Ethoxy radicals

Ethane. Ethane VPO occurs at lower temperatures than methane oxidation but requires higher temperatures than the higher hydrocarbons (121). This is a transition case with mixed characteristics. Low temperature VPO, cool flames, oscillations, and a NTC region do occur. At low temperatures and pressures, the main products are formaldehyde, acetaldehyde (HCHOiCH CHO ca 5) (121—123), and carbon monoxide. These products arise mainly through ethylperoxy and ethoxy radicals (see eqs. 2 and 12—16 and Fig. 1). 

The proposed mechanism for producing ethanol [64-17-5] from butane involves -scission of a j -butoxy radical (eq. 38). The j -butoxy radicals are derived from j -butylperoxy radicals (reaction 14 (213)) and/or through some sequence involving reaction 33. If 25% of the carbon forms ethanol, over 50% must pass through the j -butoxy radical. Furthermore, the principal fate of j -butoxy radicals must be the P-scission reaction the ethoxy radical, on the other hand, must be converted to ethanol efficiently. 

Fig. 27. Product translational energy distribution of the H-atom production channel in the secondary photodissociation of ethoxy radical at 193.3 nm. The onsets of the relevant electronic states of the possible CH3CHO product are indicated in the figure. The signals from the primary photodissociation of ethanol are labelled. (FYom Xu et al.171)...

The thermal unimolecular decomposition of ethoxy radicals (C2H5O ) was investigated at different temperatures and pressures. Under these conditions the fi-C—C scission CH3CH2O -f M CH2O - - CH3 - - M is the dominant decomposition channel. Excellent agreement between the experimental and calculated rate constants has been found. ... 

Ethoxy radical (CH3CH2O ) has enjoyed considerable interest as well. Choi et al. explored its photodissociation dynamics via photofragment translational... 

A direct cleavage of the ester to radicals similar to that observed in the vapor state94 and an internal hydrogen abstraction followed by radical formation98 have been proposed to explain the reaction in benzene. The internal hydrogen abstraction mechanism appears to be the more reasonable. Direct cleavage would yield ethoxy radicals, but no products derived from them are observed. The low quantum yield of ester disappearance observed for ethyl... 

Thynne (35) found that ethoxy radicals add to C2H4 at 70° and 160°C. ... 

Various compounds were shown to sensitize the photochemical decomposition of pyridinium salts. Photolysis of pyridinium salts in the presence of sensitizers such as anthracene, perylene and phenothiazine proceeds by an electron transfer from the excited state sensitizer to the pyridinium salt. Thus, a sensitizer radical cation and pyridinyl radical are formed as shown for the case of anthracene in Scheme 15. The latter rapidly decomposes to give pyridine and an ethoxy radical. Evidence for the proposed mechanism was obtained by observation of the absorption spectra of relevant radical cations upon laser flash photolysis of methylene chloride solutions containing sensitizers and pyridinium salt [64]. Moreover, estimates of the free energy change by the Rehm-Weller equation [65] give highly favorable values for anthracene, perylene, phenothiazine and thioxanthone sensitized systems, whilst benzophenone and acetophenone seemed not to be suitable sensitizers (Table 5). The failure of the polymerization experiments sensitized by benzophenone and acetophenone in the absence of a hydrogen donor is consistent with the proposed electron transfer mechanism. 

The use of iV-alkoxy pyridinium salts is not limited to cationic polymerization. Since, in addition to cationic species, ethoxy radicals are also formed upon direct and sensitized irradiation of pyridinium salts (see above), pyridinium salt based photoinitiating systems may be used to initiate the polymerization of vinyl monomers that are prone to free radical polymerization. Kayaman et al. [71] recently polymerized mono- and bi-functional acrylate monomers by photosensitization of pyridinium salts. It therefore appears that pyridinium salts can promote both cationic and free radical polymerization and are, thus, eminently suitable for use in hybrid systems. 

Hizal et al. [79] have recently shown an interesting variation of pyridinium salt photodecomposition in polymer synthesis. In earlier investigations it became evident that ethanol is formed by the hydrogen abstraction of primary ethoxy radical if the photolysis is carried out in strong hydrogen donor solvents such as THF ... 

The first step in the pyrolysis of the alkyl nitrates has been supposed to be O N bond fission to give NO2 and an alkoxy radical. The activation energy is 39-5 kcal for methyl nitrate and 39-9 or 34-6 kcaP <> for ethyl nitrate. If the latter value for ethyl nitrate is taken, and assumed to be i)(EtO -NO2) a value for the heat of formation of the ethoxy radical in good agreement with that given by Rebbert and Laidler is obtained, so apparently we may put D(MeO -NO2) =40 kcal, and i)(EtO -NO2) =34 kcal. In the opinion of the present author, however, the mechanism of the reaction is not sufficiently well established to allow this to be done. The discrepancy between the result of Adams and Bawn and that of Phillips 390 is large and may well be because of the different pressure ranges in which these authors worked. A further examination of the effect of pressure on rate constant is necessary before it can be taken as established that the reaction is of the first order. 

Pyrolysis of ethyl radicals was found to be relatively unimportant although the reverse is true for larger alkyl radicals such as n-propyl, isobutyl and tert-butyl. Indeed, Sampson estimated that almost half the isobutane consumed gives radicals whose fate is pyrolysis [155]. On the other hand, radical—radical reactions are important and many of the minor products are believed to be formed from further reactions of ethoxy radicals formed in reaction (52)... 

All the major products of both the slow decomposition and explosive reaction can be accounted for qualitatively by reactions of the ethoxy radicals formed in the initial step which involves fission of the 0—0 bond. The reaction scheme is... 

At about 500 °K disproportionation is favoured as the products are mainly ethanol and acetaldehyde. Explosion leads to higher temperatures, and more ethoxy radicals decompose yielding more ethane and formaldehyde. 

The thermal decomposition of alkyl nitrates has received continuous attention over a number of years. The introduction of improved techniques has required the periodic re-evaluation of data on these very complex reactions . The substrate of choice has been ethyl nitrate. Early studies on the thermal decomposition suggested that the reaction became complex after the initial fission into ethoxy radicals and nitrogen dioxide . 

Gutman, D., N. Sanders, and J. E. Butler (1982). Kinetics of the reactions of methoxy and ethoxy radicals with oxygen. J. Phys. Chem. 86, 66-70. 

Organic radicals are generated when superoxide or the hydroxyl radical indiscriminately extract electrons from other molecules. Organic peroxy radicals are intermediates of chain reactions, such as hpid peroxidation. Other organic radicals, such as the ethoxy radical, are intermediates of enzymatic reactions that escape into solution (see Table 24.2). 

Fig. 11 Laser excitation spectrum of the ethoxy radical showing rotational structure with the indicated assignments.

Elmaimouni, L., C. Bourbon, C. Fittschen, J. P. Sawerysyn, P. Devolder Reaction rates of the C2H5O (ethoxy) radical with O2 and NO, in P.M. Borrell, P. Borrell, T. CvitaS, W. Seiler (eds), Proc. EUROTRAC Symp. 94, SPB Academic Publ., The Hague 1994, pp. 153-157. 

Fig. 2. Densities of states as function of the freely distributable energy for ethoxy radicals calculated with the Beyer Swinehart (B S) algorithm or the Whitten-Rabinovitch (W-R) equation. Calculations were done with DenSum from the MultiWell distribution.

Ethoxy radicals (C2H5O) are rather unstable and dissociate even at temperatures as low as 400 K in less than a millisecond. Therefore, the thermal dissociation of ethoxy is suitable to demonstrate the performance of different reaction analysis programs including MultiWell. The reaction channel leading to CH3 and CH2O, 10.5kcal/mol, has... 

The yield of ethoxy radicals has been estimated as GfCjHjO) = 1.50(15). 

---