Chapter 26: Problem 96
The order of reactivities of the following alklyl halides for a \(\mathrm{S}_{\mathrm{N}}^{2}\) reaction is (a) \(\mathrm{RF}>\mathrm{RCl}>\mathrm{RBr}>\mathrm{RI}\) (b) \(\mathrm{RF}>\mathrm{RBr}>\mathrm{RCl}>\mathrm{RI}\) (c) \(\mathrm{RCl}>\mathrm{RBr}>\mathrm{RF}>\mathrm{RI}\) (d) \(\mathrm{RI}>\mathrm{RBr}>\mathrm{RCl}>\mathrm{RF}\)
Short Answer
Expert verified
The order is (d) \(RI > RBr > RCl > RF\).
Step by step solution
01
Understand the SN2 Reaction
The S\(_N^{2}\) reaction mechanism involves a nucleophile approaching the electrophilic carbon opposite to the leaving group due to steric and electronic factors. This process results in the inversion of configuration at the carbon and typically happens in one concerted step.
02
Evaluate Leaving Group Ability
In an S\(_N^{2}\) reaction, the leaving group ability is crucial. The better the leaving group, the faster the reaction. Iodide (\(I^-\)) is a better leaving group compared to bromide (\(Br^-\)), chloride (\(Cl^-\)), and fluoride (\(F^-\)) due to its larger size, lower basicity, and better ability to stabilize the negative charge.
03
Determine the Reactivity Order
Taking the leaving group abilities into account, the alkyl halides reactivity order for an S\(_N^{2}\) reaction is based on the ease with which the leaving group departs. Hence, \(RI\) > \(RBr\) > \(RCl\) > \(RF\), making option (d) the correct choice.
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.
Nucleophilic Substitution
In organic chemistry, nucleophilic substitution reactions are a fundamental class of reactions where a nucleophile replaces a leaving group on a carbon atom. The term "nucleophile" refers to a species rich in electrons that seeks out a positively charged or electron-deficient carbon center. These reactions can happen through various mechanisms, with the S\(_N^{2}\) mechanism being one of the most common, especially for primary and secondary alkyl halides.
An S\(_N^{2}\) reaction occurs in one concerted step with the nucleophile attacking the carbon from a direction opposite to the leaving group, leading to a characteristic inversion of configuration, often referred to as the "Walden inversion." This reaction type is bimolecular at the rate-determining step, meaning the reaction rate depends on both the concentration of the nucleophile and the substrate. Understanding nucleophilic substitution is crucial for predicting reaction outcomes in synthetic and chemical applications.
An S\(_N^{2}\) reaction occurs in one concerted step with the nucleophile attacking the carbon from a direction opposite to the leaving group, leading to a characteristic inversion of configuration, often referred to as the "Walden inversion." This reaction type is bimolecular at the rate-determining step, meaning the reaction rate depends on both the concentration of the nucleophile and the substrate. Understanding nucleophilic substitution is crucial for predicting reaction outcomes in synthetic and chemical applications.
Leaving Group Ability
In nucleophilic substitution reactions, the ability of the leaving group to depart effectively is a critical factor in determining the reaction's speed and feasibility. A good leaving group is typically a stable species after it has left, which means it should ideally be a weak base and able to stabilize the negative charge.
For halogens as leaving groups, iodide (\(I^-\)) is the most effective, followed by bromide (\(Br^-\)), chloride (\(Cl^-\)), and lastly fluoride (\(F^-\)). This happens because iodide is larger and less basic compared to other halogens. As a result, it can stabilize the negative charge more effectively once it leaves the substrate. Meanwhile, fluoride ions are poor leaving groups due to their high electronegativity and strong basicity. By understanding the nature of leaving groups, chemists can better predict and control reaction pathways.
For halogens as leaving groups, iodide (\(I^-\)) is the most effective, followed by bromide (\(Br^-\)), chloride (\(Cl^-\)), and lastly fluoride (\(F^-\)). This happens because iodide is larger and less basic compared to other halogens. As a result, it can stabilize the negative charge more effectively once it leaves the substrate. Meanwhile, fluoride ions are poor leaving groups due to their high electronegativity and strong basicity. By understanding the nature of leaving groups, chemists can better predict and control reaction pathways.
Reaction Mechanisms
The reaction mechanism of an S\(_N^{2}\) reaction consists of a simultaneous bond-forming and bond-breaking process. It's a one-step reaction, where the nucleophile attacks the electrophile, pushing out the leaving group. This "concerted" process doesn't involve any intermediates - the process is straightforward yet elegant.
Understanding this involves understanding why S\(_N^{2}\) reactions prefer certain conditions. The kinetics are second-order, which implies that both the nucleophile and the electrophilic carbon on the substrate influence the reaction rate. Steric hindrance can drastically slow down or prevent the reaction since it involves a direct, backside attack, meaning that bulkier groups around the electrophile make it harder for the nucleophile to approach. This is why primary alkyl halides are often most suitable for S\(_N^{2}\) reactions.
Understanding this involves understanding why S\(_N^{2}\) reactions prefer certain conditions. The kinetics are second-order, which implies that both the nucleophile and the electrophilic carbon on the substrate influence the reaction rate. Steric hindrance can drastically slow down or prevent the reaction since it involves a direct, backside attack, meaning that bulkier groups around the electrophile make it harder for the nucleophile to approach. This is why primary alkyl halides are often most suitable for S\(_N^{2}\) reactions.
Alkyl Halides
Alkyl halides are organic compounds containing halogen atoms attached to an alkyl group. These halides play a critical role in nucleophilic substitution reactions like the S\(_N^{2}\) mechanism. Variations in their structure can significantly impact the reaction mechanisms and the efficiency of such reactions.
The nature of the halogen impacts leaving group quality, with iodide delivering the best results for S\(_N^{2}\) reactions due to its efficient leaving group ability. The carbon-halogen bond's strength also affects the reactivity; for example, a C-I bond is weaker than a C-F bond, making alkyl iodides more reactive under the conditions necessary for an S\(_N^{2}\) reaction. Additionally, the alkyl group itself, which can range from methyl to tert-butyl, influences reactivity due to steric effects. Primary alkyl halides are typically most reactive in S\(_N^{2}\) scenarios due to less steric hindrance.
The nature of the halogen impacts leaving group quality, with iodide delivering the best results for S\(_N^{2}\) reactions due to its efficient leaving group ability. The carbon-halogen bond's strength also affects the reactivity; for example, a C-I bond is weaker than a C-F bond, making alkyl iodides more reactive under the conditions necessary for an S\(_N^{2}\) reaction. Additionally, the alkyl group itself, which can range from methyl to tert-butyl, influences reactivity due to steric effects. Primary alkyl halides are typically most reactive in S\(_N^{2}\) scenarios due to less steric hindrance.
Organic Chemistry Reactivity
Organic chemistry reactivity encompasses the principles and trends that determine how and why organic molecules react in certain ways. Reactivity in the context of nucleophilic substitution involves evaluating both the leaving group and the nucleophile.
For S\(_N^{2}\) reactions, the combination of a strong nucleophile and a good leaving group results in a fast and efficient reaction. The trend among halides sees iodides reacting quicker due to their excellent leaving group ability. This reactivity is essential for organic synthesis, where understanding these patterns allows chemists to construct complex molecules methodically.
Steric effects and electronic factors also play roles in dictating reactivity. Molecules with less steric hindrance and the right electronic environment will react more favorably under S\(_N^{2}\) conditions. Through mastering these concepts, students and chemists alike can predict outcomes and optimize reaction conditions for better yields and more efficient synthesis.
For S\(_N^{2}\) reactions, the combination of a strong nucleophile and a good leaving group results in a fast and efficient reaction. The trend among halides sees iodides reacting quicker due to their excellent leaving group ability. This reactivity is essential for organic synthesis, where understanding these patterns allows chemists to construct complex molecules methodically.
Steric effects and electronic factors also play roles in dictating reactivity. Molecules with less steric hindrance and the right electronic environment will react more favorably under S\(_N^{2}\) conditions. Through mastering these concepts, students and chemists alike can predict outcomes and optimize reaction conditions for better yields and more efficient synthesis.
