para-Nitroaniline is an order of magnitude less basic than

Step 1 of 2

(a) First, we’re asked to explain the observed difference in basicity.

If para-nitroaniline acts as a base, the amino group becomes protonated and positively charged. The lone pair of electrons on the neutral amino group reaches out and attacks an acid species, abstracting a proton. The relative availability of this lone pair is, therefore, the critical determining factor. When there is a nitro group (a powerful electron-withdrawing substituent) in the para position, resonance structures indicate that the amine lone pair is spread out over numerous atoms. If the nitro group is in the meta position, resonance structures can still be drawn but the resonance structure with the lone pair still on the amino group is a larger resonance contributor.

In other words, there is less electron density on the amine group of para-nitroaniline than there is on the amine group of meta-nitroaniline. It’s therefore a weaker base - roughly ten times weaker.