p-Nitroaniline is an organic compound widely used in dyes, pigments, and pharmaceutical intermediates. Understanding its molecular weight is essential for handling, formulation, and regulatory purposes.
This article explains the molecular weight of p-nitroaniline together with key physical properties, calculation methods, and practical implications for industry and laboratory work.
| Property | Symbol or Unit | Value for p-Nitroaniline | Notes |
|---|---|---|---|
| Molecular Formula | C6H6N2O2 | - | 6 carbon, 6 hydrogen, 2 nitrogen, 2 oxygen atoms |
| Molecular Weight | g·mol-1 | 138.12 | Sum of atomic masses based on IUPAC standard atomic weights |
| Exact Mass | Da | 138.0429 | Calculated using the most abundant isotopes |
| Appearance | - | Affected by purity and particle size | |
| Melting Point | °C | 147–150 | Useful indicator for identity and purity assessment |
Calculating the Molecular Weight of p-Nitroaniline
The molecular weight of a compound is the sum of the atomic weights of all atoms in its molecular formula. For p-nitroaniline, the formula is C6H6N2O2, which indicates six carbon atoms, six hydrogen atoms, two nitrogen atoms, and two oxygen atoms per molecule.
Using average atomic masses from the periodic table—carbon 12.01, hydrogen 1.008, nitrogen 14.01, and oxygen 16.00—the calculation proceeds stepwise. Multiply each atomic mass by the number of atoms of that element, then add the products together.
This calculation yields a molecular weight of approximately 138.12 g·mol-1, which is the standard value cited in chemical catalogs, safety data sheets, and analytical methods.
Importance of Accurate Molecular Weight in Analytical Chemistry
Accurate molecular weight is critical when converting between moles and grams in synthesis, quality control, or regulatory testing. Errors in molecular weight propagate directly into concentration calculations and can affect reaction stoichiometry.
In chromatography and spectroscopy, molecular weight helps interpret retention times, spectral peaks, and calibration curves. Using certified reference values ensures comparability across laboratories and compliance with pharmacopeial standards.
For p-nitroaniline, certified reference materials and authoritative databases list 138.12 g·mol-1 as the accepted molecular weight, supporting consistent method validation and reporting.
Practical Applications and Handling Considerations
p-Nitroaniline is employed as an intermediate in the synthesis of pharmaceuticals, agrochemicals, and colorants. Knowing its molecular weight is essential for scaling up reactions, designing dosing procedures, and assessing environmental fate.
In formulation work, accurate dosing depends on correct molar conversions, particularly when p-nitroaniline is used in trace quantities alongside other reagents. Small deviations in molecular weight can lead to significant errors in high-precision applications.
Storage conditions, purity grades, and supplier specifications should always be reviewed to ensure that the molecular weight used matches the material actually in use, accounting for potential impurities or isotopic variations.
Comparison with Related Aromatic Amines
Comparing p-nitroaniline with similar compounds clarifies how molecular structure influences molecular weight and physical behavior. Substituents on the aromatic ring change both mass and properties such as solubility and melting point.
| Compound | Molecular Formula | Molecular Weight (g·mol-1) | Key Property |
|---|---|---|---|
| Aniline | C6H7N | 93.13 | Lower mass, higher solubility |
| p-Nitroaniline | C6H6N2O2 | 138.12 | Higher mass due to nitro group |
| 4-Nitroaniline | C6H6N2O2 | 138.12 | Isomer with same mass |
| Nitroaniline mixture | - | ~138 (average) | Varies with isomer ratio |
Key Takeaways and Recommendations for Working with p-Nitroaniline
- Use the standard molecular weight of 138.12 g·mol-1 for calculations and documentation.
- Verify purity and certificate of analysis before critical applications.
- Cross-check conversions between mass and moles using authoritative reference data.
- Consider isotopic substitution effects when designing labeled studies or mass spectrometry experiments.
- Document source and version of molecular weight values to ensure reproducibility across projects.
FAQ
Reader questions
How do I verify the molecular weight of p-nitroaniline in the lab?
Compare your experimentally determined mass spectrum or high-resolution measurement against the reference value of 138.12 g·mol-1, ensuring instrument calibration is current.
Can isotopic labeling change the molecular weight of p-nitroaniline?
Yes, isotopic substitution with stable atoms such as 13C or 15N increases the exact mass; always specify isotope enrichment when preparing labeled compounds.
Why does the melting point vary slightly between batches of p-nitroaniline?
Melting point shifts can arise from differences in purity, crystal habit, or residual solvents, even when the molecular weight remains constant at 138.12 g·mol-1.
How does molecular weight affect dosing in pharmaceutical synthesis?
Incorrect molecular weight leads to wrong molar amounts, potentially reducing yield, generating by-products, or compromising drug substance specification and batch consistency.