Hydroperoxyalkyl radicals

Isomerization of Alkylperoxy Radicals. Alkylperoxy radicals can isomerize to hydroperoxyalkyl radicals (19, 25, 45, 55), which subsequently react to regenerate free radicals capable of attacking C H2 + 2. 

Hydroperoxyalkyl radicals can react in several ways. First, of course, alkylperoxy radical isomerization is reversible (Reaction —4). Secondly, several modes of decomposition (Reaction 5) occur, giving O-hetero-cycles, alkenes, and saturated and unsaturated aldehydes and ketones. (Alcohols can also be formed by the decomposition of the alkylperoxy-alkyl radicals which result from isomerization by group transfer in alkylperoxy radicals.) These modes of decomposition have been enumerated (24, 25) only examples need be given here. Each mode of decomposition gives one or more product molecules plus one free radical, acting, therefore, as a propagation step. Moreover, the radical produced... 

The third important mode of reaction of hydroperoxyalkyl radicals is addition to molecular oxygen (Reaction 6). 

Addition of HOo to Alkenes. Although Reaction 7 is frequently postulated as a reaction which, with Reaction 2, constitutes the primary chain-propagation process during the oxidation of alkanes at temperatures >450°C., at lower temperatures competing reactions of H02 may render Reaction 7 ineffective (33). These competing reactions are the addition of H02 to an alkene to give a hydroperoxyalkyl radical (Reaction 8) and its bimolecular self-destruction (Reaction 9). 

Reaction 8 may, therefore, be the major chain-propagating reaction of H02 between 250° and 400°C. The radicals produced will, of course, undergo the same fates as those produced in Reaction 4, regenerating (eventually) alkyl radicals. The main difference between the alkene-H02 addition route and the alkylperoxy radical isomerization route is that in the former case the hydroperoxyalkyl radicals formed are necessarily a-radicals—i.e., radicals in which the unpaired electron is borne by a carbon atom adjacent to that bearing the hydroperoxy group, such as... 

It appears, then, that alkylperoxy radical isomerization is capable of producing hydroperoxyalkyl radicals during the oxidation of all alkanes and that alkene-hydroperoxy radical addition will serve a similar function during the oxidation of those alkanes which contain a high proportion of primary C—H bonds. It remains to determine the proportion of hydroperoxy alkyl radicals arriving by each route as equilibrium is approached. 

Calculation of this proportion, from the values of rate constants and equilibrium constants given in the Appendix, shows that it is always of the order of 1%. The route to hydroperoxyalkyl radicals via alkenes is therefore a minor one at temperatures of ca. 550°K. This is especially true with respect to the oxidation of large alkanes such as 2-methyl-pentane. 

Large alkyl radicals are oxidized to alkylperoxy radicals which isomerize to hydroperoxyalkyl radicals the decomposition of these gives molecular products and hydroxyl radicals which attack alkanes unselec-tively to regenerate alkyl radicals. The alkene-H02 addition route is unimportant. 

Small alkyl radicals, such as ethyl, are oxidized predominantly to the corresponding alkene because the isomerization of an a-hydroperoxy-alkyl radical to the corresponding alkylperoxy radical competes successfully with its decomposition to an oxiran and a hydroxyl radical. The formation of alkenes via a-hydroperoxyalkyl radicals (and not vice-versa) cannot be excluded, however. 

More recently, Lucquin and co-workers [63, 64] have shown from studies of the oxidation of n-butane and isobutane that the alkene theory is in fact at variance with experiment. Thus, on this theory the negative temperature coefficient is seen as a direct consequence of the increasing instability with temperature of the hydroperoxyalkyl radical, viz. 

On the basis of the alkene theory the hydroperoxalkyl radical initially formed must necessarily be the a-hydroperoxyalkyl radical, e.g. for the oxidation of n-butane... 

Methyloxetan can only be formed if the a-hydroperoxyalkyl radical isomerizes to the but-2-ylperoxy radical, which then re-isomerizes to yield the j3-hydroperoxyalkyl radical, reactions (—14a), (14/3) and (16/3),... 

The simple decomposition of a hydroperoxyalkyl radical to an O-heterocycle with elimination of OH is an irreversible unimolecular process, e.g. 

Thermodynamic and kinetic parameters for the decomposition of hydroperoxyalkyl radicals by cyclization at 600 °K (from ref, 107) C H2 00H O-heterocycle + OH... 

Kinetic parameters for the -scission decomposition of a-hydroperoxyalkyl radicals... 

Reaction (14) is of course a reversible reaction and hence the reverse isomerization of the hydroperoxyalkyl radical will compete with its decomposition and further oxidation, viz. 

The estimated values of fe 14 for oc, /3, 7 and 6-H transfer are given in Table 3 (p. 275) from which it can be seen that the reverse reaction is faster than the forward reaction. However, it can be seen by compEirison with Table 4 (p. 278) that the reverse isomerization of 7 and 5-hydro-peroxyalkyl radicals is much slower than their decomposition to tetra-hydrofurans and tetrahydropyrans, respectively. Reactions (—147) and (—145) will therefore be of little significance. On the other hand, the reverse isomerization of a- and j3-hydroperoxyalkyl radicals would be expected faster than either their decomposition to oxirans and oxetans, respectively or than their decomposition to the conjugate alkene and j3-scission products respectively. Reactions (—14a) and (—14j3) would thus be expected to be important. As discussed in Sect. 3.2.2, however,... 

Further oxidation of hydroperoxyalkyl radicals by addition of oxygen to give hydroperoxyalkylperoxy radicals, viz. 

It was pointed out in Sect. 3.2.2(a)(iv) that on the basis of, 6-scission decompositions of hydroperoxyalkyl radicals equal yields of lower carbonyl compounds and their corresponding lower alkene would be expected, but that this is not found in experiment where the yield of the... 

A pseudo-steady-state treatment was developed for this scheme in which it was assumed that the chains are long, that the rates of formation of the products are controlled by the rates of propagation of the chains, that the formation of alkyl radicals occurs only via reaction (12) and that the sum of the rates of further reactions of hydroperoxyalkylperoxy radicals is Isirge compared with their rate of formation. In order to simplify the mathematical treatment, it was also necessary to assume that reactions (2) and (—4,1) Eire slow and that the more complex decomposition reactions of hydroperoxyalkyl radicals can be ignored. 

The rate of formation of a product via hydroperoxyalkyl radical decomposition is then given by... 

Comparison of the ease of formation of -hydroperoxyalkyl radicals for different parent alkanes at 600 °K... 

It can be seen from Table 3 that the most facile isomerizations are those which involve 1 5 H-transfer (i.e. the formation of a 6-membered transition state ring) to form a j3-hydroperoxyalkyl radical, e.g. 

The ease of formation of /3-hydroperoxyalkyl radicals from the alkane increases with molecular weight as shown in Table 16. Thus, for example, isomerization involving 1 5 H-transfer is impossible for ethylperoxy and prop-2-ylperoxy radicals, while isomerization of the pent-2-ylperoxy radicals leads to the lowest molecular weight hydroperoxyalkyl radical which can be formed by initial attack at a secondsiry C—H bond followed by isomerization involving 1 5 H-transfer from another secondary C—H bond. 

H-transfer is always ca. 10 faster than 1 4 H-transfer at 600 °K (see Table 3), so it will predominate when the molecular structure of the fuel permits. Simple estimation of the relative concentrations of the hydroperoxyalkyl radicals derived from propane, n-butane and n-pentane illustrates this. Thus, if the relative frequency of attack by OH at primary, secondary and tertiary C—H bond is taken as 2 3 5 [102], then the relative concentrations of propyl, butyl and pentyl radicals may be obtained. The equilibrium constant for reaction (3)... 

Summation of the concentrations of like hydroperoxyalkyl radicals calculated in this way shows that the relative concentrations of a-hydro-peroxyalkyl radicals decreases rapidly with increase in molecular weight (Table 17). 

Since cyclization of 7-hydroperoxyalkyl radicals and j3-scission of /3-hydroperoxyalkyl radicals are ca. 10 times faster than 3-scission of cyclization of o-hydroperoxyalkyl radicals, product formation via j3- and 7-hydroperoxyalkyl radicals will increase rapidly. 

The major product from the oxidation of n-heptane [83, 84] is the conjugate 0-heterocycle 2-methyl-5-ethyltetrahydrofuran. The predominant chain cycle therefore involves initial attack at a secondary C—H, followed by addition of oxygen, 1 6-hydrogen transfer from another secondary C—H and decomposition of the 7-hydroperoxyalkyl radical by simple cyclization and loss of OH, e.g. 

The yields of the corresponding /3-scission products are much smaller and clearly /3-scission decomposition of 7-hydroperoxyalkyl radicals, reaction (2I7) below, does not compete effectively with their decomposition by cyclization to tetrahydrofurans. 

In contrast, compounds arising from /3-scission of C—C bonds are the major products formed during the oxidation of 3-ethylpentane, their yields being ca. 3 times as large as those of the corresponding oxetans. /3-Scission decomposition of /3-hydroperoxyalkyl radicals competes effectively, therefore, with their decomposition by cyclization to oxetans. 

The values of logi given in this scheme on the arrows are those estimated by Fish [107] for cyclization and by Cullis and co-workers [86] for jS-scission decompositions of hydroperoxy-3-ethylpentyl radicals at 600 °K.) The product distribution will depend, therefore, on the relative rates of formation of a-,/3- and 7-hydroperoxyalkyl radicals formed from the alkane, which will of course depend upon its structure. 

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