Rank each set of compounds in order of increasing basicity.
(d) pyrrole, imidazole, 3-nitropyrrole

Verified step by step guidance

1. Step 1: Understand the concept of basicity. Basicity refers to the ability of a compound to donate a lone pair of electrons to accept a proton (H⁺). In organic chemistry, the basicity of nitrogen-containing heterocycles is influenced by factors such as electron density, resonance, and the presence of electron-withdrawing or electron-donating groups.
2. Step 2: Analyze pyrrole (structure i). Pyrrole has a nitrogen atom with a lone pair of electrons that is delocalized into the aromatic ring system. This delocalization reduces the availability of the lone pair for protonation, making pyrrole less basic.
3. Step 3: Analyze imidazole (structure ii). Imidazole contains two nitrogen atoms, one of which has a lone pair that is not involved in aromatic delocalization. This lone pair is readily available for protonation, making imidazole more basic than pyrrole.
4. Step 4: Analyze 3-nitropyrrole (structure iii). The nitro group (-NO₂) is a strong electron-withdrawing group that decreases the electron density on the nitrogen atom in the pyrrole ring. This further reduces the basicity of 3-nitropyrrole compared to pyrrole.
5. Step 5: Rank the compounds in order of increasing basicity. Based on the analysis, 3-nitropyrrole is the least basic due to the electron-withdrawing nitro group, followed by pyrrole with delocalized lone pair electrons, and imidazole is the most basic due to the availability of a lone pair for protonation.

Key Concepts

Basicity in Organic Chemistry

Basicity refers to the ability of a compound to accept protons (H+) in a chemical reaction. In organic chemistry, basicity is influenced by the availability of lone pairs on nitrogen atoms, which can bond with protons. Compounds with more electron-donating groups tend to be more basic, while those with electron-withdrawing groups are less basic.

Aromaticity and Basicity

Aromatic compounds, such as pyrrole and imidazole, exhibit unique stability due to resonance and delocalization of electrons. This aromatic character can affect basicity; for instance, in pyrrole, the nitrogen's lone pair is involved in the aromatic system, making it less available for protonation compared to imidazole, where the nitrogen's lone pair is more accessible.

Electron-Withdrawing and Electron-Donating Groups

Electron-withdrawing groups (EWGs), like nitro groups (-NO2), decrease basicity by stabilizing the positive charge that forms upon protonation, making it less favorable. Conversely, electron-donating groups (EDGs) enhance basicity by increasing electron density on the nitrogen atom, facilitating protonation. Understanding these effects is crucial for ranking the basicity of the given compounds.