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

-If 3-bromo-2,4-dimethylpentane (shown below) was used as a substrate in the experiments described in the passage, what would the observation be for reaction with NaI at 25°C?

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

-If 3-bromo-2,4-dimethylpentane (shown below) was used as a substrate in the experiments described in the passage, what would the observation be for reaction with NaI at 25°C?

Quizplus · Accepted Answer

11ecba2d_1287_14ef_a3bf_bfe8ec04c7f9_MD0008_00 Steric hindrance arises when a molecule is large and bulky, and can inhibit a reaction from occurring or stop the reaction from taking place at a particular part of a molecule. In nucleophilic substitutions (S_N1 and S_N2), the nucleophile needs to be able to access the electrophilic carbon easily, but steric hindrance from the substrate or the nucleophile (or both) can prevent this. The extent of steric hindrance a substrate has dictates in which nucleophilic substitution reaction it is likely to participate. Compounds with tertiary leaving groups (sterically hindered) are good substrates for S_N1 reactions because they involve formation of a carbocation, which is stabilized by three substituents. Molecules with primary leaving groups are generally good substrates for S_N2 reactions because the reaction is concerted and undergoes backside attack (nucleophile attacks on the opposite side of the leaving group). Molecules with secondary leaving groups are an intermediate case that can participate in S_N1 or S_N2 reactions, depending upon factors such as steric hindrance. 3-Bromo-2,4-dimethylpentane is a secondary alkyl halide with two isopropyl groups bonded to the electrophile for the nucleophilic substitution reaction. The passage states the reaction with NaI is an S_N2 reaction. Although secondary alkyl halides can undergo S_N2 reactions, the large bulky isopropyl groups will block the nucleophile's access to the electrophile. Therefore, 3-bromo-2,4-dimethylpentane will not react with NaI at room temperature because the electrophile is too sterically hindered. (Choices A and B) The central carbon of the isopropyl group (electrophile) is secondary rather than tertiary. Precipitate formation would be observed if the reaction occurred, but it does not because the electrophilic carbon is sterically hindered. (Choice D) Although an S_N2 reaction does not occur between 3-bromo-2,4-dimethylpentane and NaI, S_N2 reactions can occur at sp³ carbons. Educational objective: The steric hindrance of a substrate must be considered when determining whether it can undergo a particular reaction. The extent of a substrate's steric hindrance can dictate which nucleophilic substitution (S_N1 or S_N2) it is likely to undergo. S_N1 reactions permit sterically hindered electrophiles whereas S_N2 reactions require easily accessible electrophiles.

Passage A Group of Students Studied the Reactivity of Four Alkyl