The unimolecular elimination

In 1935, a second mechanism for elimination was recognised and was called the unimolecular (El) mechanism . The presence of a base is not required and the reaction proceeds through a two-step process (16). Initially, dissociation of the C -X bond, aided by an ionising solvent, occurs and the carbonium ion formed undergoes rapid unimolecular decomposition to the olefin, viz. 

The reaction is first-order in the substrate. The steady-state approximation for the carbonium-ion concentration gives 

Evidence for the mechanism often depends on the observation that the elimination is not accelerated by the addition of base, other than the effect of the latter on the reaction medium. Addition of the common ion e.g. X = halide) also slows the reaction by a mass action effect on the first step. 

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Wu, F., and R. W. Carr, Time-Resolved Observation of the Formation of CF20 and CFCIO in the CF2C1 + 02 and CFCI2 + 02 Reactions. The Unimolecular Elimination of Cl Atoms from CF2CIO and CFCI20 Radicals, J. Phys. Chem., 96, 1743-1748 (1992). 

Additional compelling evidence for the unimolecular elimination mechanism (4)... 

An ab initio study on the unimolecular elimination reactions of methacrylonitrile has revealed a direct four-centre elimination of HCN and three-centre elimination of H2 channels.17 A methylcyanoethylidene intermediate has also been identified. 

A generalized rate expression for the reaction of allylic chloride (RC1) with metal stabilizers (MX2) includes terms for the unimolecular elimination of HC1 from RC1 and for the bimolecular reaction of RC1 with all metal containing species. 

TABLE 6. Kinetic parameters for the unimolecular elimination of HC1 from pyrolysis of ZCH(C1)CH3 at 360 °C... 

The rate-determining elimination step in an ElcB reaction is unimolecular, so you might imagine it would have a first-order rate equation. But, in fact, the rate is also dependent on the concentration of base. This is because the unimolecular elimination involves a species—the anion—whose concentration is itself determined by the concentration of base by the equilibrium we have just been discussing. Using the following general ElcB reaction, the concentration of the anion can be expressed as shown. 

As well as conventional kinetic methods, the shock-tube method and the toluene vapour-flow method have also been used for studying the unimolecular elimination of hydrogen iodide from alkyl iodides. Table 12 summarises Arrhenius Parameters for these reactions. 

Pyrolysis is a technique that uses heat to transform macromolecules into a series of lower molecular weight volatile products characteristic of the original sample. The initiation reactions involved in the pyrolysis of most organic materials are the result of free radical reactions initiated by bond breaking or by the unimolecular elimination of simple molecules such as water or carbon dioxide [172-175]. The products of pyrolysis are identified by gas chromatography, often in combination with mass spectrometry. 

If planar carbonium ions were the intermediates in El reactions in the cyclohexyl series, menthyl and neomenthyl compounds should give the same product ratios. However, the olefin distribution is quite different in the two El processes and the stereospecificity is less marked than in the E2 reactions of these substrates (Table 15). Whereas the concerted eliminations always show anti stereospecificity, the unimolecular eliminations only exhibit this preference when a tertiary beta hydrogen is trans to the ionising group (e.g. neomenthyl series). Possibly in this case the tertiary hydrogen aids ionisation by forming a type of non-classical bridged intermediate, viz. 

The efifect of solvent on the reaction rate is small (Smith el al., 1961) and so it is likely that the polarity of the transition state differs little from that of the original azide. There is therefore probably little electrostatic contribution to the volume of activation. If a partial bond has formed between the group R and the nitrogen atom in the transition state we should expect the volume of activation to be negative, and if the slow step is the unimolecular elimination of nitrogen without appreciable migration of the group R then the volume of activation should be fairly small and positive. 

But if an alkyl halide can form a stable carbocation, the unimolecular elimination reaction is faster. It s referred to as unimolecular because the slow step has only one molecule in the transition state, just like substitution reactions. 

El Reaction (Section 7.8) The unimolecular elimination reaction. The ionization of the starting material is followed by the loss of a proton to base. The product has a new it bond. 

In this case sulfuranyl peroxyls serve just as the intermediates in electron transfer from one-electron reduced sulfites, sulfinic acids, or sulfoxides to molecular oxygen. This hypothetical reaction is analogous to the unimolecular elimination of HOO from a-hydroxyalkyl peroxyl radicals that is known to proceed with a rate constant of ca. 10 s" in room temperature solutions [104]. 

Rotinov A, Chuchani G, Andres J, Domingo LR, Safont VS (1999) A combined expraimental and theoretical study of the unimolecular elimination kinetics of 2-alkoxyjHX)pionic acids in the gas phase. Chem Phys 246 1-12... 

Secondary and tertiary alcohols dehydrate by the unimolecular elimination pathway (El), discussed in Sections 7-6 and 9-2. Protonation of the weakly basic hydroxy oxygen forms an alkyloxonium ion, now containing water as a good potential leaving group. Loss of H2O supphes the respective secondary or tertiary carbocations and deprotonation furnishes the alkene. The reaction is subject to all the side reactions of which carbocations are capable, particularly hydrogen and alkyl shifts (Section 9-3). 

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