One-step concerted mechanism

As mentioned above, the replacement of bromine or chlorine in haloforms by fluorine decreases the rate coefficient of carbanion formation, k. The overall rate of basic hydrolysis, however, increases because fluorine stabilizes the carbene intermediate. Because of the increase in /cj, the ratio /ca/it-i of halo-forms CHX2F is in the order of 2-10. 

KINETIC DATA FOR THE BASIC HYDROLYSIS OF HALOFORMS CHXF, IN AQUEOUS SOLUTION  

14) exceeds the isotope effects found in trihalocarbanion formation (p. 387), a fact which is also consistent with a concerted mechanism. 

Seyferth et have studied the kinetics of the reaction of phenylfbromo-dichloromethyl)mercury with olefins, in which CCI2 is transferred to the olefins with formation of gew-dichlorocyclopropanes in high yield. The initial rates are essentially independent of olefin concentration and proportional to the first power of the mercurial concentration. They do, however, show a small dependence on the nature of the olefin, the observed reaction rates decreasing as the nucleophilicity of the olefin decreases (Table 11). 

KINETICS OF OLEFIN-PhHgCClaBr REACTIONS AT 39.0°C IN BENZENE SOLUTION  

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The synthesis of tetrazole proceed through a one-step concerted mechanism. 

The stereochemistry of the dienophile is preserved in the product, the substituted cyclohexene. This fact is consistent with a one-step concerted mechanism. 

To illustrate, suppose that methane chlorination appeared to proceed by way of a one-step concerted mechanism ... 

Pericyclic reactions are a class of reactions that include some of the most powerful synthetically useful reactions such as the Diels-Alder reaction. Pericyclic reactions often proceed with simultaneous reorganization of bonding electron pairs and involve a cyclic delocalized transition state. They differ from ionic or free radical reactions as there are no ionic or free radical intermediates formed during the course of the reaction. They proceed by one-step concerted mechanisms and have certain characteristic properties (although there are some exceptions to all these rules). 

Studies of the kinetic parameters indicate that it takes place by a one-step concerted mechanism and that the norcaradiene is in fact a transition complex having a partial C(2) - C(7) bond and elongated HC-CO bonds [152]. 

Reaction occurs either by a one-step concerted mechanism, or via aziridine-1-thiones which isomerize to the 2-thiazolines under the reaction conditions. However, direct evidence for the formation of these intermediates has yet to be produced. ... 

A one-step, concerted mechanism must retain stereochemistry... 

A computational study on the mechanism and kinetics of the pyrolysis of 2-nitrophenyl azide is in agreement with a one-step concerted mechanism (63)-(65) and turns down various intermediates which have been discussed in the literature. The OT-xylylene moiety has been applied as a rigid spacer in intramolecular glycoside bond formation This application is highly face-selective and efficient in generating 14- and 15-membered macrocycles. 

The characteristics of Sn2 and SnI reactions are compared in Table 9.3. Remember that the 2 in Sn2 and the 1 in SnI refer to the molecularity of the reaction (the number of molecules involved in the transition state of the rate-determining step), and not to the number of steps in the mechanism. In fact, the opposite is true an Sn2 reaction proceeds by a one-step concerted mechanism, whereas an SnI reaction proceeds by a two-step mechanism with a carbocation intermediate. 

The pure cis addition of the reactants can be easily explained by a one-step concerted mechanism. However, an added restriction becomes necessary to explain the other two intermediates. Here, a rotation around a single bond in the intermediate may alter the geometry of the product. For this not to happen, a partial-bond formation may be invoked. Thus, in the case of a zwitterionic intermediate 193, only partial charges may be developed at the termini. Similarly, with the diradical form 194 only partial radical character may be invoked. 

Xr to Z is a nucleophilic association, thus symbolized by A . This association does not involve the two carbon atoms which are core-atoms (the two C atoms will become double bonded). If Z = H, the symbol A j, indicates that the hydron (proton, deuteron, or triton) dissociates from one of two carbon atoms ( core-atoms ). Absence of punctuation tells us that the one step ( concerted ) mechanism has taken place. 

The one step concerted mechanism, which does not include free radicals, is encountered in p eliminations of olefins, especially when metals of higher atomic weights are involved - ... 

The mechanism of the D-A rxn is believed to be a one-step, concerted, non-synchronous process. 

There are, broadly speaking, three possible mechanisms that have been considered for the uncatalyzed Diels-Alder reaction. In mechanism a there is a cyclic six-centered transition state and no intermediate. The reaction is concerted and occurs in one step. In mechanism b, one end of the diene fastens to one end of the dienophile first to give a diradical, and then, in a second step, the other ends become fastened. A diradical formed in this manner must be a singlet that is, the... 

By far the commonest elimination mechanism is the one-step concerted (E2) pathway exhibiting, e.g. for the base-induced elimination of HBr from the halide RCH2CH2Br (21), the rate law ... 

More than one mechanism can account for the experimental observation of the Diels-Alder reaction.521,522,528 However, most thermal [4 + 2]-cycloadditions are symmetry-allowed, one-step concerted (but not necessarily synchronous) process with a highly ordered six-membered transition state.529 Two-step mechanisms with the involvement of biradical or zwitterion intermediates can also be operative.522,528... 

The retention of olefin geometry in the oxidation of cis and trans alkenes408 suggests a one-step concerted [2 + 2]-cycloaddition process, suprafacial in the alkene, and antarafacial in singlet oxygen.363 The strong solvent dependence, however, observed in many cases, points to a stepwise mechanism involving a perepoxide intermediate 403,409... 

E2 mechanism (Sections 8.3, 8.4) An elimination mechanism that goes by a one-step concerted process, in which both reactants are involved in the transition state. E2 is an abbreviation for Elimination Bimolecular. ... 

Of the 18 systems, some of which are unstable and must be generated in the reaction has been accomplished for at least 15, but not in all cases with a carbon-carbon double bond (the reaction also can be carried out with other double bonds ). Not all aUcenes undergo 1,3-dipolar addition equally well. The reaction is most successful for those that are good dienophUes in the Diels-Alder reaction (15-60). The addition is stereospecific and syn, and the mechanism is probably a one-step concerted process, as illustrated above, " largely controlled by Frontier Molecular Orbital considerations. " In-plane aromaticity has been invoked for these dipolar cycloadditions. " As expected for this type of mechanism, the rates do not vary much with changes in solvent, " although rate acceleration has been observed in ionic liquids. " Nitrile oxide cycloadditions have also been done in supercritical carbon dioxide. There are no simple rules... 

If the lifetime of the alkyl radical is extremely short, in other words, if a certain given radical is very unstable (i.e., very reactive), the lifetime of a radical pair, consisting of an alkyl and a ketyl radical, after electron transfer and homolysis, would be extremely short. Subsequently, electron transfer and radical combination follow each other, with almost no separation no differentiation can be made between a two-step electron-transfer mechanism and a one-step polar mechanism. For steric reasons, one would expect the following sequence of reactivities in a concerted mechanism tertiary < secondary < primary, whereas the reverse is to be expected for an electron-transfer mechanism, in view of the stability of radicals. 

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