Question

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

Short answer

Ortho nitrophenol is steam volatile due to intramolecular hydrogen bonding, making it easier to vaporize.

Step-by-step solution

Understand the Compounds Involved

Ortho and para nitrophenols are structural isomers that differ in the position of their nitro group relative to the hydroxyl group on a benzene ring. Ortho nitrophenol has the nitro group adjacent to the hydroxyl group, while in para nitrophenol, the nitro group is opposite to the hydroxyl group on the benzene ring.

Analyze Hydrogen Bonding

The volatility of a compound in steam distillation is influenced by its ability to form intermolecular hydrogen bonds. In ortho nitrophenol, there is intramolecular hydrogen bonding (bonding within the molecule) between the hydroxyl group and the nitro group. This type of bonding reduces the intermolecular hydrogen bonding with water.

Consider Effects on Volatility

Para nitrophenol, on the other hand, primarily forms intermolecular hydrogen bonds with other molecules or solvent, which enhances its interaction with water and decreases its volatility compared to ortho nitrophenol. The lack of substantial intermolecular interactions in ortho nitrophenol makes it more volatile.

Conclusion Based on Volatility

Ortho nitrophenol is steam volatile because it has stronger intramolecular hydrogen bonding, thereby reducing its intermolecular attractions and making it easier to vaporize during steam distillation.

Key concepts

Ortho Nitrophenol

Ortho nitrophenol is a chemical compound that belongs to the family of nitrophenols, which are derivatives of phenol. In this case, the nitro group \(-NO_2\) is positioned adjacent to the hydroxyl group \(-OH\) on the benzene ring, making it an ortho isomer.

One significant feature of ortho nitrophenol is its ability to form intramolecular hydrogen bonds. This occurs when the hydrogen atom of the hydroxyl group bonds with the oxygen of the nitro group that is located right next to it. This seems inside the molecule, thereby significantly influencing its physical properties.

Due to intramolecular hydrogen bonding, ortho nitrophenol tends to have decreased intermolecular hydrogen bonding opportunities with water molecules. This reduction in molecular interaction allows ortho nitrophenol to be more readily vaporized, making it steam volatile, which is an essential trait when performing separation using steam distillation.

Para Nitrophenol

Para nitrophenol is another member of the nitrophenol family, differentiated by the unique positioning of its functional groups on the benzene ring. In para nitrophenol, the nitro group \(-NO_2\) is placed opposite the hydroxyl group \(-OH\), which results in different interaction dynamics compared to ortho nitrophenol.

Unlike ortho nitrophenol, para nitrophenol does not form significant intramolecular hydrogen bonds. Instead, it engages in intermolecular hydrogen bonds with other para nitrophenol molecules and with surrounding solvent molecules like water.

This capability of forming tight intermolecular associations restricts its tendency to evaporate easily, thus making para nitrophenol less volatile compared to its ortho counterpart. This characteristic of high intermolecular bonding is why para nitrophenol is less suitable for steam distillation.

Intramolecular Hydrogen Bonding

Intramolecular hydrogen bonding occurs within a single molecule, binding atoms tighter and affecting the molecule's overall characteristics.

In the case of ortho nitrophenol, this type of bonding takes place between the hydroxyl group \(-OH\) and the nitro group \(-NO_2\), which are positioned next to each other on the benzene ring. This molecular arrangement encourages a loop-like bonding internally.

Through intramolecular hydrogen bonding, a molecule reduces its potential to interact externally with other molecules — particularly through intermolecular interactions like hydrogen bonding with water. This internal bonding configuration results in easier vaporization, which impacts both volatility and its ability to undergo steam distillation efficiently.

Intermolecular Forces

Intermolecular forces are interactions that occur between molecules, dictating many properties such as boiling and melting points, solubility, and volatility. These forces include hydrogen bonds, dipole-dipole interactions, and van der Waals forces.

In para nitrophenol, intermolecular hydrogen bonds are dominant due to the positioning of the hydroxyl and nitro groups. This spatial arrangement allows the molecules to create stronger connections with each other or with solvent molecules.

These repeated intermolecular interactions result in increased boiling points and reduced volatility, making para nitrophenol less likely to evaporate under steam distillation. Understanding intermolecular forces helps to explain why ortho nitrophenol, with its lessened intermolecular hydrogen bonding, is more steam volatile.