Methyl propionate, radical attack

The polar effect was at first invoked to explain various directive effects observed in aliphatic systems. Methyl radicals attack propionic acid preferentially at the a-position, ka/kp = 7.8 (per hydrogen), whereas chlorine " prefers to attack at the /3-position, ka/kp = 0.03 (per hydrogen). In an investigation of f-butyl derivatives, a semiquanti-tative relationship was observed between the relative reactivity and the polar effect of the substituents, as evidenced by the pK, of the corresponding acid. In the case of meta- and / ara-substituted toluenes, it has been observed that a very small directive effect exists for some atoms or radicals. When treated by the Hammett relation it is observed that p = —0.1 for H , CeHs , P-CH3C6H4 and CHs . On the contrary, numerous radicals with an appreciable electron affinity show a pronounced polar effect in the reaction with the toluenes. Compilation of Hammett reaction constants and the type of substituent... 

Polar effects can also be important in atom transfer reactions. 4 In an oft-cited example (Scheme 13), the methyl radical attacks the weaker of the C—H bonds of propionic acid, probably more for reasons of bond strength than polar effects. However, the highly electrophilic chlorine radical attacks the stronger of the C—H bonds to avoid unfavorable polar interactions. As expected, the hydroxy hydrogen remains intact in both reactions. 

Concurrently, the hydroperoxide may be converted to methyl ethyl ketone (MEK). If the initial radical attack is at the primary rather than the secondary carbon, the process makes propionic and formic acids. Reaction conditions can be changed to produce more MEK at the expense of some acetic acid. The maximum acetic acid/MEK ratio is 6.5-7 on a weight basis. If ethyl acetate is also formed, the ratio can go down to acetic acid/(ethyl acetate + MEK) of 3.6-4, with MEK being about 55 percent of the byproduct. 

Hydroquinone was beneficial to the reaction outcome, its effect being attributed to the suppression of radical side-reactions. The proposed mechanism consists of iniHal electrophilic attack of the metal onto the C—H bond of the arene to give an arylruthenium species with concomitant proton release, followed by alkene insertion into the Ru—C bond, -hydride elimination to liberate the cinnamate and a ruthenium hydride, and finally regeneration of the active catalyst either by insertion of another alkene into the Ru-H bond protonation of the alkylruthenium complex and elimination of methyl propionate (under an... 

Methyl radicals preferentially attack the hydrogen atoms on C-2 of propionic acid (389). On the other hand, chlorine atoms preferentially attack the hydrogen... 

Fig. 5-5 Interactions for the attack of methyl and chlorine radicals on propionic...

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