Unimolecular Transformations

In this section, we can consider many different transformations. Unimolecular, radical, reduction, ring expansion, hydrolysis, and dechlorination reactions can all be described. The first two reactions in this list have already been addressed (Sections 6.11.2.2, 6.11.4.1, and 6.11.4.2). 

Unimolecular reaction (Section 11.4) A reaction that occurs by spontaneous transformation of the starting male-rial without the intervention of other reactants. For example, the dissociation of a tertiary alkyl halide in the S l reaction is a unimolecular process. 

The transformation of the hydrophobic periphery composed of bromo substituents into a hydrophilic wrapping of carboxylic acid functions was achieved by reacting 31 with (i) n-butyllithium and (ii) carbon dioxide. The polymer-analogous transformation provides water soluble, amphiphilic derivatives of 31 which constitute useful covalently bonded unimolecular models for micellar structures. 

In the event, treatment of a rapidly stirred solution of 3 and sodium acetate in MeOH-tbO at 38 °C with PdCl2 results in the fomation of carpanone (1) in 46% yield. The ordered unimolecular transition state for the oxidative coupling reaction furnishes putative bis(quinodimethide) 2 stereoselectively. Once formed, 2 readily participates in an intramolecular Diels-Alder reaction4 to give carpanone (1). Two new rings and all five contiguous stereocenters are created in this spectacular sequential transformation.5... 

The transformation of the terminal bromo substituents to carboxylic acid functions with (i) n-butyl lithium (ii) carbon dioxide, provides water soluble derivatives of 47 which are interesting as models for unimolecular micelles. 

Several groups confirmed spectroscopically the existence of an ion-pair complex Fe -HOJ, which may decompose unimolecularly at high [H2O2]/ [Fe(III)] but become transformed to FeO " " at lower [H202]/[Fe(III)]. FeO " is attacked by a second molecule of H2O2 to provide an alternative route for decomposition, viz. 

Newkome, G.R., Moorefield, C.N., Keith, J.M., Baker, G.R., and Escamilla, G.H. (1994) Chemistry of micelles. 37. Internal chemical transformations in a precursor of a unimolecular micelle boron supercluster via site-specific addition of BioH14 to cascade molecules. Angew. Chem., Int. Ed. Engl. 33, 666-668. 

New synthetic transformations are highly dependent on the dynamics of the contact ion pair, as well as reactivity of the individual radical ions. For example, the electron-transfer paradigm is most efficient with those organic donors yielding highly unstable cation radicals that undergo rapid unimolecular reactions. Thus, the hexamethyl(Dewar)benzene cation radical that is generated either via CT activation of the [D, A] complex with tropylium cation,74... 

The coefficients of the balanced overall equation bear no necessary relationship to the exponents to which the concentrations are raised in the rate law expression. The exponents are determined experimentally and describe how the concentrations of each reactant affect the reaction rate. The exponents are related to the ratedetermining (slow) step in a sequence of mainly unimolecular and bimolecular reactions called the mechanism of the reaction. It is the mechanism which lays out exactly the order in which bonds are broken and made as the reactants are transformed into the products of the reaction. 

The frequency with which the transition state is transformed into products, iT, can be thought of as a typical unimolecular rate constant no barrier is associated with this step. Various points of view have been used to calculate this frequency, and all rely on the assumption that the internal motions of the transition state are governed by thermally equilibrated motions. Thus, the motion along the reaction coordinate is treated as thermal translational motion between the product fragments (or as a vibrational motion along an unstable potential). Statistical theories (such as those used to derive the Maxwell-Boltzmann distribution of velocities) lead to the expression ... 

The fcpj characterizes a unimolecular transformation of the 7t-complex between the cation and the double-bond of the monomer, to the newly formed cation, which now has the... 

The thermal and photochemical activations of EDA complexes by electron transfer are both enhanced when the radical ions D+- or A--(either paired or free) undergo a facile first-order (unimolecular) transformation such as fragmentation, rearrangement, bond-formation, etc., which pulls the redox equilibrium and thus renders the competition from the energy-wasting back electron transfer less effective (compare Scheme 5). Critical to the quantitative evaluation of the reaction dynamics is the understanding that the typical [D+% A--] intermediates, as described in... 

For cycloheptatriene and a series of its derivatives various thermal unimolecular processes, namely conformational ring inversions, valence tautomerism, [1,5]-hydrogen and [l,5]-carbon shifts, are known. An example of such multiple transformations was described65 which can provide a facile approach to new polycyclic structures by a one-step effective synthesis (yields up to 83%) of the two unique ketones 156 and 157. The thermolysis of the neat ether 151 at 200 °C for 24 h gives initially the isomeric allyl vinyl... 

In the present review, a new variation on an existing experimental method will be used to show how accurate unimolecular dissociation rate constants can be derived for thermal systems. For example, thermal bimolecular reactions are amenable to study by use of several, now well-known, techniques such as (Fourier transform) ion cyclotron resonance spectrometry (FTICR), flowing afterglow (FA), and high-pressure mass spectrometry (HPMS). In systems where a bimolecular reaction leads to products other than a simple association adduct, the bimolecular reaction can always be thought of as containing a unimolecular... 

It has been shown that the transformation 254 - 251 -t- 253 proceeds with first-order kinetics and that increase in solvent polarity is associated with an increase in the rate. It is proposed that the reaction involves a novel unimolecular heterolytic scission of the sulfur—sulfur bond in the symmetrical disulfide (254). The kinetics of the alkaline hydrolysis of anhydro-2-mercapto-4,5-diphenyl-l,3,4-thiadiazolium... 

In this part of the chapter, we will briefly outline the main types of CL reactions which can be functionally classified by the nature of the excitation process that leads to the formation of the electronically excited state of the light-emitting species. Direct chemiluminescence is the term employed for a reaction in which the excited product is formed directly from the unimolecular reaction of a high-energy intermediate that has been formed in prior reaction steps. The simplest example of this type of CL is the unimolecular decomposition of 1,2-dioxetanes, which are isolated HEI. Thermal decomposition of 1,2-dioxetanes leads mainly to the formation of triplet-excited carbonyl compounds. Although singlet-excited carbonyl compounds are produced in much lower yields, their fluorescence emission constitutes the direct chemiluminescence emission observed in these transformations under normal conditions in aerated solutions ... 

The unimolecular decomposition of 1,2-dioxetanes and 1,2-dioxetanones (a-peroxylac-tones) is the simplest and most exhaustively studied example of a thermal reaction that leads to the formation, in this case in a single elementary step, of the electronically excited state of one of the product molecules. The mechanism of this transformation was studied intensively in the 1970s and early 1980s and several hundreds of 1,2-dioxetane derivatives and some 1,2-dioxetanones were synthesized and their activation parameters and CL quantum yields determined. Thermal decomposition of these cyclic peroxides leads mainly to the formation of triplet-excited carbonyl products in up to 30% yields. However, formation of singlet excited products occurs in significantly lower yields (below... 

The latter number incorporates just the chemical step(s) of formation of triazole within cucurbituril. Since the product release step apparently is at least 100-fold slower than the actual cycloaddition, the net catalytic acceleration should be adjusted downward by that amount. An instructive alternative estimation of kinetic enhancement is to compare the extrapolated limiting rate for cycloaddition within the complex (i.e. cucurbituril saturated with both reactants, k — 1.9xl0 s ) with the uncatalyzed unimolecular transformation of an appropriate bifunctional reference substrate as in Eq. (3) (k, = 2.0x 10 s ). Such a comparison of first-order rate constants shows that the latter reaction is approximately a thousandfold slower than the cucurbituril-engendered transformation. This is attributable to necessity for freezing of internal rotational degrees of freedom that exist in the model system, which are taken care of when cucurbituril aligns the reactants, and concomitantly to an additional consideration which follows. 

Fiedler, D., Bergman, R.G. and Raymond, K.N. (2004) Supramolecular catalysis of a unimolecular transformation Aza-Cope rearrangement within a self-assembled host. Angew. Chem., Int. Ed., 43 (48), 6748-6751. 

Pericyclic reactions are unimolecular, concerted, uncatalyzed transformations. They take place in a highly stereoselective manner governed by symmetry proper-ties of interacting orbitals. - Characteristic of all these rearrangements is that they are reversible and may be effected thermally or photochemically. The compounds in equilibrium are usually interconverted through a cyclic transition state,224 although biradical mechanisms may also be operative. A few characteristic examples of pericyclic rearrangements relevant to hydrocarbon isomerizations are presented here. 

The interpretation of a spectrum from a dynamical point of view can also be applied to a spectrum containing a broad feature associated with direct and/or indirect dissociation reactions. From such spectra dynamics of a dissociating molecule can also be extracted via the Fourier transform of a spectrum. An application of the Fourier transform to the Hartley band of ozone by Johnson and Kinsey [3] demonstrated that a small oscillatory modulation built on a broad absorption feature contains information of the classical trajectories of the vibrational motion on PES, so-called unstable periodic orbits, at the transition state of a unimolecular dissociation. 

A unimolecular reaction is one in which the absolute rate of change is proportional to the first power of the concentration of the reacting substance. The fraction of the total number of the molecules in the system which change in unit time is therefore independent of the concentration, and thus, in gaseous systems, cannot be proportional to the nuinber of collisions undergone in unit time by the molecules. It must therefore be concluded that, whether or not previously received collisions have done anything to put the molecule into an abnormal condition, the actual chemical transformation is an event happening to the isolated molecule. 

For a reason which will soon be evident, we will redefine the unimolecular reaction as a transformation in which the fraction undergoing change in unit time is independent of the pressure over the ordinary range of pressures, without prejudice to what may happen at extremely low pressures. The reservation is very important. 

Another mechanism has been suggested by Christiansen and Kramers in which activation is by collision and yet there is an apparently unimolecular reaction. It depends upon the possibility that the products of reaction, possessing the energy corresponding to the chemical heat of reaction as well as the original heat of activation, are able immediately to activate fresh molecules of reactant. In this way reaction-chains are set up. The assumption is made that every molecule of product can at once activate by collision a fresh molecule of the reactant. In this way each activated molecule removed from the system by chemical transformation is replaced by a new activated molecule. 

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