated chemical composition and conformation, whereas those of the reactants can be studied independently of the reaction. For organic nucleophilic substitution reactions, the Hughes-lngold rales permit qualitative predictions on the behavior of the rate when the polarity increases in a series of solvents, as is shown in Reichardt (Reichardt, 1988). 

The arene oxide valence tautomer of oxepins in principle should undergo nucleophilic substitution reactions (Sn2) which are characteristic of simple epoxides. In reality oxepin-benzene oxide (7) is resistant to attack by hard nucleophiles such as OH-, H20, NH2- and RNH2. Attempts to obtain quantitative data on the relative rates of attack of nucleophiles on (7) in aqueous solution hqye been thwarted by competition from the dominant aromatization reaction. 

Let us now look at some examples to illustrate what we have discussed so far to get a feeling of how structural moieties influence the mechanisms, and to see some rates of nucleophilic substitution reactions of halogenated hydrocarbons in the environment. Table 13.6 summarizes the (neutral) hydrolysis half-lives of various mono-halogenated compounds at 25°C. We can see that, as anticipated, for a given type of compound, the carbon-bromine and carbon-iodine bonds hydrolyze fastest, about 1-2 orders of magnitude faster than the carbon-chlorine bond. Furthermore, we note that for the compounds of interest to us, SN1 or SN2 hydrolysis of carbon-fluorine bonds is likely to be too slow to be of great environmental significance. 

Explain in words what a nucleophilic substitution reaction is. At what kind of atoms do such reactions primarily occur What is(are) the mechanism(s) and the corresponding rate law(s) of such reactions ... 

What are the major factors determining the rates of nucleophilic substitution reactions Q 13.3... 

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