Redox Reactions
 
 
 
SECTIONS
  
     

OXIDATION AND REDUCTION HALF REACTIONS

Redox reactions involve both an oxidation half reaction and a reduction half reaction. In electron transfer reactions the electrons come from one compound (the donor) and are received by another (the acceptor). The electrons are donated by the oxidation half reaction and accepted by the reduction half reaction. As shown below, both the donor and acceptor need to be present for the electrons to transfer.

The electrochemical cell

A good example for illustrating the concept of half reactions is an electrochemical cell (a simple battery). In an electrochemical cell, chemical redox reactions drive the movement of electrons through a wire—generating electricity. We can better understand how this works by examining the half reactions involved.

In a zinc–copper electrochemical cell, when zinc metal is placed in a zinc sulfate solution it donates electrons, which travel along the metal wire and are accepted by copper ions in a copper sulfate solution. The loss of electrons by the zinc metal causes Zn2+ ions to be formed, and the gain of electrons by the Cu2+ ions results in the formation of copper metal. This electron flow, or electricity, occurs only when the zinc and copper sources are connected to one another in a complete circuit. To complete the circuit, the two solutions are connected by a salt bridge that permits the passage of K+ and Cl– ions. The overall charge of both the zinc and copper baths needs to stay neutral. As negatively charged electrons leave the zinc bath, they are replaced by Cl– ions that move in from the KCl solution. As positively charged copper ions combine with incoming electrons to make neutral copper metal, K+ ions move into the copper bath to replace the lost positively charged Cu2+ ions.

In each vessel of the electochemical cell, individual oxidation or reduction reactions are occuring. These half reactions are separated by a wire that serves to move the electrons and couple both into a redox reaction. Each of these two separated reactions are called the "half reactions," for they each represent half of the total redox reaction: one an oxidation, the other a reduction. The half reactions are shown below.

Zn    Zn2+ + 2 e– (oxidation half reaction)
Cu2+ + 2e–     Cu (reduction half reaction)

The half reaction

Redox reactions are very important to living organisms. As an example, for animals under nonstrenuous conditions, aerobic metabolism reduces O2 to generate ATP to power the muscles. During vigorous exercise when muscles consume O2 faster than it can be replenished by circulating blood, muscles can keep working hard by fermenting pyruvate, a byproduct of glucose metabolism. The overall reaction is

pyruvate + NADH + H+    lactate + NAD+

This redox reaction consists of two half reactions:

pyruvate + 2 H+ + 2 e–    lactate pyruvate gains e–  (reduction half reaction)
NADH + H+    NAD+ + 2H+ + 2 e– NADH loses e– (oxidation half reaction)

The NAD+ generated is used in other metabolic reactions to generate more ATP. The lactate (lactic acid) produced by this reaction is believed to be responsible for the “burn” that you feel in muscles that you worked too hard.

Chemistry and biochemistry textbooks list half reactions in tables similar to the one shown below.

REDUCTION POTENTIALS
Half Reaction E°' (Volts)
O2 + 2 H+ + 2 e–      H2O 0.816 V
SO42– + 2 H+ + 2 e–      SO32– + H2O 0.480 V
fumarate + 2H+ + 2 e–      succinate 0.030 V
acetaldehyde + 2 H+ + 2 e–      ethanol – 0.163 V
oxaloacetate + 2 H+ + 2 e–      malate – 0.175 V
FAD + 2H+ + 2 e–      FADH2 – 0.180 V
NAD+ + 2H+ + 2 e–      NADH + H+ – 0.180 V
pyruvate + CO2 + 2H+ + 2 e–      malate – 0.330 V

Notice how half reactions are always listed as reductions, that is, as gaining electrons. An oxidation half reaction is simply the reverse of the corresponding reduction reaction. Notice also that each half reaction is accompanied by its reduction potential, E°'. The significance of this value will be examined in the next section.

The reduction half reactions in the table show the compounds gaining electrons, as you would expect, but also note that these organic reactions are shown as also gaining protons (H+ ions). Remember that for redox reactions of biological compounds, hydrogen atoms are often being transferred. For example, recall the reaction for methane combustion that we encountered in the first section:

CH4 + 2O2 CO2 + 2H2O
methane
oxygen
carbon
dioxide
water

The oxygen atom gained hydrogen atoms and was reduced. Since a hydrogen atom is simply a proton and an electron combined, reduction can be thought of as gaining an electron or as gaining a hydrogen. Another way to think about this is that if a compound gains a hydrogen atom, then it is gaining an electron (reduction) as well as a proton.

Example 3

Using the half reaction table above, split the following biochemical redox reaction into its constituent half reactions. Identify the oxidation half reaction and the reduction half reaction when the redox reaction proceeds as written (from left to right):

succinate + FAD    fumarate + FADH2

Answer:

Looking at the half reaction table, we find two appropriate half reactions:

(1) fumarate + 2H+ + 2e–    succinate
(2) FAD + 2H+ + 2e–    FADH2

We can reverse reaction (1) and add it to reaction (2) to produce the overall reaction:

(1 reversed) succinate    fumarate + 2H+ + 2e–
(2) FAD + 2H+ + 2e–    FADH2

(Sum) succinate + FAD    fumarate + FADH2

Reaction (1) is written as losing electrons, and is thus the oxidation half reaction. Reaction (2) is written as gaining electrons, and is thus the reduction half reaction.