Does Substitution Occur Only at Allylic Positions or All Positions in Cyclohexene Reaction with Cl2 under UV Light?
The radical chlorination of cyclohexene with chlorine (Cl2) in the presence of UV light initially involves addition across the double bond (positions 1 and 2), but with excess chlorine and continued UV irradiation, substitution occurs at allylic and other ring positions, leading to extensive chlorination.
Radical Mechanism Initiation
UV light causes homolytic cleavage of the Cl-Cl bond:
Cl2 → 2 Cl•
These chlorine radicals (Cl•) are highly reactive and initiate the radical chain reaction.
Primary Reaction Steps
- Addition at the Double Bond: A chlorine radical attacks the double bond of cyclohexene, forming a carbon-centered radical intermediate.
- Radical Propagation: The intermediate reacts with another Cl2 molecule to yield 1,2-dichlorocyclohexene and regenerates a chlorine radical.
| Step | Reaction | Product |
|---|---|---|
| Initiation | Cl2 → 2 Cl• | Chlorine radicals |
| Propagation 1 | Cl• + cyclohexene → cyclohexene radical | Carbon-centered radical intermediate |
| Propagation 2 | Radical + Cl2 → 1,2-dichlorocyclohexene + Cl• | Product A: 1,2-dichlorocyclohexene |
Position of Initial Substitution
The primary substitution or addition takes place specifically at the carbons involved in the double bond (positions 1 and 2). The chlorine radicals add across the alkene, converting it into 1,2-dichlorocyclohexene. This is an electrophilic radical addition typical of alkenes under radical conditions.
Further Substitution at Other Positions
When excess chlorine and continuous UV irradiation are present, the reaction extends beyond the double bond carbons. Additional radical chlorinations occur at:
- Allylic positions (carbons adjacent to the double bond)
- Saturated carbons of the ring
This leads to extensive chlorination, producing compounds like 1,2,3,4,5,6-hexachlorocyclohexane, also known as Lindane.
Comparison with Cyclohexane Chlorination
Cyclohexane, lacking a double bond, undergoes less selective radical chlorination. Substitution occurs at various ring carbons relatively unselectively because the entire ring comprises saturated carbons without allylic stabilization. In contrast, cyclohexene favors substitution at allylic positions under radical conditions, especially beyond initial addition.
Summary of Selectivity
| Substrate | Chlorination Site | Reaction Outcome |
|---|---|---|
| Cyclohexene + Cl2 + UV (Initial) | Double bond carbons (1,2-positions) | 1,2-Dichlorocyclohexene (addition) |
| Cyclohexene + Excess Cl2 + UV | All carbons including allylic and saturated positions | Poly-chlorinated cyclohexane (e.g., Lindane) |
| Cyclohexane + Cl2 + UV | All ring carbons (unselective) | Multiple chlorinated cyclohexanes |
Key Takeaways
- UV light cleaves Cl2 to form chlorine radicals initiating radical chlorination.
- Initial substitution in cyclohexene targets the double bond carbons, producing 1,2-dichlorocyclohexene.
- Excess Cl2 and prolonged UV exposure cause substitution at allylic and saturated carbons, leading to full ring chlorination.
- Cyclohexene chlorination via radicals is partially selective, favoring allylic/alkene sites initially.
- Cyclohexane chlorination is non-selective, leading to substitution at multiple ring positions.
On Reaction of Cyclohexene with Cl2 in Presence of UV Light: Does Substitution Happen Only at the Allylic Position or Everywhere?
Here’s the straight answer: When cyclohexene reacts with chlorine (Cl2) under UV light, initial substitution happens primarily at the double bond carbons, but if excess chlorine and UV light are present, substitution can extend to all positions on the ring—including allylic and saturated carbons. The reason? It’s all about that free radical party!
Let’s break it down, because chemistry is much more fun with details—and a little bit of radical drama.
Free Radical Mechanism Ignites the Reaction
UV light acts like a matchstick that sets off the molecular fireworks. It breaks the Cl-Cl bond in chlorine gas through a process called homolytic cleavage. This generates two chlorine radicals (Cl•), which are highly reactive species looking for a partner to share their unpaired electron.
Cl2 → 2 Cl• (chlorine radicals formed)
These radicals are the main characters in our story. They seek out cyclohexene molecules, ready to initiate a chain of reactions. This initiates a free radical mechanism, which is quite different from simple ionic additions you see in regular chemistry classes.
First Stop: The Double Bond—Where Addition Happens
Now, cyclohexene isn’t your average alkene. It has a double bond that’s begging for attention. When the chlorine radical meets cyclohexene, it adds across that double bond, creating a carbon radical intermediate:
- Cl• + cyclohexene → cyclohexene radical (carbon-centered)
- cyclohexene radical + Cl2 → 1,2-dichloro-cyclohexene + Cl•
The key product here is 1,2-dichloro-cyclohexene. This comes from the initial addition of chlorine atoms across the double bond carbons (positions 1 and 2). This step dominates early in the reaction.
So if you were wondering, “Does substitution happen only at the allylic position?” — well, at first, they’re busy adding at the double bond itself. No allylic love just yet. This is a classical electrophilic addition setup but given a radical twist by UV light.
But Wait, There’s More: Excess Chlorine Invites Substitution Everywhere
Enter stage right: excess chlorine and more UV light. This turns the 1,2-dichloro-cyclohexene into the opening act because the radicals get super aggressive, going on a chlorination spree.
Now the chlorine radicals don’t just stop at the double bond vicinity. They start replacing hydrogens all over the cyclohexane ring—including:
- Allylic positions (the carbons next to the double bond)
- Saturated carbons further away
- Basically every carbon on the ring
This results in the formation of a fully chlorinated molecule: 1,2,3,4,5,6-hexachlorocyclohexane, also known famously as Lindane. It’s like the radical mechanism decided, “Why pick favorites? Let’s chlorinate everything!”
Comparison with Cyclohexane: Selectivity Matters
To understand selectivity better, consider cyclohexane instead of cyclohexene. In cyclohexane, chlorination via radicals is much less selective. The radicals attack all hydrogens pretty indiscriminately, resulting in random substitution across the ring.
With cyclohexene, even though radicals exist, they initially focus more on the double bond and especially on allylic positions since these are more reactive. Why? Allylic hydrogens have lower bond dissociation energies, making hydrogen abstraction easier.
This subtle difference underlines how the presence of an alkene unit directs radical substitution preferentially. The double bond is like a beacon, attracting radical additions first before the rest of the ring is attacked.
Why Does This Matter? Practical Perspectives
Understanding this subtle dance of radicals isn’t just academic. It has practical implications in organic synthesis and industrial chemistry.
- Designing selective chlorination: If you want to target allylic chlorination without messing with the whole molecule, careful control of chlorine concentration and UV exposure is essential.
- Minimizing over-chlorination: Excess chlorine and prolonged UV lead to fully chlorinated products, which might be undesirable in some cases due to toxicity or reactivity.
- Producing valuable intermediates: 1,2-dichloro-cyclohexene can be a stepping stone in synthesizing fine chemicals, agrochemicals, or even pharmaceuticals.
So whether you’re a chemist in a lab or just curious about how light and chlorine make molecules dance, these reaction details help you predict and control product formation better.
In Summary: The Chlorination Chronicles of Cyclohexene
The reaction begins with addition at the double bond, forming 1,2-dichloro-cyclohexene. This step predominates when chlorine or UV light are limited.
As chlorine and UV light increase, radical substitutions spread to all ring carbons. This leads to fully chlorinated cyclohexane derivatives like Lindane.
This pattern contrasts with cyclohexane, where radical chlorination is non-selective from the start.
Does substitution happen only at allylic positions due to radical mechanisms? Initially, no—it’s focused on the double bond carbons. But over time and with excess reagents, substitution extends to allylic and all other positions in the ring.
Still Curious?
Ever wondered why radicals love the double bond first, then go wild everywhere? Or how controlling light intensity and chlorine pressure can fine-tune your reaction outcomes? These questions invite chemists to dive deeper into reaction kinetics and radical stability studies.
In the realm of radical chlorination under UV light, cyclohexene serves as a fascinating example of how subtle differences in molecular structure guide the path of chemical transformations.
1. Does chlorine substitution on cyclohexene under UV light occur only at the allylic position?
No. Initially, chlorine radicals add across the double bond (1,2-positions). However, substitution is not limited to allylic sites alone.
2. Why does chlorination happen mainly at the double bond carbons first?
Chlorine radicals add to the alkene double bond carbons forming 1,2-dichloro-cyclohexene. This is a typical radical addition to the more reactive double bond.
3. Can substitution occur at other ring positions in cyclohexene during chlorination?
Yes. With excess Cl₂ and UV light, radical substitution occurs at allylic and all saturated carbons, eventually forming fully chlorinated products.
4. How does cyclohexene chlorination compare with cyclohexane chlorination under UV light?
Cyclohexene shows selectivity, favoring allylic substitution after initial addition. Cyclohexane substitution is less selective and can occur broadly on different ring carbons.
5. What products form when cyclohexene reacts with Cl₂ under UV light?
- 1,2-dichloro-cyclohexene via addition at the double bond.
- 1,2,3,4,5,6-hexachlorocyclohexane from further substitution on all ring carbons with excess chlorine.
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