Hydrogen atom abstraction from 2-propanol

Photolysis of [Rh(tfacac)3] (tfacac is the unsymmetrically substituted 1,1,1-trifluoromethyl-acac) reveals the existence of two photoinduced reaction paths the relative efficiency of the two paths is dramatically solvent dependent.1140 In cyclohexane, mer- cis isomerization is the only observed photoreaction, but if ethanol or 2-propanol is added to the solvent, the photoisomerization efficiency decreases, and photodecomposition occurs. The nature of the photodecomposition products is not specified, but the enhanced photoreactivity in the presence of tri(n-butyl)stannane, a hydrogen atom donor, and flash and continuous photolysis studies in mixed-solvent systems strongly implicate hydrogen atom abstraction from the solvent as a key step in the photodecomposition of wer-[Rh(tfacac)3] and suggests that the photo reactive states have considerable radical character .1140 Analysis of quantum efficiencies implies that at least two distinct photoproduced excited states must be involved. 

The hydrogen atoms abstract the methine hydrogen from 2-propanol to give hydrogen and 2-propanol radical... 

Propanol. 2-Propanol was chosen as a model for a simple alcohol. Irradiation under vacuum gave hydrogen and acetone as major products along with smaller amounts of methane, acetaldehyde, carbon monoxide, tert-butyl alcohol, and ethanol. These minor products must arise mainly from acetone. Acetaldehyde may be formed either by direct photolysis of 2-propanol, hydrogen atom abstraction by acetyl radical, or /3-scission of the 2-propyloxy radical. The formation of tert-butyl alcohol implies the presence of methyl and 2-hydroxy-2-propyl radicals. 

The spin trapping of NO as aminoxyl radicals (ARs) Rj-NO -R was observed by electron spin resonance (ESR) spectroscopy in various radical reactions. For example, such ARs are formed in the course of the photodecomposition of 2,2 -azobisisobutyronitrile (AIBN) in aliphatic alcohols (methanol, ethanol, 2-propanol) [15]. In this case, Rj is the radical NhC-CICHj) from AIBN, and R is derived from alcohol molecules as a result of a hydrogen atom abstraction by Rj. 

In 2-propanoI, the quantum yield for photolytic conversion of benzophenone to the coupled reduction product is 2.0. The reason is that the radical remaining after abstraction of a hydrogen atom from 2-propanol transfers a hydrogen atom to ground-state benzophenone in a nonphotochemical reaction. Because of this process, two molecules of benzophenone are reduced for each one that is photoexcited ... 

The chlorine atom in 4-chloropyridine can be replaced photochemically by the dimethyl ketyl radical755. Irradiation of 4-chloropyridine in a 4 1 mixture of 2-propanol and water gives a low yield (2%) of 2-(4 -pyridyl)-2-propanol. Sensitization by benzophenone increases the yield to 25%, but the product is now accompanied by 6% of diphenyl-(4-pyridyl)methanol. The major product is believed to be formed via hydrogen abstraction from 2-propanol by photoexcited pyridine. Protonated pyridines do not undergo this abstraction process, and accordingly the product yield decreases under acidic conditions. The radical (259) formed from the pyridine will combine (at position 4) with the dimethyl ketyl radical (260) and elimination of HC1 from the adduct (261) completes the reaction (equation 194). 

A novel method for generating semidione radicals has recently been reported by Monroe, Weiner, and Hammond 178) who found that the quantum yield for photoreduction of camphorquinone in 2-propanol was markedly enhanced when benzophenone was added and the solution irradiated at wavelength 3660 A where most of the light was absorbed by benzophenone. Instead of benzpinacol formation, the dione underwent photoreduction. Similar enhancement was not observed with jw-methoxy-acetophenone which does not abstract hydrogen from 2-propanol. The conclusion was that the ketyl radical, formed in the efficient H-abstraction reaction of benzophenone, transferred a hydrogen atom to camphorquinone to generate the semidione radical. It was suggested that this phenomenon be called "chemical sensitization . 

The benzophenone-sensitized photo-oxygenation of 2-propanol (53) may be considered as a typical example of a Type I process, in which the electronically excited benzophenone initiates a free radical oxidation by abstracting a hydrogen atom from 2-propanol. The initiation is then followed by O2 addition to the 2-hydroxyisopropyl radical and reaction... 

This step is followed by the abstraction of a hydrogen atom (complete with one electron) from a 2-propanol molecule by a benzophenone molecule in the Ti level, forming a... 

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