2-Hydroxy-2-propyl radicals

Heberger K and Fischer H 1993 Rate constants for the addition of 2-hydroxy-2-propyl radical to alkenes in solution Int. J. Chem. Kin. 25 913-20... 

Simple alkyl radicals such as methyl are considered to be nonnucleophilic. Methyl radicals are somewhat more reactive toward alkenes bearing electron-withdrawing substituents than towards those with electron-releasing substituents. However, much of this effect can be attributed to the stabilizing effect that these substiments have on the product radical. There is a strong correlation of reaction rate with the overall exothermicity of the reaction. Hydroxymethyl and 2-hydroxy-2-propyl radicals show nucleophilic character. The hydroxymethyl radical shows a slightly enhanced reactivity toward acrylonitrile and acrolein, but a sharply decreased reactivity toward ethyl vinyl ether. Table 12.9 gives some of the reactivity data. 

CH3)2CDOH it is slowed by a factor of ca 5. This confirms that the primary reaction of 3BA with isopropyl alcohol is abstraction of the methine hydrogen to give BAH, which is observed spectroscopically, and the 2-hydroxy-2-propyl radical. These radicals combine and disproportionate to form the stable products. 

Since the quantum yields of 2-propanone and benzopinacol both are nearly unity when the light intensity is not high, it is clear that two of the radicals, 4, must be formed for each molecule of 2 that becomes activated by light. This is possible if the 2-hydroxy-2-propyl radical formed by Equation 28-7 reacts with 2 to give a second diphenylhydroxymethyl radical ... 

This reaction is energetically favorable because of the greater possibility for delocalization of the odd electron in 4 than in the 2-hydroxy-2-propyl radical. 

The 2-hydroxy-2-propyl radical anion is a stronger reductant than the hydroxymethyl radical anion. Thus, a number of reactions are readily given by the for-... 

The data shown in Scheme 12.7 confirm that addition of (substituted) benzoyl radicals is slower than that of counter radicals (Scheme 12.7b), and therefore the benzoyl adducts have weak signals in the TR ESR spectra. A similar conclusion was obtained by FT TR ESR study of the photolysis of IRG2959 in the presence of NBA (Schemes 12.1 and 12.3)." ° ESR signals of two products of photolysis of IRG2959, namely, 2-hydroxy-2-propyl radical and substimted benzoyl radical, demonstrate quite different time dependences in the presence of acrylate. The signal of 2-hydroxy-2-propyl radical disappears faster with an increase in acrylate concentration, whereas benzoyl radical is practically unaffected by the presence of acrylate in concentrations up to 0.1 M." ° However, one can obtain only estimations of kinetic rate constants A add... 

A suggestion was made that A add of the 2-hydroxy-2-propyl radicals of DAR to MA (Schemes 12.1 and 12.3) decreases with the viscosity increase,which would be a sign of a diffusion-controlled or a diffusion-enhanced reaction.In fact, an increase in viscosity 1200 times leads to a decrease in A add only 5 times and not by orders of magnitude, which does not allow the classification of the addition as almost a diffusion-controlled (diffusion-enhanced) reaction. 

The observed E/A pattern means that Fm is negative. Because x is positive for a T-precursor, the J value should be negative. This negative J value has often been observed for neutral radical pairs. From Problem 5-11, the HF intensity ratio of the CEDEP spectmm of the 2-hydroxy-2-propyl radical in this reaction is obtained to be -1 -2 /3 -5 /3 0 sVs 2>/3 1, which is -l -3.46 -8.66 0 8.66 3.46 1. This ratio is quite different from that for the thermal equilibrium, 1 6 15 20 15 6 1, as shown in Problem 2-3. 

Problem 5-11. Calculate the HF intensity ratio of the CIDEP spectrum of the 2-hydroxy-2-propyl radical for the multiplet effect where Ag = 0 and p is positive. 

To study the addition of 2-hydroxy-2-propyl radicals to alkenes, Batchelor and Fischer employed a ketone as the source of these radicals, which then develop multiplet polarizations from F-pairs the normal reaction product is isopropanol. The addition of an alkene scavenges some part of the radicals and thus decreases the isopropanol polarizations. Time-resolved CIDNP experiments yield the addition rate. The advantage of analysing multiplet CIDNP instead of net CIDNP is that relaxation effects are avoided. The method was reported to be applicable for scavenging rate constants between and 10 M s . ... 

Radical Additions. Batchelor and Fischer [109] measured absolute rate constants kaM for addition of 2-hydroxy-2-propyl radicals 21 to alkenes by a new method, using time-resolved CIDNP. Photoexcited 2,4-dihydroxy-2,4-dimethylpentan-3-one 20 served as a source of these radicals. Chart XI summarizes the relevant steps of the underlying reaction scheme in a simplified manner. Their key idea was to evaluate not the net polarizations, which may be strongly distorted by nuclear spin relaxation in the radicals, but the multiplet polarizations, which had previously [110] been shown to be uninfluenced by relaxation at the paramagnetic stage provided that the... 

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. 

Hydroxy-2-methylpropiothenone la is known for about ten years as an effective photoinitiator (1). A benzoyl and a (2-hydroxy-2-propyl) radical are formed by a photcxhemic l cL-cleavage (2-4). 

The results of Muralidharan and Espenson (1984) utilize an ingenious preparation of the 2-hydroxy-2-propyl radical to study the kinetics of both oxidation of Eu(II) and reduction of Eu(III) ... 

A comparative time-resolved CW EPR and FT EPR investigation has been carried out for the addition of 2-hydroxy-2-propyl radicals to acrylate and methacrylate monomers by Weber et al. The 2-hydroxy-2-propyl radicals... 

Figure 12 Time-resolved FT EPR spectrum (a) and CW EPR spectrum (b) with a delay of 2 ps after the laser pulse of 100 mM n-butyl methacrylate in the acetone-2-propanol system, (c) Simulation of the spectrum. Peaks marked x are assigned to the unreacted 2-hydroxy-2-propyl radical (Reprinted with permission from Ref. 172)...

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