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348 Electron Transfer Reactions

Phlogiston An eighteenth-century principle applied to burned and to ores when they were reduced to metals. The
allchemicalprocesses;itcanbesensiblyidentifiedwith reaction above could be described as a phlogiston transfer
the modern concept of the electron. reaction in which zinc becomes dephlogisticated. Hydro-
∗
π and π orbitals Orbitals formed by the overlap of p gen gas was described by Cavendish about 1770 as pure,
atomic orbitals on nonmetals and t 2g d orbitals on tran- gaseous phlogiston, since it was combustible and capa-
sition metals to provide pathways for electron transfer. ble of reducing ores to metal. Lavoisier’s discovery that
Reducing agent Atom or molecule (e.g., a metal atom) combustible materials (reducing agents) gained weight by
that undergoes oxidation when it transfers electrons to adding oxygen when they burned in air led to the concept
an oxidizing agent. of oxidation as addition of oxygen and reduction as re-
Solvated electron Free electron solvated by a polar sol- moval of oxygen or to oxidation–reduction reactions as
vent such as NH 3 or H 2 O in which it has a long enough oxygen transfer reactions:
lifetime to be studied as a chemical entity—electron
2Zn + O 2 → 2ZnO
donor or reducing agent.
Tunneling Quantum-mechanical electron transfer that and
occurs with little or no activation energy and where no
Zn + CuO → ZnO + Cu.
obvious physical pathway for the electron is apparent.
Early in the twentieth century the structure of metal
2−
2+
oxides was deduced to be ionic (Zn O ). In electrol-
ELECTRON TRANSFER REACTIONS are chemical ysis cells and batteries, addition of electrons at cath-
−
reactions in which electrons leave a donor atom or a reduc- odes(Cu 2+ + 2e → Cu)effectedreductionofmetalcom-
ing agent and join a receptor atom or an oxidizing agent. pounds to metal, while at anodes metals were corroded,
−
This article describes the concepts applied to such reac- forming ionic compounds (Zn → Zn 2+ + 2e ) by los-
tions from ancient times to present discussions concerning ing electrons. This led to the general concept that the
the states of electrons in atomic and molecular orbitals, metal was oxidized when it lost electrons to form posi-
potentials for initiating transfer, and studies designed to tive ions and that positive metal ions were reduced when
allow postulation of orbital pathways. A tentative effort they gained electrons to form the metal. Table I shows a
is made to develop a scheme for classifying such reac- history of concepts used to describe oxidation–reduction
tions in terms of the nature of the pathways employed and reactions.
of the numbers of electrons transferred and numbers of
pathways used in any given activated state. An early re-
A. Identifying Phlogiston as Electrons
sult of supercomputer calculations describing the multiple
pathways for an electron to follow from the surface of the There is irony in the fact that the phlogiston theory, which
protein molecule myoglobin to a heme iron center deep in was generally applicable to nearly all chemical systems
the interior of the molecule is discussed. and processes, was replaced by the oxygen theory, appli-
cablespecificallyonlytoclassicalmetallurgyandcombus-
tion. If one substitutes the word electrons for phlogiston
I. ELECTRON TRANSFER AND in appropriate writings, the early theory makes complete
OXIDATION–REDUCTION sense. Attempts to identify phlogiston as a weighable sub-
stance had to fail because electrons do not exit separately
In the reaction of zinc metal with dilute mineral acid, zinc from the atoms and molecules of substances and they are
“dissolves” with the evolution of hydrogen gas: a very small fraction of the weight. All chemical phenom-
ena are explicable in terms of electrons as constituents of
Zn + 2H → Zn 2+ + H 2 . (1)
+
atoms and molecules, consistent with the general applica-
Zn reduces H to H 2 and is oxidized to Zn 2+ by transfer of bility of the older phlogiston theory.
+
two electrons to two hydrogen ions, which forms hydro-
gen molecules. The term reduction goes back to ancient
B. Observation of Electrons and
metallurgy, when ores were said to be reduced to metals.
Electron Transfer Processes
In 1700 the fact that metallurgical reducing agents (char-
coal, metals, wood, etc.) also burned in air led to inclusion Electrons may be observed in electron beams and are un-
of such reagents in the phlogiston theory then being ap- doubtedly released as free charged particles at the high
plied to nearly all chemical phenomena. Reducing agents temperatures found in the sun and stars. However, on
were thought to give up phlogiston to the air when they the surface of the earth they are bound to atomic nuclei