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Chapter 15: Problem 49

What reagents are needed to convert cyclopentene into (a) bromocyclopentane; (b) \(trans\)-1 ,2-dibromocyclopentane; (c) 3-bromocyclopentene?

Short Answer

Expert verified
(a) HBr, (b) Br2 in CCl4, (c) NBS with light or a radical initiator.

Step by step solution

01

Identify the conversion for bromocyclopentane

To convert cyclopentene to bromocyclopentane, we need to perform an electrophilic addition reaction. Cyclopentene will react with hydrogen bromide (HBr). HBr will add across the double bond in cyclopentene, resulting in the formation of bromocyclopentane.
02

Identify the conversion for trans-1,2-dibromocyclopentane

To convert cyclopentene into trans-1,2-dibromocyclopentane, an anti-addition reaction needs to occur. This is achieved by using bromine (Br2) in an inert solvent such as carbon tetrachloride (CCl4). The reaction proceeds via a bromonium ion intermediate and the addition of the second bromine atom occurs on the opposite side of the initial one.
03

Identify the conversion for 3-bromocyclopentene

To obtain 3-bromocyclopentene from cyclopentene, an allylic bromination is required. This can be achieved by using N-bromosuccinimide (NBS) in the presence of light or a radical initiator such as benzoyl peroxide. This allows for the selective bromination at the allylic position, which is the carbon adjacent to the double bond.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Electrophilic Addition
Electrophilic addition is a fundamental reaction in organic chemistry, especially with alkenes like cyclopentene. It involves an electrophile, a species that loves electrons, reacting with a nucleophile, which is rich in electrons.
When cyclopentene undergoes electrophilic addition with hydrogen bromide (HBr), the 蟺-bond in the alkene attacks the hydrogen atom, breaking the H-Br bond and forming a carbocation.
The bromide ion then attacks the positively charged carbon atom, resulting in bromocyclopentane. This step-by-step process showcases the beauty of chemical transformations by using simple reagents to create useful organic compounds.
Anti-Addition Reaction
In the context of bromination of cyclopentene, an anti-addition reaction refers to the addition of bromine atoms on opposite sides of the double bond.
This phenomenon can be observed when using bromine ( Br2) dissolved in an inert solvent like carbon tetrachloride (CCl4).
The following sequence of events occurs during this reaction:
  • Formation of the bromonium ion intermediate 鈥� the double bond interacts with a bromine molecule, leading to a three-membered cyclic bromonium ion.
  • The remaining bromide ion attacks the opposite side where the first bromine is attached, opening the bromonium ring.
The result is the trans-1,2-dibromocyclopentane, a distinct stereochemistry due to bromine atoms being added from opposite sides (anti-fashion).
This gives the product its unique properties and is highly valued in organic synthesis.
Allylic Bromination
Allylic bromination is a process that selectively brominates the allylic position 鈥� the carbon next to the double bond.
It's particularly useful in converting cyclopentene into 3-bromocyclopentene. This reaction often employs N-bromosuccinimide (NBS) and requires either light or a radical initiator like benzoyl peroxide.
During this reaction, the mechanism involves:
  • Formation of bromine radicals facilitated by light or the radical initiator.
  • Abstraction of an allylic hydrogen atom, leading to an allylic radical.
  • Bromine radical reacts with this new radical, resulting in the formation of the bromo-alkene.
This approach provides high selectivity for allylic positions and avoids unnecessary bromination at other sites, making it a valuable technique in synthetic chemistry.
Bromination Reactions
Bromination is a fundamental reaction that involves the addition of bromine to an organic molecule, often changing its properties.
Bromination reactions vary widely based on the type of starting material and desired product, such as the examples seen with cyclopentene.
Key types of bromination include:
  • Electrophilic bromination involving polar addition, resulting in products like bromocyclopentane.
  • Radical bromination, often used in processes like allylic bromination and leading to selective functional group transformation.
These reactions utilize different mechanisms and conditions to achieve the desired outcomes.
Understanding the specific bromination reaction type and its mechanism is crucial for designing efficient synthesis in organic chemistry applications.

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Most popular questions from this chapter

Radical chlorination of \(CH_3CH_3\) forms two minor products \(\textbf{X}\) and \(\textbf{Y}\) of molecular formula \(C_2H_4Cl_2\) . a. Identify the structures of \(\textbf{X}\) and \(\textbf{Y}\) from the following \(^1H\) NMR data: Compound \(\textbf{X}\): singlet at 3.7 ppm Compound \(\textbf{Y}\): doublet at 2.1 and quartet at 5.9 ppm b. Draw a stepwise mechanism that shows how each product is formed from \(CH_3CH_3\).

(a) Draw the structure of polystyrene, the chapter-opening molecule, which is formed by polymerizing the monomer styrene, \(C_6H_5CH = CH_2\). (b) What monomer is used to form poly(vinyl acetate), a polymer used in paints and adhesives?

Is it possible to prepare 5-bromo-1-methylcyclopentene in good yield by allylic bromination of 1-methylcyclopentene? Explain.

When two monomers (\(\textbf{X}\) and \(\textbf{Y}\)) are polymerized together, a copolymer results. An alternating copolymer is formed when the two monomers \(\textbf{X}\) and \(\textbf{Y}\) alternate regularly in the polymer chain. Draw the structure of the alternating copolymer formed when the two monomers, \(CH_2 = CCl_2\) and \(CH_2 = CHC_6H_5\), are polymerized together.

Treatment of a hydrocarbon \(\textbf{A}\) (molecular formula \(C_9H_{18})\) with \(Br_2\) in the presence of light forms alkyl halides \(\textbf{B}\) and \(\textbf{C}\), both having molecular formula \(C_9H_{17}Br\). Reaction of either \(\textbf{B}\) or \(\textbf{C}\) with \(KOC(CH_3)_3\) forms compound D \((C_9H_{16})\) as the major product. Ozonolysis of \(\textbf{D}\) forms cyclohexanone and acetone. Identify the structures of \(\textbf{A-D}\).
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