IIT JEE , AIEEE Forum - Chemistry , Organic Chemistry

un.niranjan

Please :-
Give the order of the base strength of :-

aniline, o-, m-, & p- alkoxy aniline.

A detailed explanation too is requested!

un.niranjan

plz rate me!

DON007

hey.........

to determine

basic strength: ..........we have to see.........the stability of its conjugate............

add

H+ to aniline it becomes NH3+ ............................and check its

stability OR.........IS A +R effectiing group ............therefore it will produce negative charge..............and stabilize it...............same with ortho................

but for meta charge - will not come...............

therefore basic strength goes:

para > ortho > aniline > meta............

hope i m clear...............

thanku

un.niranjan

para > ortho > aniline > meta............
Here, shouldn't meta-aniline and aniline be interchanged because alkoxy is an overall electron releasing group, isn't it?

madhusudan.chavan

9330

1.Electron donating substituents (EDG) tend to increase and electron withdrawing substituents (EWG) tend to decrease the basicity of aniline & aryl amines.

2.The base weakening effect of EWG & base strengthening effect of EDG is more marked at para position than at meta position.

3.o-substituted anilines are usually weaker bases than aniline regardless of the nature of the group (EDG or EWG).

Groups like ?OCH₃ , -OH , etc., exert both ?I & +R effect. When such a group is present at p-position, +R effect outweighs ?I effect and thus the overall effect is electron-donating. As a result, p-methoxy aniline is a stronger base than aniline.

However, when ?OCH₃ group is present at meta position it can exert only ?I effect, since resonance effect does not occur at meta position. This decreases electron density on nitrogen atom and hence m-OCH₃ aniline is weaker base than aniline.

Thus the order of basicity is

p-OCH₃ (pkb 8.71) > Aniline (pkb 9.38 ) > o- OCH₃ (pkb 9.51) > m- OCH₃ ( pkb 9.80)

unnandu

Thanks a lot for a clear explanation Mr.Madhusudan Chavan

Ask & Discuss Questions with Community & Experts