Passage
A group of students studied the reactivity of four alkyl halides (R−X) in two nucleophilic substitution reactions. Nucleophilic substitution reactions can go through one of two mechanisms. The S_N2 reaction requires substitution of a leaving group with a nucleophile in a single step (Reaction 1) , and the S_N1 reaction requires elimination of the leaving group and formation of an intermediate before nucleophilic substitution can occur, resulting in a two-step process (Reaction 2) .R−X + NaI → R−I + NaX↓Reaction 1R−X + AgNO₃ → R⁺ + AgX↓ + HOCH₂CH₃ → R−OCH₂CH₃ + HNO₃Reaction 2Experiment 1The students performed the S_N2 reaction (Reaction 1) by the addition of a few drops of each alkyl halide into a separate test tube containing 1 mL of 15% solution of NaI in acetone at 25°C. After mixing, they monitored the reaction for the formation of a precipitate (sodium halide salt) . If no precipitate formed after 5 minutes, they raised the temperature of the reaction to 50°C and again checked for the formation of a precipitate. Formation of a precipitate indicated the substrate underwent nucleophilic substitution. The results observed for the Finkelstein reaction are shown in Table 1.Table 1 Results of the S_N2 Reaction
Experiment 2The students repeated the procedure followed in Experiment 1 except they added a few drops of each alkyl halide into a separate test tube containing 1 mL of 1% solution of AgNO₃ in ethanol at 25°C. The results observed for this reaction are shown in Table 2.Table 2 Results of the S_N1 Reaction

-Which of the following statements is NOT supported by the results in Tables 1 and 2?

Question

Passage
A group of students studied the reactivity of four alkyl halides (R−X) in two nucleophilic substitution reactions. Nucleophilic substitution reactions can go through one of two mechanisms. The S_N2 reaction requires substitution of a leaving group with a nucleophile in a single step (Reaction 1) , and the S_N1 reaction requires elimination of the leaving group and formation of an intermediate before nucleophilic substitution can occur, resulting in a two-step process (Reaction 2) .R−X + NaI → R−I + NaX↓Reaction 1R−X + AgNO₃ → R⁺ + AgX↓ + HOCH₂CH₃ → R−OCH₂CH₃ + HNO₃Reaction 2Experiment 1The students performed the S_N2 reaction (Reaction 1) by the addition of a few drops of each alkyl halide into a separate test tube containing 1 mL of 15% solution of NaI in acetone at 25°C. After mixing, they monitored the reaction for the formation of a precipitate (sodium halide salt) . If no precipitate formed after 5 minutes, they raised the temperature of the reaction to 50°C and again checked for the formation of a precipitate. Formation of a precipitate indicated the substrate underwent nucleophilic substitution. The results observed for the Finkelstein reaction are shown in Table 1.Table 1 Results of the S_N2 Reaction

Experiment 2The students repeated the procedure followed in Experiment 1 except they added a few drops of each alkyl halide into a separate test tube containing 1 mL of 1% solution of AgNO₃ in ethanol at 25°C. The results observed for this reaction are shown in Table 2.Table 2 Results of the S_N1 Reaction

-Which of the following statements is NOT supported by the results in Tables 1 and 2?

Quizplus · Accepted Answer

11ecba2d_1286_519d_a3bf_077bd4453472_MD0008_00 Results from experiments can be compared to draw conclusions about aspects of the reactions performed, including substrate reactivity, leaving group ability, and plausible intermediates formed. Observations for the S_N2 reaction conditions (Table 1) show that 1-bromobutane (a primary halide) formed a precipitate whereas tert-butyl chloride (a tertiary halide) did not react, even after heating. Conversely, results for the S_N1 reaction conditions (Table 2) record that tert-butyl chloride formed a heavy precipitate, but 1-bromobutane only formed a trace amount of precipitate after heating. This data demonstrates that S_N1 and S_N2 reactions are dependent on substrate substitution because primary and tertiary alkyl halides react only in certain conditions. (Choice A) The passage states that Experiment 2 is an S_N1 reaction that involves carbocation formation. Because a heavy precipitate was observed for crotyl chloride, it must form a resonance-stabilized carbocation to react with AgNO₃. This is an exception: Primary alkyl halides can participate in S_N1 reactions, and crotyl chloride reacted because it is allylic. (Choice B) 1-Bromobutane is a primary alkyl halide and formed a precipitate in the reaction with NaI (S_N2), but it formed only a minute amount in the reaction with AgNO₃ (S_N1), even after heating. Based on these observations, it can be concluded that primary alkyl halides undergo S_N2 reactions more readily than S_N1 reactions. (Choice D) 1-Bromobutane and 1-chlorobutane are both primary alkyl halides that are structurally identical except for the halide leaving group. Based on the observations that 1-bromobutane formed a precipitate at room temperature but 1-chlorobutane had to be heated to form just a trace precipitate in an S_N2 reaction, it can be concluded that bromide is a better leaving group than chloride and that bromide comparatively enhances reactivity in less favorable reactions. Educational objective: Results from experiments can be compared to draw conclusions about aspects of the reactions performed, including the substrate reactivity, leaving group ability, and plausible intermediates formed. In particular, experimental results show that nucleophilic substitution reactions (S_N1 and S_N2) are dependent on substrate substitution.

Passage A Group of Students Studied the Reactivity of Four Alkyl