The rate of the reaction of methyl iodide with quinuclidine was measured in nitrobenzene, and then the rate of the reaction of methyl iodide with ­triethylamine was measured in the same solvent. The concentration of the reagents was the same in both experiments.
b. The same experiment was done using isopropyl iodide instead of methyl iodide. Which reaction had the larger rate constant?

Verified step by step guidance

1. Step 1: Analyze the chemical structures provided. Quinuclidine (1-azabicyclo[2.2.1]hepta-2,5-diene) is a bicyclic amine with a rigid structure and a lone pair of electrons on the nitrogen atom, making it highly nucleophilic. Triisobutylamine, on the other hand, is a bulky tertiary amine with steric hindrance around the nitrogen atom, which reduces its nucleophilicity.
2. Step 2: Consider the reaction mechanism. The reaction of alkyl iodides with amines typically proceeds via an SN2 mechanism, where the nucleophile attacks the electrophilic carbon of the alkyl iodide, displacing the iodide ion. The rate of an SN2 reaction depends on the nucleophilicity of the amine and the steric hindrance around the electrophilic carbon.
3. Step 3: Compare the alkyl iodides. Methyl iodide is a small molecule with minimal steric hindrance, making it highly reactive in SN2 reactions. Isopropyl iodide, however, has increased steric hindrance due to the presence of two methyl groups attached to the electrophilic carbon, which slows down the SN2 reaction.
4. Step 4: Predict the effect of steric hindrance on the rate constant. Quinuclidine, being less sterically hindered and more nucleophilic, will likely react faster with both methyl iodide and isopropyl iodide compared to triisobutylamine. However, the steric hindrance of isopropyl iodide will have a greater impact on the reaction with triisobutylamine due to its already reduced nucleophilicity.
5. Step 5: Conclude which reaction has the larger rate constant. For isopropyl iodide, the reaction with quinuclidine will have a larger rate constant compared to the reaction with triisobutylamine, as quinuclidine is less sterically hindered and more nucleophilic, allowing it to overcome the steric hindrance of isopropyl iodide more effectively.

Key Concepts

Nucleophilicity

Nucleophilicity refers to the ability of a nucleophile to donate an electron pair to an electrophile during a chemical reaction. In this context, quinuclidine and triethylamine are both nucleophiles reacting with methyl iodide and isopropyl iodide. The strength of the nucleophile affects the reaction rate, with more nucleophilic species typically leading to faster reactions.

Rate Constants and Reaction Rates

The rate constant (k) is a proportionality factor in the rate law of a reaction, indicating how quickly a reaction proceeds at a given temperature. The rate of a reaction is influenced by the nature of the reactants, solvent, and temperature. Comparing the rate constants of the reactions involving methyl iodide and isopropyl iodide helps determine which reaction is faster under the same conditions.

Solvent Effects

The choice of solvent can significantly influence the rate of a reaction by stabilizing or destabilizing reactants, intermediates, or transition states. Nitrobenzene, a polar aprotic solvent, can enhance the nucleophilicity of the nucleophiles involved in the reactions. Understanding how solvent properties affect reaction mechanisms is crucial for predicting the outcomes of the experiments described.