Effervescence of CO2 with aq NaHCO3

Due to resonance effect(delocalisation of negative charge) phenoxide ion is less basic than alkoxide ion. This means that phenol is more acidic than alcohol.

NaHCO3 is the basic sodium salt of carbonic acid, H2CO3 (pKa1 = 6.3, pKa2 =10.25) a diprotic acid which exists only in small amounts in aqueous solution & CO2+H2OH2COlies mainly on the backward side (this is also the reason why CO2 evolution is observed).

Any acid having pKa less than 6.3 i.e., more acidic than H2CO3,  give effervescence of CO2 with aq. NaHCO3.

Any acid having pKa less than 10.2 are soluble in Na2CO3 & give effervescence of CO2.

Benzoic acid has a lower pKa value of 4.19 and hence Benzoic acid & almost all its derivatives are able to liberate CO2 from aq NaHCO3 or Na2CO3.  C6H5CO2H+NaHCO3C6H5CO2Na+CO2+H2O

Phenol has pKa 10 whereas Ortho-nitrophenol or para-nitrophenol have pKa  nearly  7.2 & m-nitrophenol has pKa =8.4. Hence these are not soluble in NaHCO3. However, these are soluble in Na2CO3.

However, 2,4-Dinitrophenol has a pKa of 4.1 & picric acid (2,4,6-trinitrophenol) has pKa = 0.4 due to strong electron-withdrawing effect of -O2 groups.

3,4-Dihydroxycyclobut-3-ene-1,2-dione, also known as squaric acid is a fairly strong diprotic acid, with pKvalues of 1.5 and 3.4, respectively & hence also give effervescence of CO2 with aq. NaHCO3. The acidity can be attributed to the stabilization of the dianion by resonance.

Hence 2,4-Dinitrophenol, picric acid, squaric acid etc stronger acids does evolve CO2 from aq NaHCO3 or Na2CO3.
Smaller the pKa ,Stronger the acid.

Carboxylic acid            pKa
HCOOH                         3.75
CH3COOH                    4.76
CH2ClCO2H                 2.86   CHCl2CO2H                 1.29    CCl3CO2H                    0.65
CF3CO2H                     0.23
HO2CCO2H                  1.27
C6H5CO2H                   4.2


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