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Bromination of hexene in presence of UV light or heat
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Nmra Shah
Bromination of hexene in presence of UV light or heat
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:
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:
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).
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.
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:
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:
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).
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.
"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. Thats 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.More
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:
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:
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).
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.
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:
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:
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).
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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