Oxygen radicals, radical strength reactivity

Before a discussion of specific reactions, let s briefly consider the nature of the carbonyl excited state. For ,ir states, a good model for the carbonyl excited state is the biradical-like structure shown in Eq. 16.54, especially for the triplet state. What should we expect from such a structure There are two reactive centers, a carbon-based radical and an oxygen radical. To first order they are independent, one electron in the plane and the other in the it system (see Figure 16.7). The oxygen radical should be extremely reactive. Recall the high bond strength of the O-H bond discussed in Chapter 2, and how this makes hydroxyl radical a... 

We discussed how the n,TT states of carbonyls, especially the triplets, have considerable radical character. In addition, oxygen-centered radicals are very reactive because of the high strengths of O-H and O-C bonds. Hydrogen atom abstraction should be expected, and indeed it is a common reaction. A typical reaction involves a carbonyl n,TT state undergoing a bimolecular hydrogen atom abstraction, and that is the basis for the common process called photoreduction. 

The presence of the OH group in alcohols makes alcohol combustion chemistry an interesting variation of the analogous paraffin hydrocarbon. Two fundamental pathways can exist in the initial attack on alcohols. In one, the OH group can be displaced while an alkyl radical also remains as a product. In the other, the alcohol is attacked at a different site and forms an intermediate oxygenated species, typically an aldehyde. The dominant pathway depends on the bond strengths in the particular alcohol molecule and on the overall stoichiometry that determines the relative abundance of the reactive radicals. 

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