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Chapter 15: Problem 21

Draw the products of each reaction. a. C1CCCC1 \(\frac{\mathrm{NBS}}{\mathrm{hv}}\) b. \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{3} \quad \frac{\mathrm{NBS}}{\mathrm{hv}}\) c. \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{3} \quad \stackrel{\mathrm{Br}_{2}}{\longrightarrow}\)

Short Answer

Expert verified
a. Cyclopentane remains unchanged; b. Forms 3-bromopropene; c. Forms 1,2-dibromopropane.

Step by step solution

01

Analyze Reaction (a)

The starting molecule is cyclopentane, represented by \( \text{C}_1\text{CCCC}_1 \), which is a saturated cyclic hydrocarbon. When treated with \( \text{NBS (N-bromosuccinimide)} \) under light (\( \text{hv} \)), it undergoes allylic bromination. However, since cyclopentane is a saturated alkane with no alkene group, NBS will not react under typical conditions. Therefore, the product remains cyclopentane.
02

Analyze Reaction (b)

The starting molecule is propene (\( \text{CH}_2=\text{CH}-\text{CH}_3 \)), which contains an allylic site. Allylic bromination with \( \text{NBS} \) under \( \text{hv} \) will lead to the substitution of a hydrogen atom at the allylic position next to the double bond, forming \( \text{CH}_2=\text{CH}-\text{CH}_2\text{Br} \).
03

Analyze Reaction (c)

In this reaction, propene (\( \text{CH}_2=\text{CH}-\text{CH}_3 \)) reacts with molecular bromine (\( \text{Br}_2 \)). This addition reaction proceeds through a halogenation mechanism where bromine atoms add across the double bond, resulting in the formation of 1,2-dibromopropane, \( \text{CH}_2\text{Br}-\text{CHBr}-\text{CH}_3 \).

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Key Concepts

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

NBS Bromination
N-bromosuccinimide (NBS) is a brominating agent used in organic chemistry to selectively brominate alkenes at the allylic position. This type of bromination is typically carried out under the influence of a radical initiator such as light (hv). The process involves the formation of bromine radicals, which replace a hydrogen atom at the allylic position with a bromine atom.
  • NBS is highly selective, reducing over-bromination risks.
  • It is less reactive than molecular bromine, making it more controlled.
  • Primarily used in the allylic bromination of double-bond containing hydrocarbons.
The overall goal of using NBS is to efficiently substitute allylic hydrogen with bromine, leading to the formation of valuable intermediates for further chemical transformations.
Allylic Substitution
Allylic substitution occurs when a hydrogen atom adjacent to a double bond is replaced by another atom or group, such as a bromine. This process is facilitated in the presence of NBS and light to generate bromine radicals.
  • The allylic position is reactive due to resonance stabilization of the allylic radical.
  • This provides the potential for selectivity in reactions involving unsaturated hydrocarbons.
This type of substitution is important for creating versatile intermediates needed for various organic syntheses, widening the scope for chemical modifications.
Halogenation
Halogenation is the process of adding one or more halogen atoms (like bromine) to a molecule. With alkenes, such as propene, halogenation leads to the addition of bromine atoms across a double bond.
  • In a typical bromination of an alkene, anti-addition occurs, leading to trans addition across the double bond.
  • The reaction generally involves an electrophilic addition mechanism via a bromonium ion intermediate.
  • The outcome is a dibromoalkane if two bromines add.
Halogenation broadens the chemical utility of hydrocarbons by adding functional groups that can participate in further reactions.
Cyclopentane
Cyclopentane is a saturated hydrocarbon forming a five-membered ring, described by the formula \( \text{C}_5\text{H}_{10} \).
  • Due to its saturation, cyclopentane does not possess any double bonds, limiting reactions to those with alkenes and unsaturated systems.
  • Under NBS bromination conditions (light exposure), cyclopentane remains unreacted as it has no allylic hydrogen to replace.
  • Cyclopentane is often used as a solvent or as an intermediate in reactions requiring a cyclic structure.
Understanding the reactivity limitations for cyclopentane helps rationalize its use in synthesis and reactions.
Propene
Propene, or propylene, is the simplest alkene and consists of three carbon atoms, typically represented as \( \text{CH}_2=\text{CH}-\text{CH}_3 \).
  • It has a double bond between the first and second carbon, offering reactive sites for addition reactions.
  • The presence of the double bond enables reactions such as NBS bromination, resulting in allylic brominated products.
  • In a halogenation reaction with \( \text{Br}_2 \), propene undergoes addition to produce 1,2-dibromopropane.
Propene is an essential raw material in the chemical industry, instrumental for manufacturing various polymers and chemicals.
Reaction Mechanisms
Reaction mechanisms are step-by-step descriptions detailing the transformation of reactants to products during a chemical reaction. Understanding mechanisms helps predict outcomes and troubleshoot reactions.
  • Mechanisms show the movement of electrons, often using arrows to indicate the direction.
  • In the context of NBS bromination and halogenation, radicals and electrophilic additions play key roles.
  • Recognizing common intermediates like bromonium ions or allylic radicals aids in predicting product formation.
Mastery of reaction mechanisms is vital for students and professionals to effectively engage with organic synthesis and problem-solving in chemistry.

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Most popular questions from this chapter

Draw the products formed when a chlorine atom (Cl) reacts with each species. a. \(\mathrm{CH}_{3}-\mathrm{CH}_{3}\) b. \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\) c. : \(\ddot{\mathrm{C}} \mid\). d. \(\mathrm{O}_{2}\)

Rank each group of radicals in order of increasing stability. a. \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCH}_{2} \quad \mathrm{CH}_{3} \mathrm{CH}_{2} \dot{\mathrm{CH} \mathrm{CH}}_{3} \quad\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}\).

Rank each group of radicals in order of increasing stability. a. \(\quad\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C} \mathrm{CH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2} \quad\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHC} \mathrm{CHCH}\left(\mathrm{CH}_{3}\right)_{2} \quad\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right) \dot{\mathrm{C}} \mathrm{H}_{2}\) b. CCC1CCCCC1 CC1CCCCC1 CC=C1CCCCC1

Why is a benzylic \(\mathrm{C}-\mathrm{H}\) bond unusually weak?

Draw all constitutional isomers formed when each alkene is treated with NBS \(+\) hv. a. \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{3}\) b. CC1(C)C=CCCC1 c. \(\mathrm{CH}_{2}=\mathrm{C}\left(\mathrm{CH}_{2} \mathrm{CH}_{2}\right)_{2}\)
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