Radicals aldehydes

Category 1, Simple Cleavage into Radicals.Aldehydes and ketones absorb in the 230-330-nm region. This is assumed to result from an n—>tt singlet-singlet transition. The excited aldehyde or ketone can then cleave. ... 

Lipid peroxides Lipid peroxy radicals Lipid alkoxyl radicals Aldehydes... 

Figure4.14 Schematic representation ofthe proposed mechanism ofthe photocatalytic reforming of glucose on Pt-Ti02 involving the formation of various radicals, aldehydes, and carboxylic acids. Adapted from [170] (2008) with permission from Elsevier.

For reviews of free-radical aldehyde decarbonylations, see Vinogradov Nikishin Russ. Chem. Rev. 1971, 40, 916-932 Schubert Kintner. in Patai, Ref. 189, pp. 711-735. 

Metals can play a clear but small role in such epoxidations or esterifications with a sacrificial aldehyde. First, it has long been known that the free radical aldehyde autoxidation can be initiated and therefore accelerated by metal ions. For instance, high concentrations of peracid can be obtained with dissolved Fe catalysts (5). As a rule, the presence of a metal allows autoxidation at considerably lower temperature than for the noninitiated reaction. The exact nature of the metal is not critical. Second, epoxidations by a peracid may be metal centered however, at the typical temperatures of the Mukuyiama epoxidation, the uncatalyzed reaction of peracid and olefin is usually fast enough to make the metal redundant for this second step. 

The reagent combination described for the azidoselenenylation can also be used for other reactions involving selenyl radicals. Aldehydes can be transformed into selenoesters by hydrogen atom abstraction and also methyl... 

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). 

A similar intramolecular hydroacylation is possible via reductive radical aldehyde addition to alkencs and subsequent oxidation of the resulting alcohol. Based on this observation a strategy towards the synthesis of cuparenoids [( )-a-cuparenone. ( )-epilaurene and laurene] was... 

Low-molecular mass carbonyls are among the most abundant and ubiquitous volatile organic compounds in the atmosphere. They are produced from industrial activity and incomplete combustion of fossil fuels and biomass. Many aldehydes are also emitted indoors (plastic, foam insulation, lacquers, etc.). As a source of free radicals, aldehydes play an important role in the ozone formation, in urban smog events, as well as in the photochemistry of the unpolluted troposphere. Aldehydes are recognized irritants of the eye and respiratory tract, and often, carcinogenic and mutagenic characteristics are also attributed to them. 

So, for instance, acetic acid is carbonic acid with one oxygen substituted by methyl, acetone has two oxygens replaced by methyl radicals, aldehyde is the same except one of the two methyls is replaced by hydrogen, and alcohol is aldehyde with a third oxygen atom of carbonic acid replaced by hydrogen. (In Kolbean terms, one ignores the water molecules HO to the left of the period.)... 

Thus, at destruction of P j 02 and P(2)02 radicals aldehydes and acids are with time accumulated due to the same linear law (compare (VI) and (VII) equations). 

Treatment of 2,2-dimethylselenazolidines with reactive aromatic aldehydes gives exchange of carbonyl radicals (Scheme 7Ti. 

Acetals and Acylals. Acetals, which contain the group >C(OR)2, where R may be different, are named (1) as dialkoxy compounds or (2) by the name of the corresponding aldehyde or ketone followed by the name of the hydrocarbon radical(s) followed by the word acetal. For example, CH3—CH(0CH3)2 is named either (1) 1,1-dimethoxyethane or (2) acetaldehyde dimethyl acetal. 

The carbon-oxygen double bond of the carbonyl group is opened, and the hydrogen sulfite radical is added. An increase in temperature reverses the reaction more easily for ketones than for aldehydes. 

Aldehydes are important because they are temporary reservoirs of free radicals (see eqs. 11 and 12). HCHO is a known carcinogen. Nitric acid is formed by OH attack on NO2 and by a dark-phase series of reactions initiated by O3 + NO2. Nitric acid is important because it is the second most abundant acid in precipitation. In addition, in southern California it is the major cause of acid fog. 

Oxidation begins with the breakdown of hydroperoxides and the formation of free radicals. These reactive peroxy radicals initiate a chain reaction that propagates the breakdown of hydroperoxides into aldehydes (qv), ketones (qv), alcohols, and hydrocarbons (qv). These breakdown products make an oxidized product organoleptically unacceptable. Antioxidants work by donating a hydrogen atom to the reactive peroxide radical, ending the chain reaction (17). 

Under moderate conditions, primary alkoxy radicals tend to undergo reaction 12 whereas secondary and tertiary alkoxys tend to undergo -scission. In general, the alkyl group that can form the lowest energy radical tends to become the departing radical. The -scission of secondary alkoxy radicals yields aldehydes as the nonradical products tertiary alkoxy radicals yield ketones. 

Reaction 21 is the decarbonylation of the intermediate acyl radical and is especially important at higher temperatures it is the source of much of the carbon monoxide produced in hydrocarbon oxidations. Reaction 22 is a bimolecular radical reaction analogous to reaction 13. In this case, acyloxy radicals are generated they are unstable and decarboxylate readily, providing much of the carbon dioxide produced in hydrocarbon oxidations. An in-depth article on aldehyde oxidation has been pubHshed (43). 

Acids are usually the end products of ketone oxidations (41,42,44) but vicinal diketones and hydroperoxyketones are apparent intermediates (45). Acids are readily produced from vicinal diketones, perhaps through anhydrides (via, eg, a Bayer-ViUiger reaction) (46,47). The hydroperoxyketones reportedly decompose to diketones as well as to aldehydes and acids (45). Similar products are expected from radical— radical reactions of the corresponding peroxy radical precursors. 

The cation—radical intermediate loses a proton to become, in this case, a benzyl radical. The relative rate of attack (via electron transfer) on an aromatic aldehyde with respect to a corresponding methylarene is a function of the ionization potentials (8.8 eV for toluene, 9.5 eV for benzaldehyde) it is much... 

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