On increasing the pH, phenol shows which of the following shift in UV spectra ?
(a) Bathochromic shift
(b) Hypsochromic shift
(c) Blue shift
(d) Red shift

Question

On increasing the pH, phenol shows which of the following shift in UV spectra ? 
(a) Bathochromic shift 
(b) Hypsochromic shift 
(c) Blue shift 
(d) Red shift

Accepted Answer

Phenol is an aromatic compound that can undergo changes in its UV-visible absorption spectrum when the pH of the solution is altered. This is due to the ionization of the phenolic hydroxyl group, which affects the electronic structure of the molecule.

1. **Explanation for Option 3 (a, d):**
   - **Bathochromic shift (Red shift):** When the pH is increased, phenol loses a proton to form the phenoxide ion. This ionization increases the electron density on the aromatic ring, which can lead to a bathochromic shift, also known as a red shift. This shift is characterized by the absorption of light at longer wavelengths. The increased electron delocalization in the phenoxide ion stabilizes the excited state more than the ground state, resulting in a red shift in the UV spectrum.
   - **Red shift:** This term is synonymous with a bathochromic shift. It refers to the same phenomenon where the absorption maximum moves to longer wavelengths (lower energy).

2. **Explanation for why other options are incorrect:**
   - **Option 1 (b, c):** This option suggests a hypsochromic shift (blue shift) and a blue shift. A hypsochromic shift occurs when the absorption maximum moves to shorter wavelengths (higher energy), which is not the case when phenol ionizes to form the phenoxide ion. The increased electron delocalization in the phenoxide ion leads to a bathochromic shift, not a hypsochromic shift.
   - **Option 2 (a, c):** This option includes a bathochromic shift (correct) but also a blue shift, which is incorrect. As explained, the ionization of phenol to phenoxide results in a red shift, not a blue shift.
   - **Option 4 (b, d):** This option suggests a hypsochromic shift (incorrect) and a red shift (correct). The hypsochromic shift is not observed when phenol is ionized to phenoxide; instead, a bathochromic (red) shift occurs.

In conclusion, Option 3 (a, d) is correct because increasing the pH leads to the ionization of phenol, resulting in a bathochromic shift (red shift) in the UV spectrum. The other options are incorrect as they either include a hypsochromic shift or a blue shift, which do not occur under these conditions.