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Radical elimination can be viewed as the reverse of radical addition. 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]
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 radical cyclization step usually involves the attack of a radical on a multiple bond. After this step occurs, the resulting cyclized radicals are quenched through the action of a radical scavenger, a fragmentation process, or an electron-transfer reaction. Five- and six-membered rings are the most common products; formation of smaller and ...
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).
Hyperconjugation can be used to rationalize a variety of chemical phenomena, including the anomeric effect, the gauche effect, the rotational barrier of ethane, the beta-silicon effect, the vibrational frequency of exocyclic carbonyl groups, and the relative stability of substituted carbocations and substituted carbon centred radicals, and the thermodynamic Zaitsev's rule for alkene stability.
In free radical polymerization, radicals formed from the decomposition of an initiator molecule are surrounded by a cage consisting of solvent and/or monomer molecules. [6] Within the cage, the free radicals undergo many collisions leading to their recombination or mutual deactivation. [5] [6] [9] This can be described by the following 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 .
In chemistry, chemical stability is the thermodynamic stability of a chemical system, in particular a chemical compound or a polymer. [1] Colloquially, it may instead refer to kinetic persistence , the shelf-life of a metastable substance or system; that is, the timescale over which it begins to degrade.