What is "reducing agent"

Reducing agent \Re*du"cing a`gent\, n. (Chem.) a substance that causes reduction of another substance in a chemical reaction, as by donating electrons or adding hydrogen atoms; as, lithium hydride is a powerful reducing agent.

n. (context chemistry English) Any substance that reduces, or donates electrons to, another; in so doing, it becomes oxidized.

n. a substance capable of bringing about the reduction of another substance as it itself is oxidized; used in photography to lessen the density of a negative or print by oxidizing some of the slackened silver [syn: reducer, reductant]

A reducing agent (also called a reductant or reducer) is an element or compound that loses (or "donates") an electron to another chemical species in a redox chemical reaction. Since the reducing agent is losing electrons, it is said to have been oxidized.

If any chemical is an electron donor (reducing agent), another must be an electron recipient ( oxidizing agent). A reducing agent is oxidized because it loses electrons in the redox reaction. Thus reducers are "oxidized" by oxidizers and oxidizers are "reduced" by reducers; reducers are by themselves reduced (have more electrons) and oxidizers are by themselves oxidized (have fewer electrons). A reducing agent typically is in one of its lower possible oxidation states and is known as the electron donor. Examples of reducing agents include the earth metals, formic acid, and sulfite compounds.

For example, consider the overall reaction for aerobic cellular respiration:

The oxygen (O) is being reduced, so it is the oxidizing agent. The glucose (CHO) is being oxidized, so it is the reducing agent.

In organic chemistry, reduction more specifically refers to the addition of hydrogen to a molecule, though the aforementioned definition still applies. For example, benzene is reduced to cyclohexane in the presence of a platinum catalyst:

In organic chemistry, good reducing agents are reagents that deliver H.

Historically, reduction referred to the removal of oxygen from a compound, hence the name 'reduction'. The modern sense of donating electrons is a generalisation of this idea, acknowledging that other components can play a similar chemical role to oxygen.