In some substitution reactions of aromatic compounds , Ortho and para derivatives are obtained as products. Which factor determines the occurrence of major and minor products ?

Eg. Phenol + Dil. HNO3 > o-nitro phenol +o-nitro phenol

In which Ortho is major product...

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.