Biological Oxidation-Reduction Reactions:- Oxidation-Reductions Can Be Described as Half-Reactions
Although oxidation and reduction must occur together, it is convenient when describing electron transfers to consider the two halves of an oxidation-reduction reaction separately. For example, the oxidation of ferrous ion by cupric ion,
can be described in terms of two half-reactions:
The electron-donating molecule in an oxidation reduction reaction is called the reducing agent or reductant; the electron-accepting molecule is the oxidizing agent or oxidant. A given agent, such as an iron cation existing in the ferrous (Fe2+) or ferric (Fe3+) state, functions as a conjugate reductant-oxidant pair (redox pair), just as an acid and corresponding base function as a con jugate acid-base pair. Recall from Chapter 2 that in acid base reactions we can write a general equation: proton donor ⇌H+ proton acceptor. In redox reactions we can write a similar general equation: electron donor ⇌ e- electron acceptor. In the reversible half-reaction (1) above, Fe2+ is the electron donor and Fe3+ is the electron acceptor; together, Fe2+ and Fe3+ constitute a con jugate redox pair. The electron transfers in the oxidation-reduction reactions of organic compounds are not fundamentally different from those of inorganic species:
This overall reaction can be expressed as two half reactions:
Because two electrons are removed from the aldehyde carbon, the second half-reaction (the one-electron re duction of cupric to cuprous ion) must be doubled to balance the overall equation.
