Difference Between Ortho and Para Nitrophenol: Structure, Properties & Reactivity

Nitrophenols are significant aromatic chemicals extensively applied across the pharmaceutical, chemical, and dye sectors. Of all the isomers, two of the most influential isomers—ortho nitrophenol and para nitrophenol—excite interest for their unique physical, chemical, and structural attributes. In this blog, Kajay Remedies delves into the variations between the two compounds in detail, based on their structures, properties, and reactivity patterns.

Introduction to Ortho and Para Nitrophenol

Ortho nitrophenol (otherwise known as o-nitrophenol or 2-nitrophenol) and para nitrophenol (also known as p-nitrophenol or 4-nitrophenol) are positional isomers of nitrophenol. Both have a benzene ring with a hydroxyl group (-OH) and a nitro group (-NO₂) but at different positions. The two groups appear in consecutive positions (1,2-position) in o-nitrophenol while they occur opposite each other on the benzene ring (1,4-position) in p-nitrophenol.

Structure and Molecular Formulas

The molecular formula for both para and ortho nitrophenol is the same, C₆H₅NO₃. But their structural formulas are not identical because of the positions of the substituents:

Ortho Nitrophenol Structure (2-nitrophenol): 

The nitro group and hydroxyl group are at neighboring positions on the aromatic ring.

Para Nitrophenol Structure (4-nitrophenol): 

Hydroxyl and nitro groups are at opposite positions on the benzene ring.

This difference in structure influences their intermolecular interactions, hydrogen bonding, and eventually their physical and chemical characteristics.

Physical Properties

Ortho and para nitrophenols exhibit different physical characteristics due to the differences in structure.

Ortho nitrophenol is a yellow crystalline solid with a melting point of about 45°C. Ortho nitrophenol has a distinct odor and is steam-volatile. The cis location of the hydroxyl and nitro groups causes intramolecular hydrogen bonding. These lower the strength of intermolecular hydrogen bonding, with the consequence that there is a depressed melting point and lowered water solubility.

For nitrophenol, on the other hand, the melting point is higher at around 114°C and is present as a yellow crystalline powder. It creates intermolecular hydrogen bonding because the substituents are arranged opposite to each other, thus being more polar and more water-soluble than in the case of ortho nitrophenol.

Acidity and pKa Values

The acidity of nitrophenols is directed by the electron-withdrawing effect of the nitro group. Both p-nitrophenol and o-nitrophenol are acidic compared to phenol, but p-nitrophenol tends to be more acidic because the phenoxide ion is stabilized better with resonance and inductive effects.

pKa of o-nitrophenol: ~7.2

pKa of p-nitrophenol: ~7.1

Although the difference is slight, p-nitrophenol is a slightly stronger acid and hence more ionized in aqueous solution.

Solubility

Ortho nitrophenol's intramolecular hydrogen bonding decreases its capacity to hydrogen bond with water molecules, thus is less soluble in water. Para nitrophenol, which has strong intermolecular hydrogen bonding, is more hydrophilic and thus more soluble in water.

Reactivity and Chemical Behavior

Reactivity is another area where o-nitrophenol and p-nitrophenol differ significantly. Ortho nitrophenol tends to be more reactive in electrophilic substitution reactions due to the electron density distribution created by the nitro and hydroxyl groups being adjacent.

Para nitrophenol, on the contrary, is more stable and is also prone to react at a slower rate. Its resonance structure helps in delocalization of the negative charge, stabilizing the molecule and affecting its reactivity profile.

Applications

2-nitrophenol and 4-nitrophenol both have wide industrial uses:

Ortho nitrophenol (2-nitrophenol): 

It is utilized in the preparation of dyes, agrochemicals, and pharmaceuticals. It is an intermediate used in organic synthesis.

Para nitrophenol (4-nitrophenol): 

Widely employed as a precursor for the synthesis of paracetamol (acetaminophen) and also employed in the production of pesticides and other organic chemicals.

Since they can take part in numerous chemical reactions, they are useful for chemical research and industrial processes.

Environmental and Safety Considerations

Both o-nitrophenol and p-nitrophenol are hazardous materials. They can be exposed through inhalation, ingestion, or skin contact and cause numerous health problems. Care should be taken when handling these chemicals, storing them, and disposing of them.

From the environmental perspective, nitrophenols are harmful to aquatic organisms and must be handled as pollutants if discharged in their natural form into the environment. Kajay Remedies follows stringent safety and environmental standards to ensure proper use of these chemicals.

It is important for chemists and chemical synthesis, pharmaceutical industry professionals to understand differences between o-nitrophenol and p-nitrophenol because the two compounds, despite being structurally similar, exist very differently because of the position of their functional groups.

Here at Kajay Remedies, we deal with the manufacturing and supply of top-grade nitrophenol derivatives with an uncompromising quality control process. Whether you need 2-nitrophenol or 4-nitrophenol for your intended purpose, you can rely on us as we are committed to providing you with the best.