= Autoxidation
<Autoxidation> is oxidation of a substance by molecular <oxygen>, commonly proceeding through a self-propagating <free radical> chain in organic materials. The name does not mean that initiation is unnecessary. Light, existing peroxides or other radical sources can start a chain, and changes in initiation can alter the observed rate.
A basic hydrocarbon propagation scheme writes the substrate as $\mathrm{RH}$:
$$
\mathrm{R^{\bullet}+O_2\longrightarrow ROO^{\bullet}},\qquad
\mathrm{ROO^{\bullet}+RH\longrightarrow ROOH+R^{\bullet}}.
$$
A <peroxyl radical> is converted into a <hydroperoxide> while generating a new carbon-centered radical. The two-step cycle consumes substrate and oxygen but regenerates a chain carrier. Radical–radical reactions can terminate chains; hydroperoxide decomposition can create further initiating species.
An original limiting kinetic model assumes abundant oxygen, approximately constant substrate concentration and termination dominated by pairs of peroxyl radicals. If initiation supplies radicals at rate $R_i$ and termination removes them at $2k_tP^2$, the steady radical concentration is
$$
P=\sqrt{\frac{R_i}{2k_t}},\qquad v_{\mathrm{prop}}=k_p[\mathrm{RH}]P.
$$
Quadrupling $R_i$ then doubles the propagation rate rather than quadrupling it. This scaling follows from the stated balance; it need not hold when oxygen delivery, changing substrate or other termination pathways control the experiment.
A motivating modern apparatus is the <inhibited autoxidation assay>. A primary method co-oxidizes a hydrocarbon substrate with a <BODIPY>-based signal carrier and monitors progress with a visible-light <spectrophotometer>. Comparing inhibited and uninhibited traces tests radical trapping while distinguishing reaction rate from trapping capacity.<A Continuous Visible Light Spectrophotometric Approach To Accurately Determine the Reactivity of Radical-Trapping Antioxidants by Evan A. Haidasz and others>[Primary abstract, co-autoxidation signal carriers and kinetic/stoichiometric interpretation]{ref}
A reduced signal slope can reflect chemical inhibition or inadequate oxygen supply, so controls for <oxygen-limited autoxidation>, probe response and matched initiation matter. The chain mechanism helps explain why excluding oxygen, intercepting radicals and changing peroxide decomposition can affect oxidation differently. An arbitrary antioxidant-assay signal alone does not establish product identity or a biological benefit.
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