DAT Destroyer Gen Chem 204

[Demo0710]

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Consider the following extraction:

phenyl with COOH attached
phenyl with CH3 attached
phenyl with NH2 attached
phenyl with OH attached

If NaHCO3 was added, which compound would be in the aqueous layer?

The answer is sodium benzoate since the phenyl with COOH donates the H to the NaHCO3.

Can someone help me understand the logic behind this?

Thanks!

 

[lutopie0730]

Consider the following extraction:

phenyl with COOH attached
phenyl with CH3 attached
phenyl with NH2 attached
phenyl with OH attached

If NaHCO3 was added, which compound would be in the aqueous layer?

The answer is sodium benzoate since the phenyl with COOH donates the H to the NaHCO3.

Can someone help me understand the logic behind this?

Thanks!

1)

PhCOOH + HCO3- -> PhCOO- + H2CO3 (further rxn forming CO2 and H2O)
pKa PhCOOH ~4 < pKa H2CO3 ~6

However,

2)

PhOH + HCO3- -> PhO- + H2CO3
pKa PhOH ~10 > pKa H2CO3 ~6

Bicarbonate would deprotonate benzoic acid but not phenol.
therefore, benzoate ions would be in the aqueous layer, but phenol would stay in the organic layer.


P.S.
chronological extraction steps.

1) benzoic acid + phenol + aniline + toluene in organic layer
add H2CO3
2) phenol + aniline + toluene in organic layer / benzoate in aq layer
add NaOH
3) aniline + toluene in organic layer / phenol in aq layer
add acid (it would protonate aniline)
4) toluene in organic layer / aniline in aq layer

 

[Demo0710]

The reason bicarbonate can deprotonate the benzoic acid is because its pKa is less than that of the carbonate?

 

[lutopie0730]

yes. if you want to estimate the equilibrium quickly, just compare pKa of acid from the reactants and another acid from the products.
the equilibrium would favor the acid with higher pKa (less acidic and less reactive)