While separating a mixture of ortho-and para nitrophenols by steams distillation, name the isomer which will be steam volatile? Give reasons.

When separating a mixture of ortho- and para-nitrophenols using steam distillation, the isomer that will be steam volatile is para-nitrophenol. This distinction arises from the structural differences and intermolecular interactions between the two isomers.

Understanding the Structure and Properties

To grasp why para-nitrophenol is steam volatile, we need to consider the molecular structures of both isomers:

• Ortho-nitrophenol: In this isomer, the nitro group (-NO2) is positioned adjacent to the hydroxyl group (-OH) on the benzene ring. This proximity leads to strong intramolecular hydrogen bonding, which stabilizes the molecule but also makes it less volatile.
• Para-nitrophenol: Here, the nitro group is located opposite the hydroxyl group on the benzene ring. This arrangement allows for weaker intermolecular interactions, making it more volatile and easier to vaporize during steam distillation.

Intermolecular Forces at Play

The key to understanding volatility lies in the types of intermolecular forces present in each compound. Ortho-nitrophenol's intramolecular hydrogen bonding reduces its tendency to escape into the vapor phase, whereas para-nitrophenol primarily experiences weaker van der Waals forces and some hydrogen bonding with neighboring molecules. This difference in bonding results in a lower boiling point for para-nitrophenol, enhancing its steam volatility.

Practical Implications in Steam Distillation

During steam distillation, the volatile component is carried away with the steam, allowing for effective separation. Since para-nitrophenol is more volatile, it will be preferentially distilled over ortho-nitrophenol. This characteristic is particularly useful in laboratory settings where purity and separation of compounds are crucial.

Example of Application

In a practical scenario, if you were to set up a steam distillation apparatus with a mixture of these two isomers, you would observe that the distillate collected would predominantly contain para-nitrophenol. This outcome not only highlights the importance of understanding molecular interactions but also demonstrates the effectiveness of steam distillation as a separation technique.

In summary, the steam volatility of para-nitrophenol compared to ortho-nitrophenol is primarily due to its structural arrangement, which leads to weaker intermolecular forces and a lower boiling point, making it the preferred isomer in steam distillation processes.