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Why does p-nitrophenol give sodium bicarbonate test, but o-nitrophenol does not?
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Orest Reminsky
Why does p-nitrophenol give sodium bicarbonate test, but o-nitrophenol does not?
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.
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.
Thanks Oscar, I too had the same views but the textbook i referred says that ortho nitrophenol does not give sodium bicarbonate test. Should I consider that wrong?More
Likely the book missed. Based on duissociation constants the meta isomer is the one that drags its feet. The proton is indeed hydrogeb-bonded in the ortho isomer, but the closer proximity of the nitro group to the hydroxyl group in that isomer.More
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.
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.
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