Common Reducing Agents
A reducing agent causes another substance to be reduced by donating electrons to it. Strong reducing agents are electron-rich species that readily give up electrons. They are the chemical opposite of oxidizing agents, and knowing the major ones helps you predict reaction outcomes on the MCAT.
The Big List of Common Reducing Agents
| Reducing Agent | Why It Donates Electrons | Common Context |
|---|---|---|
| Alkali metals (Li, Na, K) | Extremely low ionization energy; one valence electron easily lost | Li is the strongest reducing agent on the standard table (E = -3.04 V) |
| Alkaline earth metals (Mg, Ca, Ba) | Low ionization energy; two valence electrons | Mg ribbon burns in air (vigorous oxidation by O2) |
| Zinc (Zn) | Active metal above H2 in activity series | Used in galvanization, Daniel cell anodes |
| Iron (Fe) | Active metal above H2 | Corrodes (rusts) by losing electrons to O2 |
| Aluminum (Al) | Very active despite protective oxide layer | Thermite reaction: 2Al + Fe2O3 —> Al2O3 + 2Fe |
| Carbon (C) / Carbon monoxide (CO) | C goes from 0 to +4; CO goes from +2 to +4 | Smelting: reducing metal ores in a furnace |
| Hydrogen gas (H2) | H goes from 0 to +1 | Hydrogenation of alkenes, industrial metal reduction |
| NaBH4 (sodium borohydride) | Delivers H- (hydride with 2 electrons) | Organic chemistry: reduces aldehydes and ketones to alcohols |
| LiAlH4 (lithium aluminum hydride) | Stronger hydride donor than NaBH4 | Organic chemistry: reduces carboxylic acids, esters, and amides |
| NADH (biological) | Donates 2e- + H+ | Cellular respiration: delivers electrons to ETC |
| FADH2 (biological) | Donates 2e- + 2H+ | Cellular respiration: delivers electrons to ETC |
Active Metals as Reducing Agents
The most common reducing agents in general chemistry are the active metals. Their placement in the activity series directly reflects their reducing power:
| Metal | E (V) | Reducing Strength |
|---|---|---|
| Li | -3.04 | Strongest |
| K | -2.93 | Very strong |
| Ca | -2.87 | Very strong |
| Na | -2.71 | Very strong |
| Mg | -2.37 | Strong |
| Al | -1.66 | Strong |
| Zn | -0.76 | Moderate |
| Fe | -0.45 | Moderate |
The more negative the reduction potential, the less the metal wants to be in its reduced (metallic) form, and the more readily it donates electrons. Lithium metal is so reactive that it must be stored under mineral oil to prevent it from reacting with moisture in the air.
Hydride Reagents in Organic Chemistry
You will encounter NaBH4 and LiAlH4 extensively in organic chemistry:
NaBH4 (mild reducing agent):
- Delivers H- (hydride) to carbonyl carbons
- Reduces aldehydes and ketones to alcohols
- Does NOT reduce carboxylic acids or esters (too mild)
LiAlH4 (strong reducing agent):
- More powerful hydride donor
- Reduces aldehydes, ketones, carboxylic acids, esters, and amides
- Must be used in anhydrous conditions (reacts violently with water)
Both reagents work by delivering a hydride ion (H:-), which is hydrogen with two electrons - a powerful reducing species.
Carbon and Carbon Monoxide as Reducing Agents
In metallurgy, carbon (as coke) and carbon monoxide are used to extract metals from their ores:
Fe2O3(s) + 3CO(g) —> 2Fe(l) + 3CO2(g)
Carbon is oxidized from +2 (in CO) to +4 (in CO2), while iron is reduced from +3 to 0. This is the basis of iron smelting in a blast furnace - an industrial application of the activity series.