Textbook Question
Which of the following is the better leaving group in a polar aprotic solvent?
(c) Br ⁻ vs. I ⁻
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7. Substitution Reactions
Good Leaving Groups
6:30 minutesProblem 56a,b,c
Textbook Question
Which reaction in each of the following pairs takes place more rapidly? (EtOH is ethyl alcohol; Et2O is diethyl ether.)
a. 
b. 
c. 
Verified step by step guidance1
Step 1: Analyze the reaction mechanism for each pair of reactions. For part (a), both reactions involve the nucleophilic substitution of CH3Br. The first reaction uses HO⁻ (a strong nucleophile), while the second uses H2O (a weaker nucleophile). Stronger nucleophiles generally lead to faster reactions in nucleophilic substitution.
Step 2: For part (b), compare the leaving groups in the two reactions. The leaving group ability increases as the halide ion size increases (I⁻ > Br⁻ > Cl⁻ > F⁻). CH3I has a better leaving group (I⁻) compared to CH3Cl (Cl⁻), so the reaction with CH3I is expected to proceed more rapidly.
Step 3: For part (c), compare the nucleophiles and reaction conditions. In the first reaction, NH3 acts as a nucleophile, while in the second reaction, H2O is the nucleophile. NH3 is a stronger nucleophile than H2O, so the reaction with NH3 is expected to proceed more rapidly.
Step 4: Consider the solvent effects in each reaction. Polar protic solvents (like H2O and EtOH) can stabilize the transition state and the leaving group, which can influence the reaction rate. However, the strength of the nucleophile and the leaving group ability are the dominant factors in these cases.
Step 5: Summarize the findings for each pair of reactions. For part (a), the reaction with HO⁻ is faster. For part (b), the reaction with CH3I is faster. For part (c), the reaction with NH3 is faster. These conclusions are based on the nucleophile strength and leaving group ability.
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Video duration:
6mKey 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. Stronger nucleophiles are more reactive and can displace leaving groups more effectively. In the given reactions, the strength of the nucleophile (e.g., HO− vs. H2O) significantly influences the reaction rate, as stronger nucleophiles will lead to faster reactions.
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Leaving Group Ability
The leaving group ability is a measure of how easily a group can depart from the parent molecule during a reaction. Good leaving groups, such as bromide (Br−) and iodide (I−), stabilize the negative charge after leaving, facilitating the reaction. In the comparisons provided, the nature of the leaving group affects the overall reaction rate, with better leaving groups leading to faster reactions.
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Solvent Effects
The solvent can significantly impact the rate of a reaction by stabilizing reactants, intermediates, or products. Polar protic solvents, like water and alcohols, can stabilize ions through solvation, while polar aprotic solvents can enhance nucleophilicity. In the reactions presented, the choice of solvent (EtOH vs. Et2O) can influence the speed of the reaction by affecting the nucleophile's reactivity and the leaving group's stability.
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