Question

Mark the correct order of decreasing acidic strength of the following compounds:

• A. V > IV > II > I > III
• B. II > IV > III > I > V
• C. IV > V > III > II > I
• D. I > IV > III > II > I

Answer 1

The correct option is B II > IV > III > I > V

Phenols are acidic in nature due to resonance. The presence of electron withdrawing groups (such as: $-N{O}_2,-X,-N{R}_3^{\oplus },-CXO,-COX,-COOR,-CN)$ on the ring stabilises the phenoxide ion formed and increases the acidic nature of phenols. On the other hand, presence of an electron donating group in the ring destabilises the phenoxide ion and decreases the acidic nature of phenols.

Furthermore, meta-isomer of nitrophenol is less acidic than p- and o- isomer because the phenoxide ion is stabilised by inductive effect only. Similarly, meta-isomer of methoxyphenol is destabilised to lesser extent than the para-isomer but more stable than the phenoxide ion because of the inductive effect of oxygen atom.
The correct order of acidic strength is:
II > IV > III > I > V