Radiolytic isomerizations

The fact that ionizing radiations bring about cis-trans isomerizations has been demonstrated in several investigations. However, little has been done to establish the mechanisms, and the matter will therefore not be considered here. 

The thermal decompositions (pyrolyses) of hydrocarbons other than the cyclic ones invariably occur by complex mechanisms involving the participation of free radicals the processes are usually chain reactions. In spite of this, many of the decompositions show simple kinetics with integral reaction orders, and this led to the conclusion by the earlier workers that the mechanisms are simple. Ethane, for example, under the usual conditions of a pyrolysis experiment, decomposes by a first-order reaction mainly into ethylene and hydrogen, and the mechanism was thought to involve the direct split of the ethane molecule. Rice et however, showed that free radicals are certainly involved in this and other reactions, and this conclusion has been supported by much later work. An important advance was made in 1934 when Rice and Herzfeld showed how complex mechanisms can lead to simple overall kinetics. They proposed specific mechanisms in a number of cases most of these have required modification on the basis of more recent work, but the principles suggested by Rice and Herzfeld are still very useful. 

Thanks to this work, and subsequent work on pyrolysis mechanisms, it has become clear that three different types of elementary reactions are involved (see Chapter 2, Volume 2 for a discussion of chain reactions). 

Initiation reactions are usually unimolecular splits of this kind, and like all uni-molecular reactions they become first-order at sufficiently high pressures and second-order at low pressures. In the second-order region rates are dependent on the concentration of the third bodies, and the first-order rate coefficients are pressure-dependent. 

In the ethane pyrolysis the H atoms react with ethane with the abstraction of a hydrogen atom and the formation of an ethyl radical, viz. 

---

Lehmann, H. P., G. Stein, and E. Fischer Sensitized Radiolytic Isomerization of Stilb en Chem. Comm. 1965, 583. 

Polystyrene fflm doped with traws-stilbene has been found useful as a chemical dosimeter for ionizing radiation [68]. The dosimeter was effective for the photon energy range from 4.7 MeV to 1.25 keV (mean) and for rates above 300 rads/s. The mechanism for the dosimetry reaction, isomerization of trans- to cis-stilbene, was shown to be analogous to the mechanism proposed for radiolytic isomerization. 

The gas-phase base-induced elimination reaction of halonium ions was thoroughly investigated in radiolytic experiments22. Radiolytically generated acids C/JH5+ (n = 1,2) were allowed to react at 760 Torr with selected 2,3-dihalobutanes to form the halonium intermediates which, in the presence of trimethylamine, undergo base-induced bimolecu-lar elimination as shown in Scheme 6. This elimination reaction occurs in competition with unimolecular nucleophilic displacement to the cyclic halonium ion and subsequent rearrangement. Isolation and identification of the neutral haloalkenes formed and kinetic treatment of the experimental results indicated that 3-halo-1 -butene is formed preferentially with respect to the isomeric 2-halo-2-butenes and that the bimolecular elimination process occurs predominantly via a transition state with an anti configuration22. 

There is little doubt that, under such conditions, the large number of collisions with inert Hg molecules, which take place before a reactive encounter of the cycloalkanium ion with a cycloalkane molecule can occur, allow the isomerization process already observed in pure cycloparaffins vide infra) to proceed to completion. It seems reasonable to conclude that the formation of cycloalkanium ions could not be detected in the radiolytic experiments owing to the composition of the gaseous system. 

Much of this chapter is concerned with the thermal, photolytic, photosensitized and radiolytic decompositions of pure organic hydrocarbons. Since the action of heat on cyclic hydrocarbons usually leads to reactions that occur by simpler mechanisms than is the case with the other hydrocarbons, these reactions are considered first (Section 2). This section deals with isomerizations as well as decompositions. Section 3 is concerned with cis-trans isomerizations, in which there is twisting about a double bond. The decompositions of non-cyclic hydrocarbons are dealt with in Sections 4-7, and a final section (8) treats the effects of additives, such as inhibitors, on the various decompositions and isomerizations. 

Cationic cyclization and its reverse have particular relevance, since ring closure and ring opening have been invoked to account for a large number of biosynthetic pathways.The gaseous cyclopropylcarbinyl-cyclobutyl-homoallyl system represents an extreme instance of a rapid, reversible isomerization of this sort, which has been studied over a period of 20 years both by /1-decay of tritiated cyclobutane and by radiolytic methods. This interconversion is so fast that the three structures... 

It is worth mentioning that cis-trans isomerization can also be achieved radiolytically. Attainment of a photoequilibrium of 4,4 -NMS in benzene under y-radiation is promoted by terphenyl as scintillator [455]. Dainton et al. [456] have ascribed a 100-ns transient, produced by pulse radiolysis of trans-stilbene at room temperature, to the trans triplet state. Pulse radiolysis was applied to determine the triplet state of cycloheptatriene (tt = 6 s) which was used to mimic energy transfer to ds-stilbene [457]. 

Several recent publications indicate that the role of intermediate complexes in ionic reactions is still controversial (21, 24, 25). Our interest in this and earlier observations of persistent complexes in alkyl halides already mentioned prompted us to study ionic reactions in ethyl chloride. The previously noted mass spectrometric investigations of alkyl halides did not include the chlorides, and radiolytic studies of these compounds have been limited to the propyl and butyl chlorides which apparently isomerize (39). The present investigation consists of two phases. In the initial phase, the ion-molecule reactions for ethyl chloride were probed by the sensitive mass spectrometric methods which we have applied in recent studies of a similar nature (3,12, 28, 43). In the latter part of this study, the gas-phase radiolysis and vacuum-ultraviolet photolysis of ethyl chloride have been studied to identify those products which arise from ionic precursors. More specifically, we wished to define the behavior under radiolytic conditions of those intermediate ionic species which the spectrometric studies suggested were important, and we hoped to arrive at a reasonable conciliation of the ionic reaction information derived from these different but complementary techniques. 

---