Nucleophilic substitution reactions first-order rate equation

The elimination reaction of f-butyl bromide happens because the nucleophile is basic You will recall from Chapter 12 that there is some correlation between basicity and nucleophilicity strong bases are usually good nucleophiles. But being a good nucleophile doesn t get hydroxide anywhere in the substitution reaction, because it doesn t appear in the first-order rate equation. But being a good base does get it somewhere in the elimination reaction, because hydroxide is involved in the rate-determining step of the elimination, and so it appears in the rate equation. This is the mechanism. 

The HMPA adducts of trimethylchlorosilane (32, R = Me) and trimethylbromosilane were isolated and found to be ionic as shown. If the first step shown in equation 14 is a pre-equilibrium, the observed order for substitution is first order as expected. For racemization, the rate-limiting step is invertive attack of the second HMPA molecule on 32, such that the reaction is second-order overall with respect to nucleophile. 

Equation [2] illustrates a similar nucleophilic substitution reaction with a different alkyl halide, (CH3)3CBr, which also leads to substitution of Br by CH3COO . Kinetic data show that this reaction rate depends on the concentration of only one reactant, the alkyl halide that is, the rate equation is first order. This suggests a two-step mechanism in which the rate-determining step involves the alkyl halide only. 

Originally the difference between unimolecular and bimolecular substitution reactions was deduced from kinetic studies on a wide range of reagents. It was observed that for some reactions the overall rate of substitution depended only upon the concentration of the substrate, i.e. the species undergoing substitution, and that the rate was independent of the concentration of the attacking species, i.e. the nucleophile. The reaction, therefore, is a first order reaction that is, the sum of the indices of the concentration of the reagents in the rate equation equals one. These reactions are called unimolecular nucleophilic substitution reactions, and are given the label SN1. 

The reactions of P-donor nucleophiles with the metal carbonyl cluster Rh4COi2 have been studied over a considerable time period.It is widely accepted that the reaction is associative. This latest investigation is aimed at quantifying the effects of the electronic and steric properties of the nucleophiles upon the kinetic parameters. A rapid substitution reaction step using an excess of the nucleophile was monitored by SF spectrophotometry. Second-order rate constants were obtained from the variation of the pseudo-first-order rate constants with nucleophile concentration. Contributions to these constants from the properties steric effect, TT-activity, and, in addition, an aryl effect of the nucleophiles were assessed in a multi-parameter equation. The outcome is a successful understanding of the relative reactivities of many P-donors toward the rhodium cluster. The data were also represented by a three-dimensional potential energy surface. 

Theoretical studies of the gas-phase hydrolysis or methanolysis of methylsul-fonyl chloride indicated a concerted Sn2 process involving a four-membered cyclic transition state. The tertiary amine-catalysed hydrolysis of benzenesul-fonyl chloride was shown to be inhibited by chloride ion and a nucleophilic mechanism of catalysis was favoured. Kinetic studies" of the solvolysis of p-substituted benzenesulfonyl chlorides in aqueous binary mixtures with acetone, methanol, ethanol, acetonitrile and dioxime showed that the reactions were third order processes, with first order rate constants determined mainly by the molar concentrations of the protic solvent, so that the reaction rates appear to be dominated by solvent stoichiometry. The solvolyses in methanol and ethanol yield both an alcoholysis (ap) and a hydrolysis product (hp). Solvolyses of electron-rich arylsulfonyl chlorides, under neutral or acidic conditions, exhibited surprising maxima in solvent-dependent S values as defined by Equation 15. 

If the first-order or pseudo-first-order rate constant for a reaction under a specific reaction condition is 0.035 sec-, then such a reaction is completed almost 50% within 20 sec — an average minimum time to obtain first data point on the vs. t plot in a conventional UV-visible spectrophotometric technique. In order to test reliable and satisfactory fit of observed kinetic data for such reactions to Equation 7.24, the experimentally determined kinetic data (A b, vs. t) for a typical kinetic run on the nucleophilic substitution reaction of pyrrolidine with phenyl salicylate are shown in Table 7.7 in which the rate of reaction remains strictly first order for the reaction period of 53 half-lives. 3 The observed kinetic data in Table 7.7 have been treated with Equation 7.24 using the nonlinear least-squares technique. The kinetic parameters (k, S pp, and AJ calculated using different reaction time ranges (t to ts3, where subscripts n and 53 represent half-lives of the reaction and n < 53) with n = 0.8, 1.8, 2.4, and 3.6, are summarized in Table 7.7. The extent of reliability of the data fit to Equation 7.24 is evident from the values of and standard deviations associated with the calculated kinetic parameters (Table 7.7). A similar calculation has been carried out on kinetic data of Table 7.1 using Equation 7.25, and the results obtained are summarized in Table 7.8. 

Pseudo-first-order rate constants (k bs) for the intramolecular general base-catalyzed nucleophilic substitution reactions of ionized phenyl salicylate with ROH, R = CH3 9, CHjCHj o, HOCH2CH2 , and ROH = glucose in mixed aqueous solvents within the ROH content or concentration range of varying values, have been found to fit to the following empirical equation ... 

Chemical research, starting in the 1890s, has shown that nucleophilic substitution reactions can involve two types of mechanisms. The reaction between chloromethane and the hydroxide ion (equation 27.1) has a rate law that is first order in both the nucleophile and the electrophile. That is,... 

Kinetics of reactions of cyclic secondary amines with benzohydrazonyl halides (31) have been measured in benzene51 at 30 °C. The products result from nucleophilic substitution at the halo-carbon via an associative addition-elimination mechanism. For X = Cl or Br, the rate equation has significant terms that are both first and second order in amine, whereas two amine molecules are essential for the fluoro compounds to react. 

Kinetic studies show that hydrolysis of 1-organyl- and 1-alkoxysilatranes in neutral aqueous solutions is a first-order reaction catalyzed by the formed tris(2-hydroxyalkyl)amine13 294. As a rule, electron release and steric effects of the substituent X hinder the reaction. However, the hydrolytic stability of 1-methylsilatrane is just below that of 1-chloromethylsilatrane294. Successive introduction of methyl groups into the 3, 7 and 10 sites of the silatrane skeleton13,294 and substitution with ethyl group on C-459 retard sharply the hydrolysis rate. It was proposed294 that nucleophilic attack at silicon by water proceeds via formation of the four-centered intermediate 57 (equation 56). 

Rates are thus expected to show the first-order dependence on the concentration ofT added, and the rate enhancement (RE, Equation 9.8) for a given concentration of T is easily shown to be the corresponding product ratio [Q] / [P ]. Behaviour of this kind occurs, for example, in the solvolysis of2-propyl tosylate in aqueous ethanol [22]. Products are those of nucleophilic substitution - a mixture of 2-propanol and 2-propyl ethyl ether. When the reaction is rim in the presence of sodium azide, an excellent nucleophile, rates of disappearance of the tosylate are enhanced and 2-propyl azide is also formed. The relationships between amounts of azide product and rate enhancements for a series of different azide concentrations were accurately described by Equations 9.5-9.8. 

Sj l reaction (Sections 6.9, 6.10, 6.12, 6.13, and 6.18B) Literally, substitution nucleophilic unimolecular. A multistep nucleophilic substitution in which the leaving group departs in a unimolecular step before the attack of the nucleophile. The rate equation is first order in substrate but zero order in the attacking nucleophile. 

The systematic measurements of reaction rates of nucleophilic substitutions have shown that, depending on the structure of reactants, the reaction kinetics can follow either the first or the second order rate law. The chemical reaction rate can be expressed by kinetics equations in which the main parameter is the reaction coefficient (rate constant) k ... 

Obviously, 74 cannot be formed if iodide does not react with 66, but the collision of iodide with the cation is very fast and ionization of 64 is quite slow. Therefore, the rate of the overall process is determined by the rate of slow first step (known as the rate limiting step). A reaction that follows this rate equation is said to be unimolecular (for aU practical purposes, an ionization reaction) or first order (see Chapter 7, Section 7.11.1, for first-order reactions). The conversion of 64 to 74 is therefore a unimolecular, nucleophilic substitution and it is given the descriptor 8 1. This is an Sffl reaction. Note that unimolecular or first order in this case indicates a slow ionization reaction to form a carboca-tion intermediate. 

The most reasonable interpretation (there have been many) is to consider the hydroxide or cyanide as forming first an sp -hybridized carbon atom (a pseudobase or Reissert-type adduct, respectively) and then being transmitted from carbon to metal ion. In other words, the change in reactivity of an N-heterocycle on coordination to a metal ion is akin to that of the same N-heterocycle on classical quaternization by an organic agent such as methyl iodide. The unusual rate equation [Eq. (67) or (68)] involving the nucleophile s concentration in first- and second-order terms arises because the rates of these reactions (apparently hydrolysis or substitution by cyanide at the metal ion) are actually controlled by rates of reaction at the ligand (27 28). 

Increasing or decreasing the concentration of the nucleophile has no measurable effect on the rate. The rate equation is said to be first order, because the rate is linearly dependent on the concentration of only one compound. In such cases, the mechanism must exhibit a slow step in which the nucleophile does not participate. Because that step involves only one chemical entity, it is said to be unimolecular. Ingold and Hughes coined the term Sj l to refer to unimolecular substitution reactions ... 

Other terms that he invented include the system of classification for mechanisms of aromatic and aliphatic substitution and elimination reactions, designated SN1, SN2, El, and E2. "S" and "E" refer to substitution and elimination, respectively, "N" to nucleophilic, and "1" and "2" to "molecularity," or the number of molecules involved in a reaction step (not kinetic order, having to do with the equation for reaction rate and the concentration of reactants). Ingold first introduced some of these ideas in 1928 in a... 

Rate data for the Menshutkin reaction between strongly activated Z-substituted benzyl / -toluenesulfonates and Y-substituted lV,lV-dimethylanilines in MeCN at 35 °C fit the equation kohs = h +k2 [DMA], which is consistent with concurrent first- and second-order processes.The 5 nI constant ki is unaffected by changing the nucleophile and conforms to Yukawa-Tsuno treatment with p — -5.2 and r — 1.3. The 5 n2 constant k2 was increased by electron-donating substituents in the nucleophile and showed upward curvature when subjected to the Brown a + treatment. 

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