Chapter 12: Problem 39
Anti-Markovnikov's addition of \(\mathrm{HBr}\) is not observed in: (a) Propene (b) but-1-ene (c) but-2-ene (d) pent-3-ene
Short Answer
Expert verified
Anti-Markovnikov's addition is not observed in but-2-ene.
Step by step solution
01
Understanding Anti-Markovnikov's Rule
Anti-Markovnikov addition is a reaction where the hydrogen atom in \( \mathrm{HBr} \) adds to the more substituted carbon of the alkene, and the bromine atom adds to the less substituted carbon. This rule is applicable when peroxides are present, initiating a free radical mechanism.
02
Identifying Unsymmetrical Alkenes
Anti-Markovnikov's addition applies best to unsymmetrical alkenes. Our options are: (a) propene, (b) but-1-ene, (c) but-2-ene, and (d) pent-3-ene. First, identify which of these alkenes are unsymmetrical where one carbon atom of the double bond is more substituted than the other.
03
Analyzing Each Option
(a) Propene and (b) but-1-ene have an unsymmetrical double bond, suitable for anti-Markovnikov's addition. (d) Pent-3-ene also has a double bond where one side is less hindered. However, (c) but-2-ene has a symmetrical double bond, making anti-Markovnikov's addition not applicable.
04
Conclusion
Since anti-Markovnikov's rule is not applicable to symmetrical alkenes and but-2-ene has a symmetrical double bond, anti-Markovnikov's addition is not observed in but-2-ene.
Unlock Step-by-Step Solutions & Ace Your Exams!
-
Full Textbook Solutions
Get detailed explanations and key concepts
-
Unlimited Al creation
Al flashcards, explanations, exams and more...
-
Ads-free access
To over 500 millions flashcards
-
Money-back guarantee
We refund you if you fail your exam.
Over 30 million students worldwide already upgrade their learning with Vaia!
Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Free Radical Mechanism
The free radical mechanism is a fascinating pathway for chemical reactions, particularly in the addition of hydrogen bromide (HBr) to unsymmetrical alkenes under specific conditions that involve the presence of peroxides. This process is known as Anti-Markovnikov addition. When peroxides are present, they facilitate the formation of free radicals through the cleavage of molecular bonds, driven by the addition of energy.
Here, the mechanism begins with the generation of bromine radicals from the decomposition of peroxides. These bromine radicals react with the alkene, breaking the double bond and forming a new carbon-bromine bond at the less substituted carbon. This differs from the typical method where the addition occurs at the more substituted carbon. The next major step involves the newly formed carbon radical reacting with hydrogen from HBr, adding to the more substituted carbon. This results in the conservation of the radical, allowing the cycle to continue until termination.
The significance of free radical mechanisms in Anti-Markovnikov additions is its ability to diverge from the classical Markovnikov's rule, relying heavily on the presence of peroxides to initiate the free radical process.
Here, the mechanism begins with the generation of bromine radicals from the decomposition of peroxides. These bromine radicals react with the alkene, breaking the double bond and forming a new carbon-bromine bond at the less substituted carbon. This differs from the typical method where the addition occurs at the more substituted carbon. The next major step involves the newly formed carbon radical reacting with hydrogen from HBr, adding to the more substituted carbon. This results in the conservation of the radical, allowing the cycle to continue until termination.
The significance of free radical mechanisms in Anti-Markovnikov additions is its ability to diverge from the classical Markovnikov's rule, relying heavily on the presence of peroxides to initiate the free radical process.
Unsymmetrical Alkenes
Unsymmetrical alkenes are critical in the application of Anti-Markovnikov's rule. These alkenes have a double bond between carbon atoms that are substituted differently on either side. This structural peculiarity allows for selective reactions that are steering away from classical mechanisms.
In these molecules, one carbon atom of the double bond will have more substituents than the other. For example, in propene, one side of the double bond is bonded to two hydrogen atoms while the other side is attached to a single hydrogen atom and a methyl group. When undergoing reactions like Anti-Markovnikov's addition, the less substituted carbon is where the nucleophilic bromine adds, while the hydrogen prefers the more substituted carbon.
This type of alkene is essential in polymer chemistry and organic synthesis. Understanding which parts of the molecule are more or less substituted can help predict where reactions will occur. This leads to gaining control over product formation and improving yield specificity.
In these molecules, one carbon atom of the double bond will have more substituents than the other. For example, in propene, one side of the double bond is bonded to two hydrogen atoms while the other side is attached to a single hydrogen atom and a methyl group. When undergoing reactions like Anti-Markovnikov's addition, the less substituted carbon is where the nucleophilic bromine adds, while the hydrogen prefers the more substituted carbon.
This type of alkene is essential in polymer chemistry and organic synthesis. Understanding which parts of the molecule are more or less substituted can help predict where reactions will occur. This leads to gaining control over product formation and improving yield specificity.
Symmetrical Alkenes
Symmetrical alkenes, unlike their unsymmetrical counterparts, have identical substituents on either side of the double bond. This symmetry plays a key role in determining the products formed in reactions involving these compounds.
In symmetrical alkenes like but-2-ene, both carbon atoms involved in the double bond are equally substituted. As a result, reactions like Anti-Markovnikov addition do not favor one side over the other since there is no disparity in substitution level. With no difference between the carbons, the typical radical pathway is not applicable because there is no selectivity to exploit.
Due to this lack of differentiation, other forms of reactions that do not rely on asymmetric conditions may occur, highlighting the importance of symmetry in determining chemical reactivity and pathway choices.
In symmetrical alkenes like but-2-ene, both carbon atoms involved in the double bond are equally substituted. As a result, reactions like Anti-Markovnikov addition do not favor one side over the other since there is no disparity in substitution level. With no difference between the carbons, the typical radical pathway is not applicable because there is no selectivity to exploit.
Due to this lack of differentiation, other forms of reactions that do not rely on asymmetric conditions may occur, highlighting the importance of symmetry in determining chemical reactivity and pathway choices.
Alkene Substitution
Alkene substitution refers to the number of carbon groups or substituents attached to the carbon atoms of the alkene's double bond. This plays a pivotal role in predicting how an alkene will react in specific chemical conditions.
Alkenes with different levels of substitution react differently, particularly in reactions under the guidance of rules like Markovnikov's and Anti-Markovnikov's. Highly substituted carbons often stabilize intermediates better, affecting factors such as the location where new bonds form in the product.
In the context of Anti-Markovnikov's addition, understanding substitution is crucial. The less substituted carbon in an unsymmetrical alkene is the nucleophilic target for bromine addition, which is a major departure from many traditional reactions where more substituted alkenes would typically be preferred due to their stability. Knowing how alkene substitution affects reactivity can thus guide chemists in designing pathways for desired chemical transformations.
Alkenes with different levels of substitution react differently, particularly in reactions under the guidance of rules like Markovnikov's and Anti-Markovnikov's. Highly substituted carbons often stabilize intermediates better, affecting factors such as the location where new bonds form in the product.
In the context of Anti-Markovnikov's addition, understanding substitution is crucial. The less substituted carbon in an unsymmetrical alkene is the nucleophilic target for bromine addition, which is a major departure from many traditional reactions where more substituted alkenes would typically be preferred due to their stability. Knowing how alkene substitution affects reactivity can thus guide chemists in designing pathways for desired chemical transformations.
