To determine the correct order of basic strength for the given anilines, we need to consider the effects of the nitro group on the amino group in aniline. The nitro group is a strong electron-withdrawing group, and its position relative to the amino group significantly influences the basicity of the compound.
Understanding Basicity in Anilines
Basicity in anilines is primarily determined by the availability of the lone pair of electrons on the nitrogen atom of the amino group. When this lone pair is more available, the compound is more basic. The presence of electron-withdrawing groups, like nitro groups, decreases the electron density on the nitrogen, making it less basic.
Position of the Nitro Group
The position of the nitro group (ortho, meta, or para) affects the basicity in different ways:
- Ortho (o-nitroaniline): The nitro group is adjacent to the amino group. This proximity can lead to steric hindrance and intramolecular hydrogen bonding, which can stabilize the molecule but also reduce the availability of the nitrogen's lone pair.
- Meta (m-nitroaniline): The nitro group is positioned such that it does not directly interact with the amino group. This means that the electron-withdrawing effect is present, but it does not significantly hinder the availability of the lone pair on nitrogen.
- Para (p-nitroaniline): The nitro group is opposite the amino group. This position allows for some resonance effects, where the electron-withdrawing nature of the nitro group can delocalize the lone pair on nitrogen, making it less available for protonation.
Comparing Basic Strengths
Now, let’s analyze the basicity of each compound:
- **o-Nitroaniline:** The steric hindrance and potential for intramolecular interactions reduce its basicity significantly.
- **m-Nitroaniline:** This compound has the least steric hindrance and the nitro group does not directly affect the lone pair on nitrogen as much, making it the most basic of the three.
- **p-Nitroaniline:** While it has resonance effects that can stabilize the molecule, it still has a stronger electron-withdrawing effect than m-nitroaniline, making it less basic than m-nitroaniline but more basic than o-nitroaniline.
Final Order of Basic Strength
Based on this analysis, the order of basic strength from strongest to weakest is:
m-nitroaniline > p-nitroaniline > o-nitroaniline
Thus, the correct answer is (B) m-nitroaniline > p-nitroaniline > o-nitroaniline. This order reflects the influence of the nitro group's position on the basicity of the aniline derivatives.
