Rearrangement reactions unimolecular

The kinetic data for the reaction of primary alkyl radicals (RCH2 ) with a variety of silanes are numerous and were obtained by applying the free-radical clock methodology. The term free-radical clock or timing device is used to describe a unimolecular radical reaction in a competitive study [2-4]. Three types of unimolecular reactions are used as clocks for the determination of rate constants for this class of reactions. The neophyl radical rearrangement (Reaction 3.1) has been used for the majority of the kinetic data, but the ring expansion rearrangement (Reaction 3.2) and the cyclization of 5-hexenyl radical (Reaction 3.3) have also been employed. 

The Arrhenius expression for the reaction of the o-(allyloxy)phenyl radical (15) with (Me3 Si)3 SiH relative to this unimolecular rearrangement (Reaction 3.8) has been established [10]. A rate constant kc = 8 x 10 s has been estimated for... 

When the commodity chemical propylene oxide is heated to high temperature in the gas phase in a shock tube, unimolecular rearrangement reactions occur that generate the CsHgO isomers allyl alcohol, methyl vinyl edier, propanal, and acetone (Figure 15.9). Dubnikova and Lifshitz carried out a series of calculations to determine the mechanistic pathway(s) for each isomerization, with comparison of activation parameters to those determined from Arrhenius fits to experimental rate data to validate the theoretical protocol. 

Unimolecular radical reactions result from the instability of the first formed radical. The radicals may completely decompose or they may rearrange before reaction with other molecules or radicals present. In decomposition reactions, the radical decomposes to give a stable molecule and a new radical. In rearrangement reactions, the breaking of an adjacent C-C bond in a cyclic system leads to the concomitant formation of a new bond, usually carbonyl, and a new isomeric radical. There could also be migration of an atom, via intramolecular abstraction by the radical center, thus creating a new, isomeric radical. 

Series of First-Order Reections The phenyl rearrangement (reaction 4, Scheme 7.2) proceeds through an intermediate rather than a transition state. In this case, the unimolecular reaction becomes a series of first-order reactions ... 

Figure 6.17. Warhaftig diagram The upper portion represents the probability of a particular ion at a specified energy. The lower portion represents the rate constant of a unimolecular fragmentation of the ABCD+ radical cation as a function of internal energy the steep curve is for a direct cleavage reaction, and the shallow curve is for a rearrangement reaction. (Redrawn from F. W. McLafferty and F. TureCek, Interpretation of Mass Spectra, 4th ed.)...

With this terminology we may not only describe a fully concerted, one-step Sn2 reaction (AnDn), and a stepwise SnI reaction involving intermediate ions that diffuse apart (Dn -H An), but we may also concisely represent a stepwise reaction involving a transient ion pair (E An). The lUPAC nomenclature system can also be used to describe other substitution reactions. Among them are the SnI (substitution nucleophilic unimolecular with rearrangement) reaction, equation 1.8, which is denoted by lUPAC as an (1/Dn -I- 3/An) reaction. The numbers before the slash symbols indicate atoms involved in the dissociation and association steps. Thus, 1/Dn means that the nucleofuge dissociates from one atom (1), while the 3/An term means that the nucleophile associates at an allylic position (3). 

The inhibitor endo-6-0 has had quite a career since we first described it in 1985. First, in the molecular equivalent of "turn around is fair play", it was used by Peter Schultz and David Jackson and by Don Hilvert and his coworkers to make catalytic antibodies.25-27 Rearrangement reactions in general, and especially those for which control of the substrate conformation is important, are particularly appropriate candidates for catalytic antibodies since they are unimolecular and seldom require covalent catalysis. The antibody obtained by Schultz and Jackson is reasonably efficient, accelerating the rearrangement 10,(X)0-fold at 0 C, which is 60% as good as the enzyme in stabilizing the transition state. It is interesting to compare the enzyme and the antibody directly, and... 

Such unimolecular ion-decomposition reactions can be viewed as another field of chemistry, but fortunately for most chemists studying this book, there are many close similarities to pyrolytic, photolytic, radiolytic, and other energetic reactions, and there are even many general similarities to condensed-phase (solution) organic reactions. The largest points of difference are that ionic and often radical species are involved in each reaction in the mass spectrometer, and their combined effects sometimes appear unusual to the organic chemist. Chemists may also question the reliability of structural relationships based on rearrangement reactions. However, many of these are based on well-established chemistry and can provide key molecular information... 

Kamiya, K., and K. Morokuma (1991), Potential energy surface for unimolecular dissociation and rearrangement reactions of the ground electronic state of HFCO, J.Chem. Phys., 94, 7287-7298. 

From this expression, it is obvious that the rate is proportional to the concentration of A, and k is the proportionality constant, or rate constant, k has the units of (time) usually sec is a function of [A] to the first power, or, in the terminology of kinetics, v is first-order with respect to A. For an elementary reaction, the order for any reactant is given by its exponent in the rate equation. The number of molecules that must simultaneously interact is defined as the molecularity of the reaction. Thus, the simple elementary reaction of A P is a first-order reaction. Figure 14.4 portrays the course of a first-order reaction as a function of time. The rate of decay of a radioactive isotope, like or is a first-order reaction, as is an intramolecular rearrangement, such as A P. Both are unimolecular reactions (the molecularity equals 1). 

Tables 12.1-12.3 below give some examples of the magnitude of each term for two bimolecular reactions (Diels-Alder and Sn2 reactions, forming either one or two molecules as the product) and a unimolecular rearrangement (Claisen reaction). All values have been calculated at the MP2 level with the 6-31G(d) basis for the Diels-Alder and Claisen reactions, and the 6-31+G(d) basis for the S l reaction. The values are given in kcal/mol at a temperature of 300 K (RT = 0.60 kcal/mol).

Ideally all reactions should result from unimolecular homolysis of the relatively weak 0-0 bond. However, unimolecular rearrangement and various forms of induced and non-radical decomposition complicate the kinetics of radical generation and reduce the initiator efficiency.46 Peroxide decomposition induced by radicals and redox chemistry is covered in Sections 3.3.2.1.4 and 3.3.2.1.5 respectively. 

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