Non-concerted decomposition

Multirunctional initiators where the radical generating functions are in appropriate proximity may decompose in a concerted manner or in a way such that the intermediate species can neither be observed nor isolated. Examples of such behavior are peroxyoxalate esters (see 3.3.2.3.1) and a-hydroperoxy diazenes (e.g. 31), derived peroxyesters (65) and bis- and multi-diazenes such as 66. 

It has been reported that the a-hydroperoxy diazenes may undergo induced decomposition either by OH or H transfer.  

Initiators where the radical generating functions are sufficiently remote from each other break-down in a non-concerted fashion. Examples include the azo-peroxide (68) and the bis-diazene (67). Their chemistry is often understandable in terms of the chemistry of analogous monofunctional initiators. This class also includes the hydroperoxyketals (see 3.3.2.5). 

The use of initiators such as 68 has been promoted for achieving higher molecular weights or higher conversions in conventional polymerization and for the production of block and graft copolymers. The use and applications of multifunctional initiators in the synthesis of block and graft copolymers is briefly described in Section 7.6.1. 

---

The rates of thermal decomposition of diacyl peroxides (36) are dependent on the substituents R. The rates of decomposition increase in the series where R is aryl-primary alkyKsecondary alkyKtertiary alkyl. This order has been variously proposed to reflect the stability of the radical (R ) formed on (i-scission of the acyloxy radical, the nucleophilicity of R, or the steric bulk of R. For peroxides with non-concerted decomposition mechanisms, it seems unlikely that the stability of R should by itself be an important factor. 

Several unsuccessful attempts have been made to prepare the parent 1,4-dithiocin by valence isomerization approaches. The diepisulfide (2S2) was obtained from benzene dioxide, but decomposition at 20 °C ( i/2 30 min) gave only benzene by stepwise sulfur extrusion (74AG(E)737). 2,5-Dithiabicyclo[4.2.0]octadiene (2S3) on pyrolysis at 200 °C gave benzene (50%) and a trace of thiophene a non-concerted ring opening to dithiocin (254) followed by immediate destruction of the latter was suggested (71JA4627). 

The protonic acid-catalysed decomposition of azides is conceived of tis involving an initial protonation of the a-nitrogen atom, subsequent to which nitrogen may be eliminated either in a non-concerted or a concerted process. 

Russell (10) suggested that the bimolecular self-reaction of S-RO2 involves the concerted decomposition of a cyclic tetroxide formed by combination of the radicals. This mechanism was deduced from a consideration of the results of a kinetic and product study of the autoxidation of ethylbenzene. Thus Russell found that almost one molecule of acetophenone is produced per two kinetic chains and that CeHsCHCCHa)O2 interact to form non-radical products nearly twice as fast as CsHsCDCcHs) O2. The former result is only compatible with (29) if all the alkoxy radicals disproportionate in the solvent cage (30) while the deuterium isotope effect requires a H-atom transfer reaction to be rate controlling, which is unlikely for the radical pathway. 

Influence of ionic strength on the reaction rate constant. The influence of the ionic strength on the reaction rate constant was studied using KCl as electrolyte. The results obtained in this study are listed in Table 4, where we can see that the reaction rate constant for N-Br-alanine decomposition undergoes an increment of 40 % upon changing the ionic strength from 0.27M to IM, while in the case of N-Bromoaminoisobutyric acid the increment of the reaction rate constant is of about 12 %. This is an evidence of a non ionic mechanism in the case of the decomposition of N-Br-aminoisobutyric acid, as it is expected for a concerted decarboxylation mechanism. For the decomposition of N-Br-proline the increase on the reaction rate constant is about 23 % approximately, an intermediate value. This is due to the fact both paths (concerted decarboxylation and elimination) have an important contribution to the total decomposition process. 

The kinetics of the gas-phase elimination of ethyl and /-butyl carbazates have been studied in a static reactor system over the temperature range 220.3-341.7 °C and pressure range 21.1-70.0 torr.13 Theoretical calculations on the thermal decomposition of ethyl carbazate (4) suggest that the reaction proceeds by a concerted non-synchronous mechanism, through a quasi-three-membered ring transition state (Scheme 4). In contrast, the transition state structure for the thermal decomposition of /-butyl carbazate is an almost planar six-membered ring. 

Molecules with hydroxy substitutions to a (C -C ) bond center imdergo facile molecular eliminations in the 600-650 °K range, at normal pressures (10-700 torr), in gas phase static systems. The reactions are concerted and probably involve six-center, rather non-polar, transition states. For example, the decomposition of 3-butene-l-ol to propene and formaldehyde may be represented as... 

---