Question

Why phenol show red shift in alkaline medium

Answer 1

Answer:

P-nitrophenol shows a red shift in an alkaline medium because of a phenomenon called Bathochromic shift.

Bathochromic shift refers to the change in the spectral band position of p-nitrophenol with respect to its reflectance, absorption, transmittance, or the emission spectra of the molecule to longer wavelengths of low frequency.

Red color is present in the visible spectrum and has a longer wavelength compared to other colors, this phenomenon is called the red shift.

In an alkaline medium, p-nitrophenol shows red shift since the negatively charged oxygen molecules delocalise effectively compared to the unshared electron pair.

Answer 2

Phenol shows red shift in alkaline medium because the negative charge on the oxygen atom is delocalised effectively as compared to the lone pairs which results in a lower energy gap and higher wavelength.

Explanation:

• Red shift or Bathochromic shift is observed in UV spectroscopy when the absorbance results in a longer wavelength.
• Thus, the red shift is associated with a shift to a longer wavelength.
• When phenol is present in an alkaline medium, deprotonation of phenol occurs resulting in phenoxide ion in which a negative charge is present on the oxygen atom.
• This negative charge is delocalised over the aromatic ring more effectively than the lone pairs in the neutral molecule.
• Due to more effective delocalisation, the energy gap between HOMO and LUMO of the phenoxide ion decreases.
• As energy and wavelength are inversely proportional to each other, therefore, with the decrease in energy gap, the wavelength of absorption increases.
• Thus, it results in a shift to a longer wavelength and red shift is observed.

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