Secondary oxidation products carbonyl compounds

A number of methods are available for following the oxidative behaviour of food samples. The consumption of oxygen and the ESR detection of radicals, either directly or indirectly by spin trapping, can be used to follow the initial steps during oxidation (Andersen and Skibsted, 2002). The formation of primary oxidation products, such as hydroperoxides and conjugated dienes, and secondary oxidation products (carbohydrides, carbonyl compounds and acids) in the case of lipid oxidation, can be quantified by several standard chemical and physical analytical methods (Armstrong, 1998 Horwitz, 2000). 

According to the Cd 18-90 AOCS ° official method, the ANV is 100 times the optical density measured in a 1 cm cell, at 350 nm, of a solution containing 1.00 g of oil in 100 ml of the test solution. The measured absorbance is due to Schiff bases (167) formed when p-anisidine (166) undergoes condensation reaction with carbonyl compounds, according to equation 55. The carbonyl compounds are secondary oxidation products of lipids, such as a, S-unsaturated aldehydes and ketones derived from the hydroperoxides (see Scheme 1 in Section n.A.2.c), and their presence points to advanced oxidation of the oil. 

Secondary oxidation products oxidation indices, 656, 665-72 acid value, 672 anisidine value, 656, 666 carbonyl compounds, 656, 669-71 conjugated dienes value, 671-2 thiobarbituric acid reactive substances, 656, 666-9... 

Additionally, key secondary oxidation products contribute distinctive aromas characteristic to certain fish species. In salmon, co-oxldatlon of polyunsaturated fatty acids of fish oils with salmon-specific carotenoid pigments leads to the formation of a characterizing cooked salmon flavor compound, and changes the ratio of carbonyl compounds formed compared to that for pure fish oil. 

Carbonyl compounds in oxidized fats and oils are the secondary oxidation products that originate from decomposition of hydroperoxides. They usually have low threshold values and hence are responsible for off-flavor development in oxidized oils. Therefore, content of carbonyl compounds corresponds with sensory data. 

Carbonyl compounds in oxidized lipids are the secondary oxidation products resulting from the decomposition of the hydroperoxides. They can be quantified by the reaction with 2,4-dinitrophenylhydrazine and the resulting colored hydrazones are measured spectrophotometrically at 430-460 nm. The carbonyl value is directly related to sensory evaluation, because many of the carbonyl molecules are those responsible for off-flavor in oxidized oil. The anisidine value is a measure of carbonyl compounds that have medium molecular weight and are less volatile (Frankel 1998). It can be used to discover something about the prior oxidation or processing history of an oil. 

Conjugate addition. Trialkylstannyllithium reagents prepared in ether solution undergo predominantly 1,2-addition to cyclohexenones, but solutions prepared in THF undergo conjugate addition to almost all enones, even hindered ones. The intermediate lithium enolates can be alkylated with reactive alkyl halides. These reactions are useful because secondary alkylstannanes are converted into the corresponding carbonyl compound by oxidation with Cr03-2Py. Tertiary alkylstannanes are also oxidized by CrOa-lPy, but mixtures of alcohols and products of dehydration are formed. 

Lipid autoxidation is generally believed to involve a free- radical chain mechanism (1) initiation steps that lead to free radicals (R ), (2) propagation of the free radicals (R -I-O2 —> ROO, ROO -1-RH — ROOH-I-R ), and (3) termination steps R -H R R—R, R- ROO- ROOR, ROO ROO O 2 ROOR (or alcohol and carbonyl compound). The oxidation of lipids results in peroxides as primary oxidation products, which in turn degrade further to secondary oxidation products, including aldehydes, ketones, epoxides, hydroxy compounds, carboxylic acids, oligomers, and polymers. 

Carbonyl Compounds by Oxidation of Alcohols and Aldehydes. A critical assessment of the use of palladium catalysts in the aerobic oxidation of alcohols has concluded that Pd(OAc)2-Et3N is the most versatile and convenient catalyst system and that this often functions under especially mild conditions.There have been many other recent advances in this field and such that there is now a wealth of methods available for effecting the palladium-catalyzed oxidation of alcohols. A procedure using pyridine under an oxygen atmosphere has been shown to convert benzylic and aliphatic alcohols into the corresponding aldehydes or ketones. The yields of product are frequently over 90%. Replacing pyridine with (—)-sparteine in such processes allows for the oxidative kinetic resolution of chiral secondary alcohols. 

Chromium compounds decompose primary and secondary hydroperoxides to the corresponding carbonyl compounds, both homogeneously and heterogeneously (187—191). The mechanism of chromium catalyst interaction with hydroperoxides may involve generation of hexavalent chromium in the form of an alkyl chromate, which decomposes heterolyticaHy to give ketone (192). The oxidation of alcohol intermediates may also proceed through chromate ester intermediates (193). Therefore, chromium catalysis tends to increase the ketone alcohol ratio in the product (194,195). 

Another factor complicating the situation in composition of peroxyl radicals propagating chain oxidation of alcohol is the production of carbonyl compounds due to alcohol oxidation. As a result of alcohol oxidation, ketones are formed from the secondary alcohol oxidation and aldehydes from the primary alcohols [8,9], Hydroperoxide radicals are added to carbonyl compounds with the formation of alkylhydroxyperoxyl radical. This addition is reversible. 

Varieties of primary and secondary alcohols are selectively oxidized to aldehyde or carbonyl compounds in moderate to excellent yields as summarized in Table 3. As can be seen, /(-substituted benzyl alcohols (e.g., -Cl, -CH3, -OCH3, and -NO2) yielded > 90% of product conversion in 3-4 h of reaction time with TOP in the range of 84-155 h (entries 2-5, Table 3), Heterocyclic alcohols with sulfur- and nitrogen-containing compoimds are found to show the best catalytic yield with TOP of 1517 and 902 h for (pyrindin-2-yl)methanol and (thiophene-2-yl) methanol, respectively (entries 9 and 10, Table 3). Some of aliphatic primary alcohols (long chain alcohols) and secondary alcohols (cyclohexanol, its methyl substituted derivatives and norboman-2-ol) are also selectively oxidized by the membrane catalyst (entries 11-14 and 15-17, Table 3) with TOP values in the window of 8-... 

Other indices measure a secondary stage of oxidation, such as the anisidine value (ANV), pointing to formation of carbonyl compounds, capable of undergoing condensation reactions with p-anisidine, and the thiobarbituric acid reactive substance (TBARS) pointing to the presence of malondialdehyde (MDA) in particular. In biological systems, TBARS is of widespread use as a measure for the extent of oxidation damage. Another test for stability of oils to oxidation is based on the development of acidity as secondary product, for example, standards using the Rancimat equipment or a similar setup. 

In solution these esters undergo a variety of transformations which are dependent on the reaction conditions. In benzene, decomposition to carbon monoxide and carbonyl compounds is observed either upon direct irradiation94 or with benzophenone sensitization.33 In cyclohexane a complex product mixture is obtained.95 Addition of solvent to the carbonyl group is observed when the reaction is carried out in cyclohexene.54 At room temperature photoreduction takes place when the reaction is carried out in a secondary alcohol.96-97 However, in the case of the phenylglyoxylates quite a different reaction is observed when the reaction is carried out at elevated temperatures. The ester is reduced to the mandelate ester of the solvent alcohol, and the alcohol moiety of the ester is oxidized to the corresponding carbonyl compound. The pyruvates have not been studied at an elevated temperature. 

Oxidation of amines.1 The radical effects oxidation of primary or secondary amines to imines at 25° (4-8 hours). The products are generally isolated as the dinitrophenylhydrazones of the corresponding carbonyl compounds. 

A primary alcohol is oxidised by chromic acid to the corresponding aldehyde while a secondary alcohol yields a ketone tertiary alcohols are generally unaffected or are decomposed into non-ketonic products. Oxidation therefore provides a method for distinguishing between primary, secondary and tertiary alcohols and characterisation of the carbonyl compound provides a means of identifying the alcohol ... 

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