Radical chain — anti-Markovnikov thiol addition
Thiol-ene reaction
How the Thiol-ene reaction works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Thiol (R–SH), alkene, photoinitiator (e.g. DMPA) or AIBN. Conditions: UV (365 nm) or 60–80 °C, no oxygen, often solvent-free. Bonds formed: C–S σ bond; C–H; bonds broken: S–H; C=C π. Stereochemistry: Anti-Markovnikov; usually no stereocentre. Rate and selectivity: Propagation and chain transfer both fast; click-like.
At a glance
Step 1: Initiator radical abstracts the S–H hydrogen
A radical initiator generates a thiyl radical by abstracting a thiol's weak S–H hydrogen — the S–H bond is far weaker than a typical C–H bond, making this abstraction fast and easy even from mild initiators or just UV light.
Step 2: Thiyl radical adds to the terminal carbon
The thiyl radical adds to the less hindered, terminal carbon of an alkene — the same regiochemical preference (forming the more stable, more substituted carbon radical) seen in the Kharasch addition covered earlier — forming a new C–S bond.
Step 3: Chain transfer gives the thioether
The resulting carbon radical abstracts a hydrogen atom from another thiol molecule, giving the anti-Markovnikov thioether product and regenerating a thiyl radical to continue the chain — this reaction's speed and near-quantitative yields under simple UV light make it a favorite 'click-like' reaction for polymer and materials crosslinking.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- AIBN
- Azobisisobutyronitrile. Radical initiator: on heating releases N₂ and two 2-cyano-2-propyl radicals.
- Δ
- Heat. Reaction is run at elevated temperature.
- hν
- Light. Photochemical activation.
- Markovnikov
- Markovnikov’s rule. In HX addition, H goes to the carbon with more H’s so the more stable cation forms.
- anti-Markovnikov
- Anti-Markovnikov. The opposite regiochemistry (radical HBr addition, hydroboration).
How it compares with related reactions
| Thiol-ene reaction | Kharasch addition (peroxide-mediated HBr addition) | Barton-McCombie deoxygenation | |
|---|---|---|---|
| Reagents | Thiol (R–SH), alkene, photoinitiator (e.g. DMPA) or AIBN | HBr, ROOR (peroxide initiator) or light, alkene | 1) NaH, CS₂, CH₃I 2) Bu₃SnH, AIBN (or (TMS)₃SiH) |
| Conditions | UV (365 nm) or 60–80 °C, no oxygen, often solvent-free | Heat or hν, non-polar solvent | Toluene, 80–110 °C, radical chain |
| Stereochemistry | Anti-Markovnikov; usually no stereocentre | Anti-Markovnikov; radical chain | Stereocentre lost (planar radical) |
| Rate / selectivity | Propagation and chain transfer both fast; click-like | Chain propagation; H-abstraction from HBr is fast | Driven by strong C=O/Sn–S bonds; chain propagation |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Thiol-ene reaction?
Thiol (R–SH), alkene, photoinitiator (e.g. DMPA) or AIBN. Conditions: UV (365 nm) or 60–80 °C, no oxygen, often solvent-free.
Which bonds are formed and which are broken?
Formed: C–S σ bond; C–H. Broken: S–H; C=C π.
What is the stereochemical outcome?
Anti-Markovnikov; usually no stereocentre
What controls the rate or the selectivity?
Propagation and chain transfer both fast; click-like
Study guides that help with this reaction
- Radical chain reactions: initiation, propagation and termination
- Curved-arrow notation: rules and the mistakes students make