Answer to Question #96011 in Organic Chemistry for Krina

Question

Why are the pKa of all three nitro-phenols smaller than that of a phenol?
Explain why the pKa of m-nitrophenol is smaller than that of the other two nitrophenol?
Phenol= pKa 10
o-nitrophenol= pKa 7.2
p-nitrophenol= pKa 7.2
m-nitrophenol= pKa 4

Answer 1

1. Because nitro group is an acceptor substituent, and any acceptor substituents in the ring making substance more acidic.

2. ЯNegative mesomeric effect, which -NO₂ grouphas, is the strongest in meta-, because of the delocalisation of electrons in the ring and stabilization the anion in some of the tautometic form.