0
$\begingroup$

made using https://web.chemdoodle.com/demos/sketcher/

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?

$\endgroup$
2
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ 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? $\endgroup$ – Reeshabh Ranjan Mar 10 '16 at 6:31
  • $\begingroup$ 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. $\endgroup$ – user1825567 Mar 10 '16 at 7:51
  • $\begingroup$ Oh. I am new here. Done! $\endgroup$ – Reeshabh Ranjan Mar 10 '16 at 9:01
0
$\begingroup$

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
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.