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In radical elimination, an unstable radical compound breaks down into a spin-paired molecule and a new radical compound. Shown below is an example of a radical elimination reaction, where a benzoyloxy radical breaks down into a phenyl radical and a carbon dioxide molecule. [7] A radical elimination reaction of a benzoyloxy radical
Radicals decrease in stability as they are closer to the nucleus, because the electron affinity of the orbital increases. As a general rule, hybridizations minimizing s-character increase the stability of radicals, and decreases the bond dissociation energy (i.e. sp 3 hybridization is most stabilizing).
Although the exo radical is less thermodynamically stable than the endo radical, the more rapid exo cyclization is rationalized by better orbital overlap in the chair-like exo transition state (see below). (1) Substituents that affect the stability of these transition states can have a profound effect on the site selectivity of the reaction.
The oxyl radicals are unstable and believed to be transformed into relatively stable carbon-centered radicals. For example, di- tert -butyl peroxide ( t - Bu OO t -Bu) gives two t -butoxy radicals ( t -BuO•) and the radicals become methyl radicals (C H 3 •) with the loss of acetone .
The ethoxy and cyano groups are able to delocalize the radical ion in the transition state, thus stabilizing the radical center. The rate enhancement is due to the captodative effect. When R = H, the reaction has the largest energy of activation because the radical center is not stabilized by the captodative effect.
The substituents on carbon are limited to Mes*, however, due to the limitation of the phosphaalkyne starting material. Most diradicaloids of this type can be handled in air and display high kinetic stability due to the steric protection provided by the Mes* substituents on the carbon radical centers. Figure 7.
For example, hexanoyl chloride 1 can be brominated in the alpha-position by NBS using acid catalysis. [ 10 ] The reaction of enolates , enol ethers, or enol acetates with NBS is the preferred method of α-bromination as it is high-yielding with few side-products.
The living nature of NMP is due to the persistent radical effect (PRE). [3] The PRE is a phenomenon observable in some radical systems which leads to the highly favored formation of one product to the near exclusion of other radical couplings due to one of the radical species being particularly stable, existing in greater and greater concentrations as the reaction progresses while the other ...