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

-The substrate tert-butylchloride reacts in Experiment 2 but not in Experiment 1 because:the leaving group is bonded to a primary carbon.a stable carbocation forms in Experiment 2.the leaving group is bonded to a tertiary carbon.an unstable carbocation forms in Experiment 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

-The substrate tert-butylchloride reacts in Experiment 2 but not in Experiment 1 because:the leaving group is bonded to a primary carbon.a stable carbocation forms in Experiment 2.the leaving group is bonded to a tertiary carbon.an unstable carbocation forms in Experiment 2.

Quizplus · Accepted Answer

11ecba2d_1283_b984_a3bf_65ae42fb7aeb_MD0008_00 An S_N2 reaction is a nucleophilic substitution reaction that occurs in a single step whereas an S_N1 reaction occurs in two steps. In the first step of an S_N1 reaction, the bond between the leaving group and carbon is broken, transferring the bonding electrons to the leaving group and forming a carbocation intermediate. This is the rate-determining (slowest) step. The second step involves formation of a bond between the nucleophile and the carbocation to form the final product of the reaction. Tertiary alkyl halides undergo S_N1 reactions more readily than less substituted alkyl halides because tertiary alkyl halides form a more stable carbocation. Conversely, tertiary alkyl halides are unreactive in an S_N2 reaction because they are more sterically hindered, preventing nucleophilic attack and the breaking of the leaving group-carbon bond in one step. The substrate tert-butyl chloride is a tertiary alkyl halide, meaning the chlorine leaving group is bonded to a carbon with three alkyl substituents (a tertiary carbon) (Number III). Therefore, tert-butyl chloride is sterically hindered and cannot undergo an S_N2 reaction with NaI, as described for Experiment 1. Alternatively, breaking the bond between the chlorine and the tertiary carbon (the first step of an S_N1 reaction) causes a stable, tertiary carbocation to form (Number II), permitting tert-butyl chloride to go through an S_N1 reaction with AgNO₃ in Experiment 2. (Number I) The leaving group of tert-butyl chloride is bonded to a tertiary, rather than a primary, carbon. (Number IV) A carbocation does form in the first step of Experiment 2, but it is stable because the positive charge is on a tertiary carbon. S_N1 reactions are favored when stable carbocations can form. Educational objective: S_N1 reactions occur in two steps: carbocation formation and nucleophilic addition. These reactions readily occur with tertiary alkyl halides because they are sterically hindered and form stable carbocations.

Passage A Group of Students Studied the Reactivity of Four Alkyl