Heterocyclic aromatic amines are weaker bases than aliphatic heterocyclic amines. Campare, for example, the \(\mathrm{p} K_{\mathrm{a}}\) values for the conjugate acids of piperidine, pyri-

First, let's get familiar with the structures of piperidine, pyridine, and pyrimidine. Piperidine is an aliphatic heterocyclic amine with a six-membered ring and one nitrogen atom. Pyridine and pyrimidine are both aromatic heterocyclic amines. Pyridine has a six-membered ring with one nitrogen atom, while pyrimidine has a six-membered ring with two nitrogen atoms. The structures of these compounds can be represented like this: - Piperidine: \[ \ce{(CH2)5NH}\] - Pyridine: \[ \ce{C5H5N}\] - Pyrimidine: \[ \ce{C4H4N2}\]

In order to compare the pKa values, we need to identify the conjugate acids of piperidine, pyridine, and pyrimidine. A conjugate acid is formed when an amine molecule accepts a proton (H+). We will add a proton to the nitrogen atoms of these compounds: - Conjugate acid of piperidine: \[ \ce{(CH2)5NH2^+}\] - Conjugate acid of pyridine: \[ \ce{C5H5NH^+}\] - Conjugate acid of pyrimidine: \[ \ce{C4H4N2H^+}\]

Now, let's compare the pKa values of these conjugate acids to determine their relative basicity. The lower the pKa value, the stronger the acid, and the stronger its conjugate base. - Piperidine (aliphatic heterocyclic amine) conjugate acid pKa: 11.2 - Pyridine (aromatic heterocyclic amine) conjugate acid pKa: 5.2 - Pyrimidine (aromatic heterocyclic amine) conjugate acid pKa: 1.10 From these values, we can see that the conjugate acid of piperidine is the weakest acid, meaning that the regular piperidine is the strongest base among the three. Pyridine is the next strongest base, followed by pyrimidine.

Now that we have established the differences in basicity, we can explain it through the following factors: 1. Resonance: In aromatic heterocyclic amines like pyridine and pyrimidine, the nitrogen atom's lone pair of electrons participates in the aromatic ring's resonance. This stabilizes the ring but makes the lone pair less available for accepting protons, decreasing the basicity of the molecule. In aliphatic heterocyclic amines like piperidine, the lone pair isn't involved in resonance, making it more available for accepting protons and increasing the molecule's basicity. 2. Electron density: The electron density of the nitrogen atom also influences basicity. In pyridine, the nitrogen atom is directly connected to the aromatic ring, resulting in a lower electron density, further decreasing its basicity. In pyrimidine, there are two nitrogen atoms in the ring, which reduces the electron density even more and makes it less basic than pyridine. 3. Inductive effect: In compounds like pyridine, the electron-withdrawing nature of the aromatic ring has an inductive effect that will reduce the electron density on nitrogen, leading to less electron donation towards the proton. This contributes to the weaker basic nature of heterocyclic aromatic amines compared to the aliphatic ones. In conclusion, the main factors contributing to the weaker basic nature of heterocyclic aromatic amines are resonance and lower electron density due to aromaticity and inductive effects.