Nucleophilic displacement reactions calculation

The rate of nucleophilic displacement reactions run under PT-conditions is treated as the sum of the rate limiting homogeneous reaction RX(org) + Y (org) RY(org) + X (org) and a rapid equilibrium exchange of anions between the two phases. This equilibrium depends on their selectivity coefficient. The various factors affecting the kinetics observed in PTC reactions are discussed and the expected behavior calculated. 

The n.m.r. spectra of a series of 2-substituted tropones have been analysed, using a combination of methods the published assignments for tropolone acetate were corrected and substituent parameters were defined and compared with corresponding parameters for monosubstituted benzenes. Studies of nucleophilic displacement reactions of chlorine or of tosyloxy-groups in troponoids" reveal that with either Mc2NH in DMSO or NaSMe in ethanol, substituents in the 3-position were replaced faster than those in 2- or 4-positions neither M.O. calculations nor e.s.r. studies predicted this. 1,3-Diaza-azulenes are obtained in acceptable yields from the reaction of benzamidine and 2-(ethylthio)tropone in related work, the interesting structure (3) has been recognized. 

Studies of gas-phase S"n2 reactions at sp carbon have been made by Fourier transform ion cyclotron resonance mass spectrometry (FTlCRMS) and complemented by both semiempirical and ab initio MO calculations. The particular processes of interest involved intramolecular reactions in which neutral nucleophiles displace neutral leaving groups within cationic substrates, e.g. A-(2-piperidinoethyl)-2,4,6-triphenylpyridinium cation (59), in which the piperidino moiety is the nucleophile and 2,4,6-triphenylpyridine (60) is the leaving group. No evidence has been obtained for any intermolecular gas-phase 5) 2 reaction involving a pyridine moiety as a leaving group. The quantum mechanical treatments account for the intramolecular preference. 

It is accepted that the acmal nucleophile in the reactions of oximes with OPs is the oximate anion, Pyr+-CH=N-0 , and the availability of the unshared electrons on the a-N neighboring atom enhances reactions that involve nucleophilic displacements at tetravalent OP compounds (known also as the a-effect). In view of the fact that the concentration of the oximate ion depends on the oxime s pATa and on the reaction pH, and since the pKs also reflects the affinity of the oximate ion for the electrophile, such as tetra valent OP, the theoretical relationship between the pATa and the nucleophilicity parameter was analyzed by Wilson and Froede . They proposed that for each type of OP, at a given pH, there is an optimum pK value of an oxime nucleophile that will provide a maximal reaction rate. The dissociation constants of potent reactivators, such as 38-43 (with pA a values of 7.0-8.5), are close to this optimum pK, and can be calculated, at pH = 7.4, from pKg = — log[l//3 — 1] -h 7.4, where is the OP electrophile susceptibility factor, known as the Brpnsted coefficient. If the above relationship holds also for the reactivation kinetics of the tetravalent OP-AChE conjugate (see equation 20), it would be important to estimate the magnitude of the effect of changes in oxime pX a on the rate of reactivation, and to address two questions (a) How do changes in the dissociation constants of oximes affect the rate of reactivation (b) What is the impact of the /3 value, that ranges from 0.1 to 0.9 for the various OPs, on the relationship between the pKg, and the rate of reactivation To this end, Table 3 summarizes some theoretical calculations for the pK. ... 

In the case of the tricarbonylarene metals, enhancement of nucleophilic substitution relative to the free arene is reported 106), In contrast to earlier reports 106) Friedel-Crafts acylation of tricarbonylbenzene chromium occurs under mild conditions 18), Molecular-orbital calculations of the 7r-electron activation energies for these reactions 63) confirm enhanced nucleophilic reactivity and suggest electrophilic activity similar to that of the free arene. The nucleophilic displacement of halide by methoxide ion... 

Exercise 14-20 In the hydrolysis of chlorobenzene-1-14C with AM aqueous sodium hydroxide at 340°, the products are 58% benzenol-1-14C and 42% benzenol-2-14C. Calculate the percentage of reaction proceeding (a) by an elimination-addition mechanism, and (b) by direct nucleophilic displacement. Would you expect the amount of direct displacement to increase, or decrease, if the reaction were carried out (a) at 240° and (b) with lower concentrations of sodium hydroxide Give your reasoning. 

With an E° value of —0.75 V, entry no. 19 of Table 17, reaction between alkyl halides and alkyllithium compounds, represents a strongly exergonic electron-transfer reaction which is expected to proceed at a diffusion-controlled tate. Experimental rate constants are not available, but such reactions are qualitatively known to be very fast. As we proceed to entry no. 21, two model cases of the nucleophilic displacement mechanism, it can first be noted that the nosylate/[nosylate]- couple is electrochemically reversible the radical anion can be generated cathodically and is easily detected by esr spectroscopy (Maki and Geske, 1961). Hence its E° = —0.61 V is a reasonably accurate value. E° (PhS /PhS-) is known with considerably less accuracy since it refers to an electrochemically irreversible process (Dessy et al., 1966). The calculated rate constant is therefore subject to considerable uncertainty and it cannot at present be decided whether the Marcus theory is compatible with this type of electron-transfer step. In the absence of quantitative experimental data, the same applies to entry no. 22 of Table 17. For the PhS-/BuBr reaction we again suffer from the inaccuracy of E° (PhS /PhS-) what can be concluded is that for an electron-transfer step to be feasible the higher E° value (—0.74 V) should be the preferred one. The reality of an electron-transfer mechanism has certainly been strongly disputed, however (Kornblum, 1975). 

In the work from my own laboratory over the last ten years or so we have used a combination of experimental measurements on selected reactants and model calculations relating to a-deu-terium effects on the rates of solvolytic nucleophilic displacement on carbon to try to correlate all known results on such reactions in terms of Scheme I. 

Calculation of EM. The reference intramolecular reaction is nucleophilic attack by the anion of a carboxylic acid of pK, 3.15 on 2-phenoxy-l,3,2-dioxaphosphorinan-2-oxide. The rate constant for this reaction can be calculated as 7.67 x 10-10 dm3 mol 1 s-1 at 39° using the formula derived by Khan and Kirby (1970), and allows the direct calculation of the EM for the corresponding intramolecular reaction (COO-—P3—6n of A.5.5). The EM is assumed to be the same for the corresponding endocyclic reaction of the diphenyl ester anion (A.5.6), and has been shown not to differ significantly for endocyclic and exocyclic displacements (Bromilow etal., 1972)... 

To conclude this section on the effect of solvent on a-nucleophilicity, we refer to the current, rather controversial, situation pertaining to gas-phase smdies and the a-effect. As reported in our review on the a-effect and its modulation by solvent the gas-phase reaction of methyl formate with HOO and HO , which proceeds via three competitive pathways proton abstraction, nucleophilic addition to the carbonyl group and Sat2 displacement on the methyl group, showed no enhanced nucleophilic reactivity for HOO relative to This was consistent with gas-phase calculational work... 

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