Bromination of cyclohexene in presence of UV light or heat

![](https://i.stack.imgur.com/07S8P.jpg)


The reaction between hexene and bromine in presence of light gives 3-bromocyclohexene. Why is 1,2-dibromocyclohexane not formed instead?




Both products are formed although 3-bromocyclohexene is the major product.


Formation of major product: 3-bromocyclohexene
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Under UV light, $\ce{Br2}$ undergoes homolytic splitting to generate $\ce{Br\*}$ radicals:


$$\ce{Br2 ->[hv] 2Br\*}$$


The formation of 3-bromocyclohexene is an example of **substitution of alkanes**, which require the **free-radical mechanism**:


![Free-radical mechanism for substitution of cyclohexene with bromine](https://i.stack.imgur.com/Stazg.png)


In the first step of the upper mechanism, which is also the rate-determining step, a stable **allyl radical** is generated, which is stabilized by resonance:


![Resonance of cyclohexen-3-yl (akin to allyl)](https://i.stack.imgur.com/O3yFo.png)


As a result, the activation energy of the first step is significantly lowered.


Formation of minor product: 1,2-dibromocyclohexane
==================================================


Individual bromine radicals are not electrophilic enough to attack the double bond in the cyclohexene, so the formation of 1,2-dibromocyclohexane requires the **ions mechanism**, typical for **addition reactions** (the lower mechanism in the following diagram).


![Ion mechanism for addition of bromine to cyclohexene](https://i.stack.imgur.com/d09j9.png)


The first step in this mechanism is the rate-determining step. In this step, bromine is ionized, which requires a moderate amount of activation energy, albeit still much higher than the rate-determining step of the upper mechanism.


Conclusion
==========


Therefore, the upper mechanism occurs at a much faster rate than the lower mechanism, which makes the **major** product 3-bromocyclohexene and the minor product 1,2-dibromocyclohexane.




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**PS**: Many people think that addition reaction is very fast. It is only true in **water**, where the bromine ion is stabilized by solvation in water.




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**Disclaimer**: The mechanism I used for the addition reaction probably contradicts with your book. However, it does not matter. The point is that an ion is formed which makes the activation energy high.


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