The absorbance spectrum of phenol typically features two characteristic absorption bands within the ultraviolet (UV) region, generally falling between 200 and 360 nanometers (nm). These bands are crucial for identifying and quantifying phenol using UV-Vis spectroscopy.
Key Absorption Bands
Phenols, including phenol itself (the simplest member of the class), exhibit two primary absorption bands:
- B-band (Benzene Band or E₂ Band): This band occurs at a shorter wavelength and is typically more intense. For phenol, the B-band is observed around 210 nm. It corresponds to the electronic transitions within the benzene ring system, specifically the E₂ band, and has a high molar absorptivity.
- C-band (Conjugated or K-band): Found at a longer wavelength, this band is generally less intense than the B-band but is often more sensitive to substituent effects and solvent changes. For phenol, the C-band (also known as the B-band in some older literature or the benzene B band in general context) is typically found around 270-275 nm. It arises from the π→π* electronic transitions of the aromatic ring, influenced by the hydroxyl group.
Absorbance Characteristics of Phenol
The "exact" absorbance spectrum of phenol, while generally consistent, can show minor variations depending on factors like the solvent used, pH, and concentration. However, the characteristic wavelengths remain largely stable.
Here's a summary of the typical absorbance characteristics for phenol in an aqueous or alcoholic solution:
| Band Name | Typical Wavelength (λmax) | Molar Absorptivity (ε) (L mol⁻¹ cm⁻¹) | Description |
|---|---|---|---|
| B-band | ~210 nm | ~6,200 | High intensity, shorter wavelength |
| C-band | ~270-275 nm | ~1,450 | Lower intensity, longer wavelength |
Note: Molar absorptivity (ε) indicates how strongly a substance absorbs light at a given wavelength and is a characteristic constant for a compound under specific conditions.
Practical Implications and Influencing Factors
Understanding the absorbance spectrum of phenol is vital for various applications, particularly in analytical chemistry.
- Quantitative Analysis: The C-band around 270-275 nm is frequently used for the quantitative determination of phenol due to its presence in a less cluttered spectral region compared to the B-band, which might overlap with other common organic chromophores absorbing at lower wavelengths.
- Solvent Effects: The polarity of the solvent can influence the exact peak positions. For instance, a more polar solvent might cause a slight shift (either hypsochromic or bathochromic) in the absorption maxima.
- pH Effects: The hydroxyl group in phenol is acidic (pKa ≈ 10).
- Below pKa (acidic/neutral conditions): Phenol exists predominantly in its neutral, undissociated form, exhibiting the λmax values described above.
- Above pKa (basic conditions): Phenol deprotonates to form the phenoxide ion. This deprotonation significantly alters the electronic structure, leading to a bathochromic shift (shift to longer wavelength) and often an increase in intensity for both bands, particularly the C-band. The C-band of phenoxide might shift to approximately 290 nm. This pH-dependent shift is a powerful tool for distinguishing phenol from other aromatic compounds.
Examples of Phenol Absorbance Data
While specific values can vary, numerous studies and databases confirm the presence of these two bands. For instance, in an aqueous solution at neutral pH, the primary peak for quantification is often measured at or around 270 nm. The high intensity of the 210 nm band also makes it useful, though it requires specialized instrumentation capable of accurate measurements in the deep UV region and careful consideration of interferences.
For further details on molecular spectroscopy and its applications, you can explore resources like the UV-Vis Spectroscopy principles.
