Question

Ortho-nitrophenol L and para-nitrophenol L \(\mathrm{N}_{2}\) have different acidities, melting points, and boiling points. In all cases, the ortho- nitrophenol has lower values in these areas. Why?

Short answer

Ortho-nitrophenol has lower acidity, melting point, and boiling point values compared to para-nitrophenol due to structural differences that affect intra- and inter-molecular forces. The nitro group in ortho-nitrophenol is closer to the hydroxyl group, allowing for better stabilization of negative charge and greater acidity. Additionally, ortho-nitrophenol exhibits intramolecular hydrogen bonding within the molecule, while para-nitrophenol forms intermolecular hydrogen bonds with other molecules. Since intermolecular forces are generally stronger than intramolecular forces, para-nitrophenol exhibits higher boiling and melting points.

Step-by-step solution

Understand the structure of ortho-nitrophenol and para-nitrophenol

Both ortho-nitrophenol and para-nitrophenol consist of a phenol group (a benzene ring with an attached hydroxyl group (OH)) and a nitro group (NO2). In ortho-nitrophenol, the nitro group is adjacent to the hydroxyl group, whereas in para-nitrophenol, the nitro group is opposite (at the other end of the benzene ring) the hydroxyl group.

Evaluate the acidity of these compounds

Acidity is determined by the stability of the anion formed after donating a proton. In both isomers, after losing an H+ ion from the hydroxyl group, the negative charge is delocalized into the benzene ring and the nitro group. However, in ortho-nitrophenol, the nitro group is closer to the negative charge and can better stabilize that charge through resonance. This makes ortho-nitrophenol more stable and consequently more acidic than para-nitrophenol.

Understand the impact of intramolecular hydrogen bonding on boiling and melting points

Boiling and melting points depend on the strength of the intermolecular forces. Stronger intermolecular forces lead to higher boiling and melting points. In the case of these isomers, the type of hydrogen bonding plays a significant role. In the case of ortho-nitrophenol, intramolecular hydrogen bonding happens within the molecule between the nitro group and the hydroxyl group due to their proximity. On the other hand, para-nitrophenol, because of the distance between the nitro and hydroxyl groups, can form intermolecular hydrogen bonding with other molecules.

Discuss the effect of intermolecular forces on boiling and melting points

Intermolecular forces are generally stronger than intramolecular forces. Therefore, compounds with intermolecular hydrogen bonding, like para-nitrophenol, will have higher boiling and melting points. The intramolecular hydrogen bonding in ortho-nitrophenol can't prevent molecules from separating as effectively as the intermolecular hydrogen bonding in para-nitrophenol. This is why ortho-nitrophenol has lower boiling and melting points. So, in conclusion, the structural differences in ortho- and para-nitrophenol lead to differences in their acidity, boiling points, and melting points.

Key concepts

Intramolecular Hydrogen Bonding

Intramolecular hydrogen bonding occurs within a single molecule and plays a significant role in determining the physical and chemical properties of a compound. In ortho-nitrophenol, the hydroxyl (OH) group and the nitro (NO2) group are positioned next to each other on the benzene ring. This proximity allows them to form a hydrogen bond within the molecule.
This type of hydrogen bonding can stabilize the conformation of the molecule but does not effectively hold the molecules together. Consequently, this results in lower boiling and melting points for ortho-nitrophenol compared to its para counterpart.
Since the intramolecular hydrogen bond is an internal interaction, it doesn't contribute to the forces that need to be overcome to separate the molecules, such as during a phase change from solid to liquid or liquid to gas. Therefore, it significantly affects the compound's boiling and melting points.

Intermolecular Hydrogen Bonding

Intermolecular hydrogen bonding occurs between separate molecules and has a profound impact on a substance's boiling and melting points. In para-nitrophenol, the hydroxyl group and the nitro group are positioned at opposite ends of the benzene ring.
This arrangement prevents intramolecular hydrogen bonding but facilitates strong intermolecular hydrogen bonds with neighboring molecules.
These intermolecular forces require more energy to overcome during phase changes, leading to higher boiling and melting points compared to ortho-nitrophenol.
Thus, while intramolecular bonds stabilize the individual molecules internally, intermolecular hydrogen bonding plays a crucial role in dictating the physical state changes by binding the molecules together more tightly in the liquid and solid phases.

Resonance Effect

The resonance effect is a critical concept in understanding the acidity of aromatic compounds like nitrophenols. When ortho- or para-nitrophenol loses a proton (H+), it forms a phenoxide ion.
This ion experiences resonance, where the negative charge is shared and delocalized over the oxygen and the aromatic ring, increasing stability.
In ortho-nitrophenol, the resonance stability is enhanced due to the close proximity of the nitro and hydroxyl groups. The nitro group's electronegative nature allows it to stabilize the negative charge more effectively, increasing the overall acidity.
This is why ortho-nitrophenol is generally more acidic compared to para-nitrophenol, where the charge stabilization through resonance is not as pronounced due to the larger distance between the groups.

Boiling and Melting Points

Boiling and melting points are physical properties influenced by the types of molecular interactions present in a compound. In the case of ortho-nitrophenol and para-nitrophenol, these properties differ primarily because of the distinct types of hydrogen bonding.
Ortho-nitrophenol predominantly engages in intramolecular hydrogen bonding, affecting its ability to engage in strong intermolecular interactions. This results in relatively low boiling and melting points, as less energy is needed to change its phase.
Conversely, para-nitrophenol, with its strong intermolecular hydrogen bonding, effectively holds the molecules together, thus requiring more energy to break these interactions during melting or boiling. As a result, its boiling and melting points are higher than those of ortho-nitrophenol, reflecting the strength and importance of intermolecular forces.

Nitrophenol Isomers

Nitrophenol isomers, such as ortho-nitrophenol and para-nitrophenol, differ in the positions of their functional groups on the benzene ring, significantly affecting their properties. In ortho-nitrophenol, the nitro and hydroxyl groups are adjacent, allowing for intramolecular hydrogen bonding.
This internal bonding influences its lower boiling and melting points, due to reduced intermolecular bonding force, and higher acidity, due to enhanced resonance effect.
On the other hand, para-nitrophenol possesses functional groups on opposite ends, preventing intramolecular but allowing for strong intermolecular hydrogen bonds that result in higher boiling and melting points.
Understanding these structural differences in nitrophenol isomers provides insight into the intricate balance of intermolecular forces and electronic effects that govern the physical and chemical behaviors of aromatic compounds.