Problem 81
Question
(a) When 1 -chloro-2-pentene is reacted with concentrated solution of sodium ethoxide, the rate of reaction depends on the concentration of both allylhalide and ethoxide ion. The product of reaction is exclusively $$ \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2} \mathrm{OC}_{2} \mathrm{H}_{5} $$ (b) In dilute solution of sodium ethoxide, the reaction rate is dependent only on the concentration of allylhalide. (c) In the presence of traces of water, 1 -chloro-2-pentene is slowly converted to a mixture of 1 -chloro-2-pentene and 3-chloro-1-pentene In case (a) the mechanism exclusively followed by reaction is (a) \(\mathrm{S}_{\mathrm{N}} 1\) (b) \(\mathrm{S}_{\mathrm{N}} 2\) (c) \(\mathrm{E}\) (d) \(\mathrm{E}_{2}\)
Step-by-Step Solution
Verified Answer
The mechanism in part (a) is \( \mathrm{S}_{\mathrm{N}}2 \).
1Step 1: Analyze Reaction Conditions in Part (a)
In part (a), the reaction occurs with a concentrated solution of sodium ethoxide, and the rate depends on the concentration of both the allyl halide and the ethoxide ion. This suggests that both nucleophile and substrate are involved in the rate-limiting step, which is characteristic of a bimolecular nucleophilic substitution mechanism, or \( \mathrm{S}_{\mathrm{N}}2 \).
2Step 2: Consider the Reaction Product in Part (a)
The product given in part (a) is \( \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}\mathrm{OC}_{2} \mathrm{H}_{5} \), indicating a substitution reaction where the chlorine atom is replaced by the ethoxy group \( -OC_2H_5 \). This further supports an \( \mathrm{S}_{\mathrm{N}}2 \) mechanism.
3Step 3: Compare with Other Mechanisms in Part (a)
The question explores other potential mechanisms like \( \mathrm{S}_{\mathrm{N}}1 \), \( \mathrm{E} \), and \( \mathrm{E}_{2} \). \( \mathrm{S}_{\mathrm{N}}1 \) would involve a carbocation intermediate and rate dependence only on the substrate, which does not match part (a)'s conditions. Elimination reactions \( (\mathrm{E}, \mathrm{E}_{2}) \) would yield alkenes and are typically considered under basic conditions or with bases rather than nucleophiles.
4Step 4: Final Step: Conclude Mechanism for Part (a)
Given the dependence of the rate on both the allyl halide and ethoxide ion, as well as the substitution product being formed, the exclusive mechanism for the reaction in part (a) is \( \mathrm{S}_{\mathrm{N}}2 \).
Key Concepts
Nucleophilic SubstitutionAllyl HalideReaction KineticsConcentrated Sodium Ethoxide
Nucleophilic Substitution
Nucleophilic substitution is a fundamental concept in organic chemistry where a nucleophile replaces a leaving group in a molecule. In the context of part (a) of the exercise, the nucleophilic substitution mechanism involves the ethoxide ion functioning as the nucleophile. This ion attacks the electrophilic carbon atom that is bonded to the chlorine atom in 1-chloro-2-pentene. The chlorine atom is the leaving group, and it departs as chloride, allowing the ethoxide ion to bind in its place.
There are two main types of nucleophilic substitution mechanisms: \(\mathrm{S}_{\mathrm{N}}1\) and \(\mathrm{S}_{\mathrm{N}}2\). The term \(\mathrm{S}_{\mathrm{N}}2\) stands for bimolecular nucleophilic substitution, with the '2' indicating that two species are involved in the rate-determining step. This matches part (a) of the problem, where the reaction rate relies on both the concentration of the allyl halide and the ethoxide ion.
This type of reaction is concerted, meaning that the bond between the nucleophile and the carbon is forming, while the bond between the carbon and the leaving group is breaking simultaneously, resulting in a transition state with partial bond character.
There are two main types of nucleophilic substitution mechanisms: \(\mathrm{S}_{\mathrm{N}}1\) and \(\mathrm{S}_{\mathrm{N}}2\). The term \(\mathrm{S}_{\mathrm{N}}2\) stands for bimolecular nucleophilic substitution, with the '2' indicating that two species are involved in the rate-determining step. This matches part (a) of the problem, where the reaction rate relies on both the concentration of the allyl halide and the ethoxide ion.
This type of reaction is concerted, meaning that the bond between the nucleophile and the carbon is forming, while the bond between the carbon and the leaving group is breaking simultaneously, resulting in a transition state with partial bond character.
Allyl Halide
An allyl halide is a type of organic compound in which a halogen atom is bonded to an allylic carbon. The allylic position is adjacent to a double bond, making such compounds reactive in substitution reactions.
In the given exercise, 1-chloro-2-pentene acts as the allyl halide. The presence of the double bond in 1-chloro-2-pentene provides electron-withdrawing characteristics that stabilize negative charges through resonance. This stabilization makes the allylic position particularly suitable for nucleophilic attacks. Hence, the allylic chlorine in 1-chloro-2-pentene is the ideal target for ethoxide, the nucleophile in the reaction.
The result is a product where the allylic chlorine has been substituted by the ethoxy group \(-OC_2H_5\), leading to \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}\mathrm{OC}_{2} \mathrm{H}_{5}\).
In the given exercise, 1-chloro-2-pentene acts as the allyl halide. The presence of the double bond in 1-chloro-2-pentene provides electron-withdrawing characteristics that stabilize negative charges through resonance. This stabilization makes the allylic position particularly suitable for nucleophilic attacks. Hence, the allylic chlorine in 1-chloro-2-pentene is the ideal target for ethoxide, the nucleophile in the reaction.
The result is a product where the allylic chlorine has been substituted by the ethoxy group \(-OC_2H_5\), leading to \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}\mathrm{OC}_{2} \mathrm{H}_{5}\).
Reaction Kinetics
Reaction kinetics is the study of the rates of chemical reactions and the factors that affect these rates. In the context of this exercise, the reaction kinetics differ based on the concentration of the ethoxide ion solution.
In a concentrated solution of sodium ethoxide, as in part (a), the reaction rate depends on both the concentrations of allyl halide and ethoxide ion. This dependency on two reactants is a hallmark of the \(\mathrm{S}_{\mathrm{N}}2\) mechanism. Such second-order kinetics suggest that both the nucleophile and the substrate come together in a single transition state, leading to the reaction occurring at an appreaciably fast rate.
Conversely, in more dilute solutions of the ethoxide, the reaction rate might only rely on the concentration of the allyl halide itself, potentially suggesting a shift towards \(\mathrm{S}_{\mathrm{N}}1\)-like behavior, where the rate is determined by the unimolecular formation of a carbocation. However, in the current exercise, conditions remain firmly in favor of \(\mathrm{S}_{\mathrm{N}}2\) reaction kinetics.
In a concentrated solution of sodium ethoxide, as in part (a), the reaction rate depends on both the concentrations of allyl halide and ethoxide ion. This dependency on two reactants is a hallmark of the \(\mathrm{S}_{\mathrm{N}}2\) mechanism. Such second-order kinetics suggest that both the nucleophile and the substrate come together in a single transition state, leading to the reaction occurring at an appreaciably fast rate.
Conversely, in more dilute solutions of the ethoxide, the reaction rate might only rely on the concentration of the allyl halide itself, potentially suggesting a shift towards \(\mathrm{S}_{\mathrm{N}}1\)-like behavior, where the rate is determined by the unimolecular formation of a carbocation. However, in the current exercise, conditions remain firmly in favor of \(\mathrm{S}_{\mathrm{N}}2\) reaction kinetics.
Concentrated Sodium Ethoxide
Concentrated sodium ethoxide serves as the nucleophile in the described reaction. Sodium ethoxide is an alkoxide, which is a strong base and also a potent nucleophile. This compound is often utilized in organic synthesis for its dual reactivity.
In the case of 1-chloro-2-pentene, its concentrated nature enables the ethoxide ion to effectively attack the allylic carbon position. The high concentration leads to more ethoxide ions in the solution, increasing the likelihood of collision with the substrate and facilitating the \(\mathrm{S}_{\mathrm{N}}2\) mechanism.
Therefore, the concentrated solution ensures that the ethoxide ion is sufficiently available to simultaneously interact with 1-chloro-2-pentene, increasing the reaction rate and effectively replacing the chlorine group with an ethoxy group to form the desired product.
In the case of 1-chloro-2-pentene, its concentrated nature enables the ethoxide ion to effectively attack the allylic carbon position. The high concentration leads to more ethoxide ions in the solution, increasing the likelihood of collision with the substrate and facilitating the \(\mathrm{S}_{\mathrm{N}}2\) mechanism.
Therefore, the concentrated solution ensures that the ethoxide ion is sufficiently available to simultaneously interact with 1-chloro-2-pentene, increasing the reaction rate and effectively replacing the chlorine group with an ethoxy group to form the desired product.
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