Why is meta-nitro phenol more basic than para-nitro phenol?

The basicity of a compound is determined by its ability to donate or accept a pair of electrons. In the case of phenols, the presence of a hydroxyl group (-OH) attached to an aromatic ring is responsible for their basicity. The electronic effects of substituents on the aromatic ring can influence the basicity of phenols. In this case, we are comparing meta-nitrophenol and para-nitrophenol.

To understand why meta-nitrophenol is more basic than para-nitrophenol, we must consider the effect of the nitro (-NO2) group on the aromatic ring.

The nitro group is an electron-withdrawing group due to the presence of electronegative atoms (nitrogen and oxygen) that attract electrons towards themselves. This results in a decrease in electron density on the aromatic ring.

In meta-nitrophenol, the nitro group is situated at the meta position, meaning it is attached to an atom adjacent to the hydroxyl group. The effect of the nitro group on the hydroxyl group is less pronounced due to the distance between them. The hydroxyl group still retains a relatively higher electron density, making it more capable of donating a pair of electrons. Therefore, meta-nitrophenol is more basic compared to para-nitrophenol.

To determine the basicity experimentally, one can measure the pKa values of the compounds. The lower the pKa value (more negative), the stronger the acid and the weaker the base. Experimental measurements provide empirical evidence of the relative basicity of different compounds.