Basicity depends on availability of an electron pair to bond a proton. Correlate structural effects in these amines with their basicities.
(a) Explain this order:

Verified step by step guidance

1. Step 1: Understand the concept of basicity. Basicity in amines depends on the availability of the lone pair of electrons on the nitrogen atom to bond with a proton (H⁺). Structural factors such as resonance, inductive effects, and steric hindrance can influence the availability of this lone pair.
2. Step 2: Analyze the structure of the first compound (N(CH₃)₂ attached to a benzene ring, pKb = 8.9). The nitrogen atom is part of a dimethylamino group, which is electron-donating due to the alkyl groups. This increases the availability of the lone pair on nitrogen, making it more basic. Additionally, the lone pair is not involved in resonance with the benzene ring, further enhancing basicity.
3. Step 3: Examine the structure of the second compound (a bicyclic amine, pKb = 6.2). The nitrogen atom is part of a bicyclic system where its lone pair is partially delocalized due to resonance with the adjacent aromatic ring. This delocalization reduces the availability of the lone pair for bonding with a proton, decreasing basicity compared to the first compound.
4. Step 4: Evaluate the structure of the third compound (a pyridine-like structure, pKb = 3.4). The nitrogen atom is part of an aromatic ring, and its lone pair is fully delocalized into the aromatic system. This extensive resonance significantly reduces the availability of the lone pair for bonding with a proton, making it the least basic among the three compounds.
5. Step 5: Correlate the pKb values with basicity. A higher pKb value corresponds to lower basicity, while a lower pKb value indicates higher basicity. The order of basicity is: N(CH₃)₂ attached to benzene (most basic, pKb = 8.9) > bicyclic amine (moderately basic, pKb = 6.2) > pyridine-like structure (least basic, pKb = 3.4). This order is explained by the structural effects on the availability of the nitrogen's lone pair.

Key Concepts

Basicity of Amines

Basicity in amines refers to their ability to accept protons (H+) due to the presence of a lone pair of electrons on the nitrogen atom. The strength of an amine's basicity is influenced by the availability of this lone pair, which can be affected by the surrounding structural environment, such as steric hindrance and electronic effects.

Inductive Effect

The inductive effect describes how electron-withdrawing or electron-donating groups attached to a molecule can influence the electron density on the nitrogen atom of an amine. Electron-withdrawing groups decrease basicity by stabilizing the lone pair, while electron-donating groups enhance basicity by increasing electron density, making the lone pair more available for bonding with protons.

Steric Hindrance

Steric hindrance refers to the repulsion that occurs when bulky groups are present near the reactive site of a molecule, in this case, the nitrogen atom of an amine. Increased steric hindrance can hinder the approach of protons, thereby reducing the basicity of the amine. Understanding the spatial arrangement of substituents is crucial for predicting the basicity order among different amines.