Problem 102 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Karl Ziegler reported that alkenes react with N-bromosuccinimide (NBS) in the presence of light to give products resulting from substitution of hydrogen by bromine at the allylic position, i.e., the position next to the double bond. Let us consider the halogenation of cyclohexene Energy level diagram for allylic, vinylic and alkylic free radicals is given . In the treatment of cyclohexene with NBS, which of the following products will be formed?

Step-by-Step Solution

Answer

Treatment of cyclohexene with NBS forms 3-bromocyclohexene due to allylic bromination.

Step 1: Understanding Allylic Bromination

Alkenes that react with N-bromosuccinimide (NBS) in the presence of light undergo allylic bromination. This reaction results in the substitution of a hydrogen atom at the allylic position (the carbon adjacent to the double bond) with a bromine atom.

Step 2: Identifying Allylic Positions in Cyclohexene

Cyclohexene is a cyclic compound with one double bond. The allylic positions are the carbons adjacent to the double-bonded carbons. In cyclohexene, there are two equivalent allylic positions – the two carbons adjacent to the double bond.

Step 3: Predicting the Allylic Radical Formation

When light is applied, it initiates homolytic cleavage in NBS forming a bromine radical. This radical abstracts a hydrogen from the allylic position of cyclohexene, forming an allylic radical. This step happens because allylic radicals are more stable compared to vinylic or alkylic radicals due to resonance stabilization.

Step 4: Forming the Brominated Product

After the formation of the allylic radical, it can react with another molecule of NBS, where the bromine radical adds to the allylic radical to form the brominated product. Depending on the starting orientation, two symmetric products can technically form, but in this case, they lead to the same compound.

Key Concepts

N-bromosuccinimide (NBS)

N-bromosuccinimide, commonly abbreviated as NBS, is a chemical reagent often used for bromination reactions in organic chemistry. It is particularly favored for its ability to selectively brominate allylic and benzylic positions. NBS is known for performing this task under mild conditions, typically in the presence of light or a radical initiator.
NBS operates by generating bromine radicals. These radicals are essential in various reactions as they are highly reactive, yet they are selective enough to target specific positions on a molecule. In the presence of an alkene, such as cyclohexene, NBS enables substitution at the allylic position, giving preference to this position due to the stability of resulting radicals.

• Functions under mild conditions
• Targets allylic, benzylic positions
• Generates bromine radicals for reactions

With NBS, the reaction proceeds smoothly, making it a popular choice in the synthesis of brominated organic compounds.

Cyclohexene

Cyclohexene is a simple cyclic compound containing a single double bond. It is a six-membered ring structure with alternating carbon and hydrogen atoms, one of which is involved in the carbon-carbon double bond. This configuration allows it to participate in various chemical reactions, particularly interesting is its role in allylic bromination.
In the context of allylic bromination, cyclohexene serves as a substrate where NBS acts to replace a hydrogen atom at the allylic position with a bromine atom. The allylic position in cyclohexene refers to the carbon atoms adjacent to the double bond. This position is crucial because removing a hydrogen from here forms an allylic radical, which is stabilized by resonance.

• Consists of a six-membered ring
• Double bond presents reactive allylic positions
• Forms a basis for numerous chemical transformations

Overall, cyclohexene's structure makes it an exemplary candidate for examining allylic substitution reactions.

Free Radical Mechanism

The free radical mechanism is a fundamental concept in organic chemistry involving reactive species known as free radicals. These are atoms or molecules containing an unpaired electron, making them highly reactive. The free radical mechanism proceeds through several steps: initiation, propagation, and termination.
In the allylic bromination of cyclohexene with NBS, the initiation step involves the homolytic cleavage of a bond facilitated by light, producing a bromine radical. This highly reactive radical then participates in the reaction by abstracting a hydrogen from the allylic position to form an allylic radical.

• Begins with initiation (radical generation)
• Continues with propagation steps (reactive intermediates)
• Ends with termination (radicals combine to form stable product)

The allylic radical is then captured by another bromine radical to form the final brominated product. This predictable and controlled sequence of reactions highlights the elegance and efficiency of the free radical mechanism in organic synthesis.

Resonance Stabilization

Resonance stabilization is a key concept explaining the stability of certain chemical species. In organic chemistry, it refers to the ability of electrons in a molecule to be delocalized over two or more atoms, effectively spreading out and minimizing potential energy.
With allylic radicals, resonance stabilization is particularly relevant. Once a hydrogen atom is abstracted from cyclohexene, the resulting allylic radical enjoys increased stability due to resonance. In such radicals, the single electron that remains can be distributed over the adjacent π-system of the double bond.

• Involves electron delocalization
• Leads to greater radical stability
• Explains preference for allylic over other positions

This redistribution adds to the overall stability of the allylic radicals, allowing them to form more readily compared to other forms like vinylic or alkylic radicals, thus favoring certain reactions.