Nitration of Aniline in Conc. H2SO4 and HNO3, we can understand the formation of both para and meta nitro aniline. But what is the reason for the poor yield of ortho nitro aniline?
Under concentrated acidic medium meta is the main product (because of formation of Ar-NH3-plus , strongly meta directing). From my view ortho is less since the attaching of nitronium ion to that position may be more difficult because of steric effect from three hydrogen and less electron density(or more positive nature of ortho carbon).
I don't have any experimental proof or reference . Waiting for some scholar's answer on this interesting question.
Since aniline contains the amino group (-NH2), then it will be activating & ortho- , para-directing. The nitronium ion will favorably do the substitution on the para position due to steric factors that occur when it approaches the ortho position. An acid-base reaction is possible converting the amine to ammonium but it is insignificant. The main path will be: 1) Reaction between sulfuric & nitric acids to form nitronium ion "electrophile" & bisulfate ion. 2) Electrophilic aromatic substitution between aniline & the electrophile. 3) Deprotonation of the para hydrogen by HSO4- to re-form the sulfuric acid catalyst.
Basically because of steric issues, the amino group is orto- and para-directing, so the nitro group will be added mainly in the para position.
Thanks to all of you for the response, but can i get some more convincing reason based on this steric hindrance effect by looking at the geometry of the molecule ?
Though Amino group is ortho/para directing, in acidic conditions it forms ammonium ion (Ar-NH3+), which is strongly meta directing..Thus results in a significant meta product (about 47%). However, the formation of para-product (51%) is still not consistent with this explanation. I guess here only inductive effect and the steric effects play the role.
At ortho- owing to the strong electron withdrawing effect, the o-product is only 2%.
However, in para and meta position comparably inductive effect is less so both these products are major..
I believe it's due to a combination of steric effects (-NH3⁺ is isoelectronic with -CH3), and electrostatic repulsion between the nitronium and ammonium species. Meta is less favored than para, for the same reason, but there are two meta carbons.
Aniline has a substituent group that is activating and electron-donating, giving ortho- and para-nitrated aniline. Except when the reaction happens in an acidic environment. In an acidic environment, the amino group in aniline gets protonated and the substituent now is deactivating and electron-withdrawing, giving meta-nitrated aniline instead, and hence the low yield of ortho- and i suppose also low yield of meta-nitrated aniline.
- Badges
- Science topic
- Similar topics
- Chemistry
- Organic Chemistry