Free Radicals and Chain Reactions of Alkanes
In this chapter we learn about how alkanes react through free radicals. Even if chemistry feels difficult, don’t worry — this is written in very simple, easy English.
What is a Free Radical?
A free radical is an atom or molecule that has an unpaired electron. Because of this unpaired electron, free radicals become very reactive. They try to steal or share electrons from other molecules.
Important points:
- Free radicals can be positively charged, negatively charged or neutral.
- They take part in combustion, polymer formation, atmospheric reactions and biological processes.
- In our body, radicals like superoxide and nitric oxide help in controlling blood pressure and metabolism.
- They are also used in signalling inside the body (redox signalling).
General Mechanism of Free Radical Reactions
Free radical reactions usually occur in three steps while reacting with alkanes, such as the chlorination of methane.
1. Initiation Step
In this step, a molecule like chlorine (Cl2) absorbs sunlight or UV light and splits into two chlorine radicals.
These chlorine radicals start the reaction.
2. Chain Propagation Steps
Propagation happens in two parts:
- First, the chlorine radical removes a hydrogen atom from methane, forming a methyl radical.
- Second, the methyl radical reacts with Cl2 to form chloromethane and another chlorine radical.
This new chlorine radical again reacts with methane — this repeats like a chain. That is why we call it a chain reaction.
3. Chain Termination
The reaction stops when two radicals combine and form a stable molecule. This removes radicals from the reaction mixture.
Sometimes termination reactions form unwanted side products containing longer carbon chains.
Reactivity of Halogens
Different halogens react differently with alkanes. The order of reactivity is:
Fluorine (most reactive) > Chlorine > Bromine > Iodine (least reactive)
Meaning:
- Fluorination is too fast and difficult to control.
- Chlorination is moderate and commonly used in labs.
- Bromination is slow and needs strong UV light.
- Iodination does not work because it is not thermodynamically favourable.
Relative Stability of Free Radicals
Some free radicals are more stable than others. The general rule is:
Tertiary radical > Secondary radical > Primary radical
Radicals near functional groups like carbonyl or nitrile are also more stable.
Reactions of Radicals with Double Bonds
Radicals easily add to double bonds. Unlike ionic reactions, radicals do not depend strongly on charge differences.
Radical addition is very fast and forms stable products like α-carbonyl radicals.
Radicals in Ring-Forming Reactions
Radicals generally form five-membered rings more easily than four-membered or six-membered rings.
Stable and Persistent Radicals
Stable Radicals
Some radicals are naturally stable. Examples:
- Oxygen (O2) — exists naturally as a radical
- Nitric oxide (NO)
- Vitamin E radical (α-tocopherol radical)
Persistent Radicals
These radicals survive for longer because they are surrounded by bulky groups (crowding). This crowding prevents them from reacting quickly.
Examples include:
- Triphenylmethyl radical
- Nitroxide radicals (e.g., TEMPO)
- Fremy’s salt
Such radicals can also be produced during combustion and may contribute to pollution-related health issues.
Diradicals
Diradicals have two radical centres. Atmospheric oxygen is a well-known diradical. Its diradical nature is the reason why oxygen is relatively unreactive at room temperature and is paramagnetic (attracted to magnets).
Detailed Notes:
For PDF style full-color notes, open the complete study material below:
PATH: PHARMD/PHARMD NOTES/ PHARMD FIRST YEAR NOTES/ ORGANIC CHEMISTRY/ PHARMACEUTICAL ORGANIC CHEMISTRY/FREE RADICALS CHAIN REACTIONS OF ALKANE : MECHANISM, RELATIVE REACTIVITY AND STABILITY.