Separation of Binary Organic Mixture by Differential Extraction

To determine which binary mixture(s) can be separated into individual compounds by differential extraction using both aqueous $\text{NaOH}$ and aqueous $\text{NaHCO}_3$ as shown in the given scheme, we evaluate the acidic properties and chemical reactivities of the components in each mixture.

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1. Principles of Differential Extraction

When an organic binary mixture containing two non-polar/organic-soluble compounds is treated with an aqueous basic solution:

1. If a compound is sufficiently acidic, it reacts with the base to form a water-soluble ionic salt: $\text{Acidic Compound} + \text{Base}(aq) \longrightarrow \text{Ionic Salt}(aq) \quad (\text{Aqueous Layer})$
2. If a compound is neutral or exceedingly weakly acidic, it does not react with the base and remains in the organic solvent: $\text{Neutral/Weakly Acidic Compound} \longrightarrow \text{Unreacted} \quad (\text{Organic Layer})$

For a mixture to be successfully separated into Compound 1 and Compound 2 by both $\text{NaOH}(aq)$ and $\text{NaHCO}_3(aq)$ individually (as shown in the scheme), one compound must react with both reagents while the other compound must react with neither.

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2. Acidity and Reactivity Comparison

• Carboxylic Acids ($\text{R-COOH}$), such as Benzoic acid ($\text{C}_6\text{H}_5\text{COOH}$) and Phenylacetic acid ($\text{C}_6\text{H}_5\text{CH}_2\text{COOH}$):

  • $pK_a \approx 4 - 5$
  • React with both strong bases like $\text{NaOH}$ and weak bases like $\text{NaHCO}_3$: $\text{R-COOH} + \text{NaOH}(aq) \longrightarrow \text{R-COO}^-\text{Na}^+(aq) + \text{H}_2\text{O}$ $\text{R-COOH} + \text{NaHCO}_3(aq) \longrightarrow \text{R-COO}^-\text{Na}^+(aq) + \text{H}_2\text{O} + \text{CO}_2\uparrow$

• Alcohols ($\text{R-OH}$), such as Benzyl alcohol ($\text{C}_6\text{H}_5\text{CH}_2\text{OH}$):

  • $pK_a \approx 15$
  • Do not react with either $\text{NaOH}(aq)$ or $\text{NaHCO}_3(aq)$ due to extremely low acidity.

• Phenols ($\text{Ar-OH}$), such as Phenol ($\text{C}_6\text{H}_5\text{OH}$):

  • $pK_a \approx 10$
  • React with $\text{NaOH}(aq)$ to form sodium phenoxide: $\text{C}_6\text{H}_5\text{OH} + \text{NaOH}(aq) \longrightarrow \text{C}_6\text{H}_5\text{O}^-\text{Na}^+(aq) + \text{H}_2\text{O}$
  • Do not react with $\text{NaHCO}_3(aq)$ because carbonic acid ($\text{H}_2\text{CO}_3$, $pK_a \approx 6.37$) is a stronger acid than phenol.

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3. Evaluation of Binary Mixtures

• Option (A): $\mathrm{C_6H_5OH}$ (Phenol) and $\mathrm{C_6H_5COOH}$ (Benzoic acid)

  • Treatment with $\text{NaOH}(aq)$: Both phenol and benzoic acid react and dissolve in the aqueous phase as sodium salts. They cannot be separated.
  • $\mathbf{Incorrect}$

• Option (B): $\mathrm{C_6H_5COOH}$ (Benzoic acid) and $\mathrm{C_6H_5CH_2OH}$ (Benzyl alcohol)

  • Treatment with $\text{NaOH}(aq)$: Benzoic acid reacts to form soluble $\text{C}_6\text{H}_5\text{COO}^-\text{Na}^+$ (aqueous layer), while benzyl alcohol remains unreacted (organic layer). Separated successfully.
  • Treatment with $\text{NaHCO}_3(aq)$: Benzoic acid reacts to form soluble $\text{C}_6\text{H}_5\text{COO}^-\text{Na}^+$ (aqueous layer), while benzyl alcohol remains unreacted (organic layer). Separated successfully.
  • $\mathbf{Correct}$

• Option (C): $\mathrm{C_6H_5CH_2OH}$ (Benzyl alcohol) and $\mathrm{C_6H_5OH}$ (Phenol)

  • Treatment with $\text{NaHCO}_3(aq)$: Neither compound reacts with $\text{NaHCO}_3$; both remain together in the organic layer. They cannot be separated using $\text{NaHCO}_3(aq)$.
  • $\mathbf{Incorrect}$

• Option (D): $\mathrm{C_6H_5CH_2OH}$ (Benzyl alcohol) and $\mathrm{C_6H_5CH_2COOH}$ (Phenylacetic acid)

  • Treatment with $\text{NaOH}(aq)$: Phenylacetic acid reacts to form soluble $\text{C}_6\text{H}_5\text{CH}_2\text{COO}^-\text{Na}^+$ (aqueous layer), while benzyl alcohol remains unreacted (organic layer). Separated successfully.
  • Treatment with $\text{NaHCO}_3(aq)$: Phenylacetic acid reacts to form soluble $\text{C}_6\text{H}_5\text{CH}_2\text{COO}^-\text{Na}^+$ (aqueous layer), while benzyl alcohol remains unreacted (organic layer). Separated successfully.
  • $\mathbf{Correct}$

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Conclusion

The mixtures that satisfy the differential extraction scheme are those in (B) and (D).