# How to compare acidity in the following aniline derivatives?

Let the simplest one be a), ortho be b), meta be c) and para be d). I have confusion between ortho and meta. How to compare their acidity?

Taking the case of ortho and meta in which you have confusion:

The acidity of aniline will increase if there is a electron donating group in the compound to stabilise the conjugate positive acid formed.

Since OMe is shows +R and -I effect, it is a net electron donating group as +R effect generally dominates over -I.So in case of ortho it is able to show electron donating nature due to +R effect which makes it more basic than the meta isomer which shows electron withdrawing effect due to only -I effect as +R effect will not affect from meta position.

• I just got more confused. Can you please help me compare the basicity? Should not ortho be the most basic out of the four due to distance factor? – Reeshabh Ranjan Mar 10 '16 at 6:31
• Distance factor plays role only in I effect not in R effect.Para is the strongest although both o and p show +R because the ortho since located closer to nh2 has more - I effect which withdraws more electron and hence stabilises the conjugate base lesser than para which has very kess - I effect due to distance factor. – user1825567 Mar 10 '16 at 7:51
• Oh. I am new here. Done! – Reeshabh Ranjan Mar 10 '16 at 9:01

According to source the pKa values (of the conjugate acid) are:

compound            °C    pKa
Aniline             25    4.87
2-Methoxyaniline    25    4.53
3-Methoxyaniline    25    4.20
4-Methoxyaniline    25    5.36