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Q7.8:
While separating a mixture of ortho and para nitrophenols by steam distillation, name the isomer which will be steam volatile. Give reason.

Solution :

1. Identification of the Steam Volatile Isomer

In a mixture of $o$-nitrophenol and $p$-nitrophenol, $o$-nitrophenol (ortho-nitrophenol) is the steam volatile isomer.

2. Theoretical Basis: Intramolecular vs. Intermolecular Bonding

The difference in volatility is dictated by the nature of hydrogen bonding present in the respective molecules:

  • $o$-Nitrophenol: Exhibits intramolecular hydrogen bonding. The hydrogen atom of the hydroxyl ($-OH$) group forms a coordinate bond with the oxygen atom of the adjacent nitro ($-NO_2$) group within the same molecule. This chelation effect restricts the hydroxyl group from participating in intermolecular associations, effectively reducing the molecular surface area and intermolecular attraction forces. [Per the principles of Molecular Polarity and Hydrogen Bonding].
  • $p$-Nitrophenol: Exhibits intermolecular hydrogen bonding. Due to the para-positioning, the $-OH$ group and the $-NO_2$ group are spatially separated, forcing the hydroxyl group of one molecule to bond with the nitro group of a neighboring molecule. This results in the formation of large, associated molecular clusters or "chains." [Reference: Principles of Intermolecular Forces and Dipole-Dipole Interaction].

3. Visual Representation of Hydrogen Bonding

o-Nitrophenol (Intramolecular) OH NOâ‚‚ H-Bond p-Nitrophenol (Intermolecular) OH NOâ‚‚ H-Bond

4. Comparative Analysis of Physical Properties

Property o-Nitrophenol p-Nitrophenol
Type of H-Bonding Intramolecular Intermolecular
Molecular Association Monomeric (Isolated) Associated (Polymeric)
Vapor Pressure Higher Lower
Volatility High (Steam Volatile) Low (Non-volatile)

5. Conclusion

Because $o$-nitrophenol is monomeric, it possesses a lower boiling point and higher vapor pressure than its para-counterpart. Consequently, it vaporizes readily when steam is passed through the mixture, allowing for efficient separation through steam distillation. Conversely, $p$-nitrophenol remains in the aqueous phase due to its higher boiling point caused by extended intermolecular association.

Final Solution: The steam volatile isomer is $o$-nitrophenol because it forms intramolecular hydrogen bonds, whereas $p$-nitrophenol forms intermolecular hydrogen bonds, leading to higher association and lower volatility.


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