# Why does p-nitrophenol give sodium bicarbonate test, but o-nitrophenol does not?

Both ortho- and para-nitrophenol are less acidic than carbonic acid. But still paranitrophenol reacts with sodium bicarbonate to give carbonic acid and sodium paranitrophenoxide.

However, this is not true for ortho-nitrophenol. Any explanations?

• Hint: consider H-bonding in o-nitrophenol Jun 20 '20 at 17:31
• Thanks for your reply. All it shows is that orthonitrophenol is less acidic but my point is that why does paranitrophenol even react with sodium bicarbonate despite being less acidic than carbonic acid. Try checking the reaction and pka values to satisfy yourself Jun 20 '20 at 19:56

p-Nitrophenol does have a higher $$\mathrm{p}K_\mathrm{a}$$ than carbonic acid (7.15 vs 6.3), the two logarithms differ by less than one unit, if we render the acidic species in carbonic acid as $$\ce{CO2}$$ rather than $$\ce{H2CO3}$$. So the equilibrium constant in the reaction
$$\ce{p-C6H4(NO2)OH + HCO3- <=> p-C6H4(NO2)O- + CO2 + H2O}$$
is close to $$1$$ and a large percentage of the p-nitrophenol could react with bicarbonate.
o-Nitrophenol actually has $$\mathrm{p}K_\mathrm{a} = 7.23,$$ about the same as the para isomer. It should, in fact, react similarly with sodium bicarbonate. It is the meta isomer that has higher $$\mathrm{p}K_\mathrm{a}$$ and may not visibly react.