# How to rationalise the basicity order of phenylenediamine?

Why is the basicity of para- > meta- > aniline > ortho- for phenylenediamine?
($\mathrm pK_\mathrm a$: ortho: 9.53, meta: 9.12, para: 7.96) I tried deducing it using resonance and inductive effects but got stuck.

• Mar 19, 2015 at 20:58
• I am not being able to incorporate the ortho effect concept!Let me know please.
– user14857
Mar 20, 2015 at 5:35
• Isn't the basicity trend listed backwards? The para-analog has the lowest pKa for the conjugate acid (strongest acid), which means that the base is the weakest? Mar 20, 2015 at 23:21
• in phenylene-diamine shouldn't the basicity order be p>aniline>o>m since -NH2 is not much of a bulky group and thus doesn't exhibit ortho effect. SO with ortho effect out of the picture -NH2 is very strong +M group and effect of +M>-I (for -NH2). In ortho due to hydrogen bonding its basicity is reduced. Meta position only -I. so by that reasoning para is most basic followed by aniline followed by ortho and then meta....So p>aniline>o>m....Am i going wrong anywhere? Nov 8, 2018 at 5:18