Question

Select the member of each pair that shows the greater rate of \(\mathrm{S}_{\mathrm{N}} 2\) reaction with \(\mathrm{KN}_{3}\) in acetone. (a) CC1(Br)CCCCC1 (b) CC1(C)CCC(Br)CC1

Short answer

(a) 1-bromo-1-methylcyclohexane This compound shows a greater rate of SN2 reaction with KN3 in acetone due to its less hindered electrophilic carbon, which allows the N3- nucleophile to attack more easily than in compound (b) 1-bromo-1,1-dimethylcyclohexane.

Step-by-step solution

Convert SMILES notation to molecular structure

The SMILES notation for the given compounds are: (a) CC1(Br)CCCCC1 (b) CC1(C)CCC(Br)CC1 Convert them to molecular structures: (a) 1-bromo-1-methylcyclohexane (b) 1-bromo-1,1-dimethylcyclohexane

Analyze the factors affecting SN2 reaction rates

In an SN2 reaction, the rate-determining step involves simultaneous attack by the nucleophile (in this case, N3-) and the departure of the leaving group (Br-). Three main factors influence the SN2 reaction rate: 1. Steric hindrance: A less hindered substrate allows the nucleophile to approach and attack the electrophilic carbon more easily, leading to a faster reaction. 2. Leaving group ability: A better leaving group can leave more easily, leading to a faster reaction. 3. Nucleophile strength: A stronger nucleophile reacts more quickly with the electrophilic carbon in the substrate. For this exercise, we will focus on the steric hindrance, as the leaving group (Br-) and nucleophile (N3-) are the same for both substrates.

Compare the steric hindrance of the two substrates

Let's compare the steric hindrance of the two substrates: (a) 1-bromo-1-methylcyclohexane: The electrophilic carbon is directly attached to a methyl group and the cyclohexane ring. (b) 1-bromo-1,1-dimethylcyclohexane: The electrophilic carbon is directly attached to two methyl groups and the cyclohexane ring. Comparing the two substrates, the electrophilic carbon in (b) is more sterically hindered due to the presence of two methyl groups on it. This makes it harder for the N3- nucleophile to approach and attack the electrophilic carbon.

Select the compound with a higher SN2 reaction rate

Given the steric hindrance comparison in Step 3, compound (a) 1-bromo-1-methylcyclohexane has a less hindered electrophilic carbon, allowing the N3- nucleophile to attack more easily. Consequently, compound (a) will have a higher SN2 reaction rate with KN3 in acetone compared to compound (b). Therefore, the answer is: (a) 1-bromo-1-methylcyclohexane