Question

Quinuclidine and triethylamine are both tertiary amines. Quinuclidine, however, is a considerably stronger base than triethylamine. Stated alternatively, the conjugate acid of quinuclidine is a considerably weaker acid than the conjugate acid of triethylamine. Propose an explanation for these differences in acidity/basicity.

Short answer

Question: Explain the differences in acidity/basicity between quinuclidine and triethylamine based on their molecular structure and inductive effects. Answer: The differences in acidity/basicity between quinuclidine and triethylamine can be primarily explained by their molecular structure and inductive effects. Quinuclidine has a more rigid, constrained structure due to its bicyclic nature, which allows for better stabilization of the positive charge on nitrogen when acting as a base. This makes quinuclidine a stronger base than triethylamine, which has a more flexible structure and less effective inductive effects due to its three ethyl groups bonded to the nitrogen atom.

Step-by-step solution

Understand the molecular structures of quinuclidine and triethylamine

First, we need to be familiar with the molecular structures of both quinuclidine and triethylamine. Quinuclidine is a bicyclic compound with nitrogen as one of the bridgehead atoms, while triethylamine has a nitrogen atom attached to three ethyl groups.

Examine resonance stabilization

Next, we need to consider how resonance stabilization might differ between the two compounds. In quinuclidine, the lone pair on the nitrogen atom cannot participate in resonance with any other atoms in the molecule, as there is no adjacent double bond or other suitable functional groups. In contrast, the lone pair on the nitrogen atom in triethylamine can also not participate in resonance, since it's only bonded to ethyl groups.

Investigate inductive effects

Inductive effects can help explain differences in acidity and basicity. The inductive effect is the ability of polar bonds to transmit electronic effects through single bonds, influencing reactivity, acidity, and basicity. Quinuclidine is of a more rigid, bicyclic structure, which reduces the inductive effect of its bonds, while triethylamine has a more flexible structure, allowing the inductive effect of its bonds to act more effectively.

Relate structure to acidity/basicity

Considering the aspects mentioned above, we can relate the molecular structure of quinuclidine and triethylamine to their acidity/basicity. Since quinuclidine has a more rigid, constrained structure with less inductive effect, it can better stabilize the positive charge on the nitrogen atom when it acts as a base and accepts a proton. This makes quinuclidine a stronger base than triethylamine.

State the conclusion

In conclusion, the differences in acidity/basicity between quinuclidine and triethylamine can be primarily explained by their molecular structure and inductive effects. The more rigid, constrained structure of quinuclidine allows for better stabilization of the positive charge on nitrogen when acting as a base, making it a stronger base than triethylamine.