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

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    $\begingroup$ Please, do not use short forms such as "rxn" for "reaction". $\endgroup$ – DHMO Jan 17 '17 at 15:33
  • $\begingroup$ Where is the source that the major product is 1-bromohex-2-ene instead of 1,2-dibromohexane? $\endgroup$ – DHMO Jan 17 '17 at 15:35
  • $\begingroup$ NCERT (Indian government Book for Chemistry) $\endgroup$ – Yashpal Rana Jan 17 '17 at 15:36
  • $\begingroup$ You can find the answer here. $\endgroup$ – DHMO Jan 17 '17 at 15:36
  • $\begingroup$ The question is find the major monohalo product. But i wbt to know if . It's a general question then. Which is major product and why?? $\endgroup$ – Yashpal Rana Jan 17 '17 at 15:37
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Both products are formed although 3-bromocyclohexene is the major product.

Formation of major product: 3-bromocyclohexene

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

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)

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

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.


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.


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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  • $\begingroup$ "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." It might be relatively slow but even in aprotic solvents with polarity in the mid-range you can get quite a lot of by-product. That's exactly why you want to use NBS for allylic bromation, since the the amount of bromine at every time is very limited and this side reaction can be surpressed. $\endgroup$ – DSVA Jan 17 '17 at 22:35

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