Alkoxy radicals molecules

Metal-Catalyzed Oxidation. Trace quantities of transition metal ions catalyze the decomposition of hydroperoxides to radical species and greatiy accelerate the rate of oxidation. Most effective are those metal ions that undergo one-electron transfer reactions, eg, copper, iron, cobalt, and manganese ions (9). The metal catalyst is an active hydroperoxide decomposer in both its higher and its lower oxidation states. In the overall reaction, two molecules of hydroperoxide decompose to peroxy and alkoxy radicals (eq. 5). 

Dw-butyl pcroxyoxalatc (DBPOX) is a clean, low temperature, source of t-butoxy radicals (Scheme 3.33).136 The decomposition is proposed to take place by concerted 3-bond cleavage to form two alkoxy radicals and two molecules of carbon dioxide. 

The latter reactions with NO and N02 are less likely than the first three possibilities. With kM) 31 3 X 10-11 cm3 molecule-1 s-1 at 298 K and 1 atm pressure, the pseudo-first-order rate of reaction, /c30[NO] or 3I[N02], is 75 s l at 100 ppb of NO or N02. For a species such as (CH3)3CO that can only decompose (and then only relatively slowly), these reactions could be responsible for 10% of the alkoxy radical reaction at the relatively high NO and N02 concentrations of 100 ppb. 

The involvement of an intramolecular hydrogen abstraction in the Barton reaction is not necessarily limited to those molecules with rigid stereochemistry. In fact, simple aliphatic nitrites undergo the Barton reaction with equal ease. Thus, the principal product obtained from the photolysis of ra-octyl nitrite20 in benzene solution is the dimer of 4-nitroso-l-octanol however, photolysis of n-octyl nitrite in n-heptane20 produced a mixture of 7/-nitroso heptanes in addition to 4-nitroso-l-octanol in the ratio l 4.5,f respectively. The formation of y-nitroso heptane obviously results from an attack of the intermediate alkoxy radical on the solvent molecule. The intermediate alkyl radical then collapses, according to eq. 2. For the sake of convenience we have indi-... 

Figure 2.9 Abstraction of a hydrogen atom by an alkoxy radical from a susceptible molecule.

The first isolable product of oxidation of the triglyceride is a tasteless and odourless hydroperoxide. The hydroperoxide can undergo homolysis, usually catalysed by a metal ion, to form an alkoxy radical whose decomposition may follow either ( > or T scission, ( > Scission gives a free aldehyde with flavour and/or odour along with an intermediate free triglyceride radical that can react further. Alternatively the alkoxy radical decomposes according to () scission, leading to a bound, odourless aldehyde and an R° radical that can react with other neutral molecules, or with itself, to form R-R. 

Notice that we didn t put HBr on the list of molecules that form radicals by homolysis relative to the weak bonds we have been talking about, the H-Br bond is quite strong (just about as strong as a C-C bond). Yet we said that Br radicals were involved in the addition reaction we talked about on p. 1020. These radicals are formed by the action of the alkoxy radicals (generated by homolysis of the peroxide) on HBr—a process known as radical ... 

The alkoxy radical may also abstract a hydrogen atom from another molecule to yield an alcohol and a new free radical, as shown ... 

B. Hydrogen Atom Abstraction is the other alkoxy-radical-like reaction of triplet n,7i carbonyls. The substrate can be another molecule, in which case a pair of radicals is formed or an accessible hydrogen in the same molecule, in which case a diradical is formed. 

The above examples demonstrate that diastereoselectively labeled substrates can be used to probe stereochemical features in gas-phase reactions of flexible molecules. In addition, this approach is not confined to metal-ion chemistry and not even to charged species, cf. the Barton rearrangement of neutral alkoxy radicals mentioned above. Instead, the decisive factor for the observation of a significant SE is a rate-determining activation barrier in an appropriate energy regime. 

Studies on the photolysis of simple aliphatic and alicyclic nitrites have confirmed the need for a six-membered transition state, which decides the point of attack on a carbon chain [ 18]. The photolysis of a nitrite (9) gives an active alkoxy radical and a molecule of nitric oxide, which is itself a stable free radical. These radicals must at first be confined within a solvent cage (lo), for no significant scrambling of isotopic nitrogen occurred when a mixture of the nitrite (14) and... 

The alkyl radical initially formed reacts readily with oxygen to give the corresponding alkylperoxy radical, which may abstract hydrogen from a fuel molecule to form the alkylhydroperoxide or alternatively decompose to yield an aldehyde and an alkoxy radical. Some workers thought that this decomposition was preceded by an isomerization of the alkylperoxy radical, the activation energy of which had been estimated by Semenov [3] to be ca. 20 kcal. mole. Shtern was of the opinion that the major, if not the only, fate of the alkylperoxy radical was decomposition, but in contrast to other workers he believed that it must involve scission of a C—C bond and could not lead to the formation of a carbonyl compound and hydroxyl radical. 

The initiating step is considered to be rupture of the weakest band in the molecule, followed by breakdown of the resulting alkoxy radical, viz. 

The most widely used antioxidants are free radical scavengers that remove reactive radicals formed in the initiation and propagation steps of autoxidation. A number of natural or synthetic phenols can compete, even at low concentrations, with lipid molecules as hydrogen donors to hydroperoxy and alkoxy radicals, producing hydroperoxides and alcohols and an unreactive radical. (3-carotene reacts with per-oxy radicals, producing a less-reactive radical. These stabilized radicals do not initiate or propagate the chain reaction. 

Aldehydes Of the volatiles produced by the breakdown of the alkoxy radicals, aldehydes are the most significant flavor compounds. Aldehydes can be produced by scission of the hpid molecules on either side of the radical. The products formed by these scission reactions depend on the fatty acids present, the hydroperoxide isomers formed, and the stability of the decomposition products. Temperature, time of heating, and degree of autoxidation are variables that affect thermal oxidation (7). 

Scheme 1. Possible reactions of the autoxidation process. R is an alkyl group of an unsaturated lipid molecule. H is an a-methylenic hydrogen atom easily detachable because of the activating influence of the neighboring double bond or bonds. RO is alkoxy radical, ROO" is peroxy radical, and is an initiator.

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