Unimolecular radical reactions

Radical chain processes 772, 1063 Radical clock 1059 Radical reactions radical-molecule 1102-1111 radical-radical 1099-1102 unimolecular 1098, 1099 Radicals, formation during radiolysis 891-922... 

LFP-Probe Method. In cases where the radicals of interest do not contain a useful chromophore, the LFP technique can be modified by incorporation of a probe radical reaction that gives a product with a chromophore. The probe reaction can be unimolecular or bimolecular, a constant concentration of probe reagent is employed in the latter case. Formation of the detectable species occurs with an observed first-order or pseudo-first-order rate constant equal to k0. In the presence of another reagent X that reacts with the original radical, the rate constant for formation of detectable species is kohs = k0 + kx [X], and the bimolecular rate constant is determined (as before) by conducting the reaction at varying concentrations of X. Note that the LFP-probe technique is a direct method even though the reactant or product of interest is not monitored. 

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 competitive kinetics of Scheme 3.1 can also be applied to calibrate the unimolecular radical reactions provided that kn is a known rate constant. In particular the reaction of primary alkyl radicals with (Mc3Si)3SiH has been used to obtain kinetic data for some important unimolecular reactions such as the p-elimination of octanethiyl radical from 12 (Reaction 3.5) [12], the ring expansion of radical 13 (Reaction 3.6) [8] and the S-endo-trig cyclization of radical 14 (Reaction 3.7) [13]. The relative Arrhenius expressions shown below for the... 

The reaction has also been extended to the analogous vinyl bromides [30]. Indeed, the alkenyl bromide 77 under normal reduction conditions gave the bicyclic compound 78 in good yield by an Sni reaction given by the vinyl radical (Reaction 6.17). Under these conditions, the reduction products could not be observed which suggests a very fast unimolecular reaction. 

In the area of reaction energetics. Baker, Muir, and Andzehn have compared six levels of theory for the enthalpies of forward activation and reaction for 12 organic reactions the unimolecular rearrangements vinyl alcohol -> acetaldehyde, cyclobutene -> s-trans butadiene, s-cis butadiene s-trans butadiene, and cyclopropyl radical allyl radical the unimolecular decompositions tetrazine -> 2HCN -F N2 and trifluoromethanol -> carbonyl difluoride -F HF the bimolecular condensation reactions butadiene -F ethylene -> cyclohexene (the Diels-Alder reaction), methyl radical -F ethylene -> propyl radical, and methyl radical -F formaldehyde -> ethoxyl radical and the bimolecular exchange reactions FO -F H2 FOH -F H, HO -F H2 H2O -F H, and H -F acetylene H2 -F HC2. Their results are summarized in Table 8.3 (Reaction Set 1). One feature noted by these authors is... 

HO-initiated oxidation of the alkanes become complex with increase in carbon number. Namely, a large variety of alkyl radicals can be produced by the H-atom abstraction from the primary, secondary and tertiary C—H bonds in the parent alkane [88]. The resulting ROO ( C4) radicals have been shown by Atkinson et al. to yield R0N02 as well as RO + N02 upon reaction with NO [100-102]. A major complication in the alkane oxidation mechanism arises from the variety of competitive reaction channels that RO radicals can undergo, e.g., 02-reaction, unimolecular dissociation and internal isomerization. There have been a number of experimental and theoretical studies of these reactions [31,88]. 

A summary of the major chemical reactions of free radicals is given in Table 4.3. Broadly speaking these can be classified as unimolecular reactions of dissociations and isomerizations, and bimolecular reactions of additions, disproportionations, substitutions, etc. The complexity of many photochemical reactions stems in fact from these free radical reactions, for a single species formed in a simple primary process can lead to a variety of final products. 

The photochlorination rate expressions may be expected to be somewhat modified as a result of the inclusion of the hot radical reactions. At temperatures below 150°C. the unimolecular decomposition of the (thermalized) AC1 radical, i.e., reaction ( — 2), may be expected to be negligible and the rate laws are... 

Radical reactions involve the correlated movement of single electrons. In a unimolecular reaction like the photolytic cleavage of chlorine, one electron moves to one atom and the other electron moves to the other. In bimolecular... 

Similarly, the p-fragmentation of tertiary alkoxyl radicals [reaction (2)] is a well-known process. Interestingly, this unimolecular decay is speeded up in a polar environment. For example, the decay of the ferf-butoxyl radical into acetone and a methyl radical proceeds in the gas phase at a rate of 103 s 1 (for kinetic details and quantum-mechanical calculations see Fittschen et al. 2000), increases with increasing solvent polarity (Walling and Wagner 1964), and in water it is faster than 106 s 1 (Gilbert et al. 1981 Table 7.2). 

Vieira AJSC, Steenken S (1987a) Pattern of OH radical reaction with 6- and 9-substituted purines. Effect of substituents on the rates and activation parameters of unimolecular transformation reactions of two isomericOH adducts. J PhysChem 91 4138-4144 Vieira AJSC, Steenken S (1987b) Pattern of OH radical reaction with N6,N6-dimethyladenosine. Production of three isomeric OH adducts and their dehydration and ring opening reactions. J Am Chem Soc 109 7441-7448... 

Vieira AJSC, Steenken S (1990) Pattern of OH radical reaction with adenine and its nucleosides and nucleotides. Characterization of two types of isomeric OH adduct and their unimolecular transformation reactions. J Am Chem Soc 112 6986-6994 Vieira AJSC, Steenken S (1991) Pattern of OH radical reaction with N6,N6,9-trimethyladenine. Dehy-droxylation and ring-opening of isomeric OH-adducts. J PhysChem 95 9340-9346 Vieira AJSC, Candeias LP, Steenken S (1993) Hydroxyl radical induced damage to the purine bases of DNA in vitro studies. J Chim Phys 90 881-897... 

A large number of radical reactions proceed by redox mechanisms. These all require electron transfer (ET), often termed single electron transfer (SET), between two species and electrochemical methods are very useful to determine details of the reactions (see Chapter 6). We shall consider two examples here - reduction with samarium di-iodide (Sml2) and SRN1 (substitution, radical-nucleophilic, unimolecular) reactions. The SET steps can proceed by inner-sphere or outer-sphere mechanisms as defined in Marcus theory [19,20]. 

Substitutions by the SRn 1 mechanism (substitution, radical-nucleophilic, unimolecular) are a well-studied group of reactions which involve SET steps and radical anion intermediates (see Scheme 10.4). They have been elucidated for a range of precursors which include aryl, vinyl and bridgehead halides (i.e. halides which cannot undergo SN1 or SN2 mechanisms), and substituted nitro compounds. Studies of aryl halide reactions are discussed in Chapter 2. The methods used to determine the mechanisms of these reactions include inhibition and trapping studies, ESR spectroscopy, variation of the functional group and nucleophile reactivity coupled with product analysis, and the effect of solvent. We exemplify SRN1 mechanistic studies with the reactions of o -substituted nitroalkanes (Scheme 10.29) [23,24]. 

Like the bimolecular reactions, unimolecular reactions are often found as individual steps in complex reactions. These include the unimolecular breakdown of molecules into radicals often found as first initiation steps and propagation steps in chain reactions, e.g. 

Another common scenario in competition kinetics utilizes unimolecular radical reactions as a clock against which other reactions can be timed. Among the most commonly used free radical clocks are the cyclization of 1 -hexenyl and other radicals with double or triple bonds in the chain,33 ring opening,34 and p-elimination from alkoxyl radicals.35... 

---