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In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron. [1] [2] With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spontaneously dimerize. Most organic radicals have short lifetimes.
DNA photoionization is the phenomenon according to which ultraviolet radiation absorbed directly by a DNA system (mononucleotide, single or double strand, G-quadruplex…) induces the ejection of electrons, leaving electron holes on the nucleic acid.
The hydroxyl radical has a very short in vivo half-life of approximately 10 −9 seconds and a high reactivity. [5] This makes it a very dangerous compound to the organism. [6] [7] Unlike superoxide, which can be detoxified by superoxide dismutase, the hydroxyl radical cannot be eliminated by an enzymatic reaction.
Radical clock reactions involve a competition between a unimolecular radical reaction with a known rate constant and a bimolecular radical reaction with an unknown rate constant to produce unrearranged and rearranged products. The rearrangement of an unrearranged radical, U•, proceeds to form R• (the clock reaction) with a known rate ...
Radical reactions must be carried out under inert atmosphere as dioxygen is a triplet radical which will intercept radical intermediates. Because the relative rates of a number of processes are important to the reaction, concentrations must be carefully adjusted to optimize reaction conditions.
This radical adds O 2 to give hydroperoxyl radical (red). In a propagation step, this hydroperoxyl radical abstracts an H + atom from a new diene, generating a new pentadienyl radical and a hydroperoxide (blue). The chemical reaction of lipid peroxidation consists of three phases: initiation, propagation, and termination. [4]
The rate of the addition reaction was accelerated by the following EDGs in increasing order: H < CH 3 < OCH 2 CH 3. When R = OCH 2 CH 3, the rate of the reaction is the fastest because the reaction has the smallest energy of activation (ΔG ‡). The ethoxy and cyano groups are able to delocalize the radical ion in the transition state, thus ...
Most experimentally observed reactions are built up from many elementary reactions that occur in parallel or sequentially. The actual sequence of the individual elementary reactions is known as reaction mechanism. An elementary reaction involves a few molecules, usually one or two, because of the low probability for several molecules to meet at ...