Textbook Question
How will the rate of each of the following SN2 reactions change if it is carried out in a more polar solvent?
a.
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7. Substitution Reactions
SN2 Reaction
4:57 minutesProblem 14c
Textbook Question
Which substitution reaction takes place more rapidly?
c. 
Verified step by step guidance1
Step 1: Understand the type of substitution reaction involved. Both reactions are nucleophilic substitution reactions, where the nucleophile (CH3O− or CH3S−) attacks the electrophilic carbon in CH3CH2Cl, displacing the leaving group (Cl−).
Step 2: Compare the nucleophiles. CH3O− is an alkoxide ion, while CH3S− is a thiolate ion. Thiolate ions are generally more nucleophilic than alkoxide ions because sulfur is larger and more polarizable than oxygen, making CH3S− a stronger nucleophile.
Step 3: Consider the solvent. The solvent ethanol is protic, meaning it can form hydrogen bonds. Protic solvents tend to stabilize smaller, more basic nucleophiles like CH3O−, reducing their nucleophilicity. Thiolate ions (CH3S−), being less basic and more polarizable, are less affected by protic solvents and retain their high nucleophilicity.
Step 4: Evaluate the leaving group and substrate. CH3CH2Cl is a primary alkyl halide, which is favorable for SN2 reactions. In SN2 reactions, the rate depends on both the nucleophile and the substrate. Since CH3S− is a stronger nucleophile, the reaction with CH3S− will proceed more rapidly.
Step 5: Conclude based on the comparison. The substitution reaction involving CH3CH2Cl + CH3S− in ethanol will take place more rapidly than the reaction involving CH3CH2Cl + CH3O− due to the higher nucleophilicity of CH3S− in a protic solvent.
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Video duration:
4mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Nucleophilicity
Nucleophilicity refers to the ability of a nucleophile to donate an electron pair to an electrophile during a chemical reaction. In this context, CH3O− (methoxide) and CH3S− (thiolate) are both nucleophiles, but their reactivity can differ based on factors such as charge, electronegativity, and solvent effects. Generally, stronger nucleophiles react more rapidly in substitution reactions.
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Nucleophilic Addition
Substitution Reactions
Substitution reactions involve the replacement of one functional group in a molecule with another. In organic chemistry, these reactions can be classified as either nucleophilic substitution (S_N1 or S_N2 mechanisms) depending on the structure of the substrate and the nature of the nucleophile. The rate of these reactions is influenced by the strength of the nucleophile and the leaving group.
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Solvent Effects
The choice of solvent can significantly impact the rate of substitution reactions. Polar protic solvents, like ethanol, can stabilize ions and influence the nucleophilicity of the reactants. For example, in a polar protic solvent, nucleophiles may be less reactive due to solvation effects, which can affect the overall reaction rate and mechanism.
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01:16Identification of polarity in solvents
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