Unimolecular elimination reaction mechanism

Preliminary results on the enantioselective formation of sulfur and nitrogen mediumsized heterocycles by base-induced ring opening of hetero-oxabicyclic [3.2.1] and [3.3.1] systems have been reported.91 The reaction involves a deprotonation-C—O bond elimination sequence. The kinetics and mechanism of gas-phase unimolecular elimination reactions of some substituted aminoazoles have been studied as an aid to heterocycle synthesis.92... 

The gas-phase unimolecular elimination reactions of 2-substituted ethyl N,N-dimethylcarbamates23,24 and several heterocyclic carbamates25 have been studied using the Moller-Plesset MP2/6-31G method. On the basis of these calculations, the mechanism appears to be concerted, asynchronous, through a six-membered cyclic transition-state structure. 

El an elimination reaction mechanism in which the slow step is a self-ionization of the molecule to form a carbocation. Thus, the ratecontrolling step is unimolecular. 

Much like the S l reaction, the rate is linearly dependent on the concentration of only one compound (the substrate). This observation is consistent with a stepwise mechanism, in which the rate-determining step does not involve the base. The rate-determining step is the first step in the mechanism (loss of the leaving group), just as we saw in the Sj,jl reaction. The base does not participate in this step, and therefore, the concentration of the base does not affect the rate. Because this step involves only one chemical entity, it is said to be unimolecular. Unimolecular elimination reactions are called El reactions ... 

These reactions are often promoted by a strong base, which assists the departure of the proton. X is the leaving group. Both El and E2 mechanisms are known, as is a variant designated Elcb, for unimolecular elimination from the conjugate base of the substrate. ... 

Another important family of elimination reactions has as its common mechanistic feature cyclic TSs in which an intramolecular hydrogen transfer accompanies elimination to form a new carbon-carbon double bond. Scheme 6.20 depicts examples of these reaction types. These are thermally activated unimolecular reactions that normally do not involve acidic or basic catalysts. There is, however, a wide variation in the temperature at which elimination proceeds at a convenient rate. The cyclic TS dictates that elimination occurs with syn stereochemistry. At least in a formal sense, all the reactions can proceed by a concerted mechanism. The reactions, as a group, are often referred to as thermal syn eliminations. 

K. A. Cooper, E. D. Hughes, and C. K. Ingold, "The Mechanism of Elimination Reactions. Part III. Unimolecular Olefin Formation from Tert.-Butyl Halides in Acid and Alkaline Aqueous Solutions," JCS 140 (1937) 1280. 

Hughes, E. D., C. K. Ingold, and V. J. Shiner jr. Mechanism of elimination reactions. Part XVII. The comparative unimportance of steric strain in unimolecular olefin elimination. J. chem. Soc. [London] 7953, 3827. 

We note that in Eq. 13-11 we have introduced the El (elimination, unimolecular) reaction, which commonly competes with the SN1 reaction provided that an adjacent carbon atom carries one or several hydrogen atoms that may dissociate. We also note that similar to what we have stated earlier for nucleophilic substitution reactions, elimination reactions may occur by mechanisms between the E2 and El extremes. 

The NH acidities of some sterically hindered ureas, namely the ureido esters (93), have been reported.81 The kinetics and mechanism of the alkaline hydrolysis of urea and sodium cyanate, NaCNO, have been studied at a number of temperatures.82 Urea hydrolysis follows an irreversible first-order consecutive reaction path. Tetrahedral intermediates are not involved and an elimination-addition mechanism operates. Sodium cyanate follows irreversible pseudo-first-order kinetics. The decomposition of the carcinogen /V-mcthyl-/V-nitrosourca (19) was dealt with earlier.19 The pyrolysis of /V-acctylurca goes by a unimolecular first-order elimination reaction.83... 

Hydroxyls can act as nucleophiles, although they are less nucleophilic than amines or thiols. Under acidic conditions, hydroxyls can be eliminated in a dehydration reaction (Fig. 79). Elimination reactions can occur as an El reaction (elimination unimolecular) or E2 reaction (elimination bimolecu-lar). The El elimination mechanism proceeds through formation of a carbo-cation intermediate as the rate-determining step with loss of water whereas the E2 mechanism is second order with the base abstraction of a proton and loss of the leaving group occurring simultaneously (120). 

Starting materials that are likely to undergo a unimolecular SN1 reaction undergo elimination reactions by a unimolecular E[ mechanism. As might be expected, the rate-limiting step is the formation of the carbocation. 

The kinetics and mechanisms of gas-phase elimination of ethyl 1-piperidinecarboxyl-ate, ethyl pipecolinate, and ethyl 1-methylpipecolinate has been determined in a static reaction system.9 The reactions proved to be homogeneous, unimolecular, and obey a first-order rate law. The first step of decomposition of these esters is the formation of the corresponding carboxylic acids and ethylene. The acid intermediate undergoes a very fast decarboxylation process. The mechanism of these elimination reactions has been suggested on the basis of the kinetic and thermodynamic parameters. 

It is easy to recognize that the substitution product is formed by an SN1 mechanism, because the starting alkyl bromide is tertiary. Because the elimination reaction follows the same rate law, its rate-determining step, like that of the SN1 reaction, must also involve only a molecule of tert-butyl bromide. Therefore, the elimination reaction is described as a unimolecular elimination or an El reaction. 

The activation mechanism of phosphosulfate linkages (P—O —S)has been studied to understand the chemistry of biological sulfate-transfer reactions of phosphosul-fates of adenosine (APS and PAPS). Several phosphosul-fates were prepared and subjected to several nucleophilic reactions including hydrolysis. In general, phosphosulfates are stable in neutral aqueous mediay but become labile under acidic conditions, resulting in selective S—O fission. This S—O fission appears to occur by unimolecular elimination of sulfur trioxide, which can react with a nucleophilic acceptor, leading to a sulfate-transfer reaction. This process can be accelerated by Mg2+ ion when the solvent is of low water content. Under neutral conditions, divalent metal ions also were found to catalyze nucleophilic reactions, but these occurred on phosphorus to result in exclusive P-O fission. 

Apart from the free-radical mode of decomposition, many halogen compounds decompose by unimolecular mechanisms, the most common of these being the direct unimolecular elimination of hydrogen halide. There is evidence that these types of unimolecular reactions involve charge separation of the carbon-halogen bond in the transition state, and they have received considerable attention in recent years. 

Step A corresponds to a direct unimolecular elimination of hydrogen iodide and Step B to a concerted mechanism for which the activation energy exactly equals the overall endothermicity. These steps are followed by the rapid reaction of the alkyl iodide with hydrogen iodide such that the latter is kept at a low stationary state concentration (except presumably in the case of t-butyl iodide). Step A could be rate-determining for the pyrolyses of ethyl - /-propyl, /-butyl and acetyl... 

In a separate study of the decomposition of sulfinic acids in the absence of solvent at 200 °C the major products were sulfur dioxide and alkenes . Minor products were water, carbon dioxide, carbon monoxide, carbonyl sulfide and sulfur. The reaction is considered to proceed by a unimolecular free radical mechanism, although kinetic evidence is lacking. Olefin formation results from transfer reactions followed by elimination and one plausible pathway is... 

This reaction type is called the ElcB mechanism, which stands for unimolecular elimination conjugate base reaction, because the conjugate base of the starting material is being formed as the reactive intermediate. It is sometimes called the carbanion mechanism. As this mechanism results from the removal of a proton, it is not surprising that it is favoured by those substrates that possess an acidic hydrogen atom. Thus, would you expect the ElcB mechanism to be more prevalent in reactions that result in a carbon/carbon double bond or in reactions that result in a carbon/carbon triple bond ... 

The E2 mechanism is a concerted one-step process in which a nucleophile abstracts a hydrogen ion from one carbon while the halide is leaving from an adjacent one. The Ei mechanism is two-steps and involves a carbocation intermediate formed upon departure of the halide ion in the first step. E2 reactions are bimolecular and the reaction rate depends on the concentrations of both the alkyl halide and nucleophile. E1 reaction rates depend on the slowest step, formation of the carbocation, and are influenced only by the concentration of the alkyl halide the reaction is unimolecular. E2 reactions involve anti elimination and produce a specific alkene, either cis or trans. E1 reactions involve an intermediate carbocation and thus give products of both syn and anti elimination. 

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