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.

Short answer

Answer: Quinuclidine is a stronger base than triethylamine because of the greater electron donation from the fused ring system and less steric hindrance around the nitrogen atom, making its lone pair more accessible for protonation. Consequently, the conjugate acid of quinuclidine is a weaker acid than the conjugate acid of triethylamine.

Step-by-step solution

Identify the general structure of tertiary amines

Tertiary amines are organic compounds in which a nitrogen atom is bonded to three different carbon atoms. They are generally basic in nature due to the lone pair of electrons present on the nitrogen atom that can accept a proton.

Examine the structures of quinuclidine and triethylamine

Quinuclidine is a bicyclic compound (two fused rings) with the nitrogen atom at the bridgehead position. On the other hand, triethylamine is a simple monomolecular compound with three ethyl groups bonded to a central nitrogen atom. Quinuclidine structure: ``` N / \ C C / \ C C \ / C---C ``` Triethylamine structure: ``` N /|\ C C C | | | H H H ```

Understand the factors affecting basicity

The basicity of amines is determined by several factors: 1. Electron availability: The greater the electron density on the nitrogen atom, the more basic the amine. 2. Inductive effect: Electron-donating groups (e.g. alkyl groups) increase the basicity of amines by increasing the electron density on the nitrogen atom. 3. Steric hindrance: If the nitrogen atom is surrounded by bulky groups, the basicity of the amine can be decreased since the lone pair of electrons may be less accessible for protonation. 4. Resonance: If an amine can participate in resonance, this can lead to delocalization of the lone pair and a decrease in basicity.

Compare the basicity of quinuclidine and triethylamine

Based on the factors mentioned above, we can analyze why quinuclidine is a stronger base than triethylamine: 1. Electron availability: Both amines have a lone pair on the nitrogen atom, which can accept a proton. 2. Inductive effect: Triethylamine has three ethyl groups donating electron density to the nitrogen atom, while quinuclidine's more complex structure has a greater electron-donating effect due to the fused ring system. 3. Steric hindrance: Triethylamine has bulky ethyl groups around the nitrogen, which may cause some steric hindrance, making it less favorable for protonation. In contrast, quinuclidine's nitrogen atom experiences less steric hindrance, making its lone pair more accessible for protonation. 4. Resonance: Triethylamine cannot participate in any resonance, while quinuclidine has some resonance within the bicyclic structure. However, this resonance does not have a significant effect on the overall basicity. Taking all the factors into consideration, quinuclidine is a stronger base than triethylamine because of the greater electron donation from the fused ring system and less steric hindrance around the nitrogen atom. Consequently, the conjugate acid of quinuclidine is a weaker acid than the conjugate acid of triethylamine.