Radical inhibitors are substances that slow down or stop reactions involving free radicals. They are very important in both biological systems (to prevent oxidative damage) and in industry (to prevent unwanted polymerization or degradation). Here’s how they work:
1. How Radical Reactions Work
- Free radicals are highly reactive species with unpaired electrons.
- They react with molecules to form new radicals, creating a chain reaction.
- As long as the chain continues, the reaction keeps going (e.g., polymerization or oxidation).
2. Mechanism of Radical Inhibition
Radical inhibitors stop the chain reaction by interfering with the free radicals in one of these ways:
a) Radical Scavenging
- Inhibitors donate an electron or hydrogen atom to the radical.
- This stabilizes the radical and turns it into a non-reactive molecule.
- Example: Vitamin E in biological systems donates a hydrogen to lipid radicals, stopping lipid peroxidation.
b) Termination of Radical Chains
- Some inhibitors can react with two radicals to form a stable, non-radical product.
- This effectively removes radicals from the system and prevents the chain reaction from continuing.
c) Formation of Stable Adducts
- Certain inhibitors react with radicals to form a stable compound that cannot propagate the chain reaction.
- Common industrial inhibitors do this to prevent unwanted polymerization of monomers.
3. Examples of Radical Inhibitors
Biological Systems:
- Antioxidants like Vitamin C, Vitamin E, glutathione, and carotenoids prevent oxidative damage by neutralizing free radicals.
Industrial Applications:
- Hydroquinone, BHT (butylated hydroxytoluene), and TEMPO are used to prevent unwanted polymerization of monomers like styrene or acrylates.
- Oxygen can act as a radical inhibitor in some polymerizations by reacting with radicals to form peroxides that stop the chain.
4. Key Points
- Radical inhibitors don’t remove radicals by destroying them, but rather stabilize them or block the chain reaction.
- They are essential for controlling reactions that would otherwise proceed uncontrollably.
- In biology, they protect cells from oxidative stress. In industry, they prevent premature polymerization or product degradation.