Radicals aldehyde, loss

Mass Spectrometry Aldehydes and ketones typically give a prominent molecular- ion peak in their mass spectra. Aldehydes also exhibit an M— 1 peak. A major fragmentation pathway for both aldehydes and ketones leads to formation of acyl cations (acyliurn ions) by cleavage of an alkyl group from the carbonyl. The most intense peak in the mass spectrum of diethyl ketone, for exanple, is m/z 57, conesponding to loss of ethyl radical from the molecular- ion. 

Example More extensive substitution at the oxirane system brings additional dissociation pathways for the molecular ions. Nevertheless, one of the main reaction paths of molecular ions of glycidols gives rise to enol radical ions by loss of a aldehyde (R = H) or ketone molecule. [218] The reaction mechanism can be rationalized by the assumption of a distonic intermediate (Scheme 6.78) ... 

The health impairing and toxic elfects of oxidation of lipids are due to loss of vitamins, polyenoic fatty acids, and other nutritionally essential components formation of radicals, hydroperoxides, aldehydes, epoxides, dimers, and polymers and participation of the secondary products in initiation of oxidation of proteins and in the Maillard reaction. Dilferent oxysterols have been shown in vitro and in vivo to have atherogenic, mutagenic, carcinogenic, angiotoxic, and cytotoxic properties, as well as the ability to inhibit cholesterol synthesis (Tai et ah, 1999 Wpsowicz, 2002). 

There is some contribution due to / -scission of the alkyl radical formed by the type I process, particularly in the MIPK and tBVK polymers. Loss of carbonyl occurs from photoreduction or the formation of cyclobutanol rings, and also from vaporization of the aldehyde formed by hydrogen abstraction by acyl radicals formed in the Norrish type I process. As demonstrated previously (2) the quantum yields for chain scission are lower in the solid phase than in solution. Rates of carbonyl loss are substantially different for the copolymers, being fastest for tBVK, slower for MIPK, and least efficient for MVK copolymers (Table I and Figure 1). 

In addition to the spectroscopic investigations, there have been attempts to obtain structural and stereochemical information about radicals by chemical means.25 The approach generally taken is to generate radicals by one of the methods discussed in the next section at a carbon where stereochemistry can be determined. As an example, we may cite the experiment shown in Equation 9.6, in which an optically active aldehyde is heated in the presence of a source of radicals.26 The reaction follows the chain pathway indicated in Scheme 1 the loss of chirality indicates that the radical is either planar or, if pyramidal, undergoes inversion rapidly with respect to the rate (on the order of 10s sec-1) at which it abstracts a hydrogen atom from another molecule of aldehyde. 

McLafferty Rearrangement of Ketones and Aldehydes The mass spectrum of butyraldehyde (Figure 18-4) shows the peaks we expect at m/z 72 (molecular ion), m/z 57 (loss of a methyl group), and m/z 29 (loss of a propyl group). The peak at m/z 57 is from cleavage between the /3 and y carbons to give a resonance-stabilized carbocation. This is also a common fragmentation with carbonyl compounds like the other odd-numbered peaks, it results from loss of a radical. 

Exposure of LDL to free radicals leads to lipid peroxidation and to a progressive loss of vitamin E and carotenoid within 6h. Thereafter the polyunsaturated fatty acids 18 2 and 20 4 are degraded in a lipid-peroxidation process [17] and a large variety of aldehydes is formed the following have been identified and quantified 4-hydroxyhexanal, 4-hydroxyoctenal, 4-hydroxynonenal, propanal, butanal, pentanal, hexanal, 2,4-heptadienal and malonaldehyde (MDA). 

Pyrrolidine, piperidine and morpholine enamines of aldehydes and ketones have been the subject of a pioneering work by D. H. Williams and collaborators8. The most abundant fragment ion is produced by the loss of an alkyl radical R by allylic cleavage (Scheme 3). When R = H the loss of a H atom is more energy demanding and the... 

Consider the C-H bond in alkanes. Carbon is a more electronegative element than hydrogen. Consequently, the electron pair that forms this bond is shifted towards the carbon atom. In the extreme, an ionic representation of this bond can be given as pictured in 122 (Scheme 2.45). Within these conventions the carbon atom in an alkane can be approximated as a carbanion (oxidation level 0 by definition). Using this definition it becomes possible to apply oxidation-reduction terminology to the processes as if they occurred to ion pair 122. Thus, oxidation of 122 with the loss of one electron leads to the radical 123. With the loss of two electrons, the oxidation leads to carbocation 124. Similarly, the conversion of an alkane to an alcohol and the alcohol into an aldehyde and the aldehyde eventually to a carboxylic acid can unambiguously be classified as an oxidation sequence with the loss of two, four, and six electrons. The oxidation levels 1, 2, and 3 are ascribed respectively to these functional derivatives. The conversion of an alkane to an alkene or alkyne can be interpreted in an analogous fashion. 

A copper catalysed click (azide-alkyne cycloaddition) reaction has been used to prepare a fluorous-tagged TEMPO catalyst (Figure 7.20). TEMPO is a stable organic free radical that can be used in a range of processes. In this case, its use in metal-free catalytic oxidation of primary alcohols to aldehydes using bleach as the terminal oxidant was demonstrated. The modified TEMPO can be sequestered at the end of the reaction on silica gel 60 and then released using ethyl acetate for reuse in further reactions in this way the TEMPO was used four times with no loss in activity. 

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