Question

Ortho nitrophenol is more acidic than phenol. Explain. Draw the resonance structures of their corresponding phenoxide ion. (2)

Hint:

The \( -NO_2 \) group at ortho withdraws electrons and delocalises the phenoxide charge onto its oxygens, stabilising the anion and raising acidity.

Answer 1

Step 1: What controls acidity. The acidity of a phenol depends on the stability of the phenoxide ion left behind after losing \( H^+ \). The more stable (more delocalised) the phenoxide ion, the more easily \( H^+ \) is released and the stronger the acid.
Step 2: Effect of the nitro group. The nitro group \( (-NO_2) \) is a strong electron withdrawing group (it shows both negative inductive, -I, and negative resonance, -M, effects). When placed at the ortho position of phenol, it pulls electron density away and, more importantly, allows the negative charge of the ortho-nitrophenoxide ion to be delocalised onto the oxygen atoms of the \( -NO_2 \) group as well as onto the ring.
Step 3: Comparison. In the plain phenoxide ion the negative charge is spread only over the ring carbons (ortho and para positions). In the ortho-nitrophenoxide ion the charge is spread over the ring and additionally onto the electronegative nitro-oxygen atoms. This extra delocalisation makes ortho-nitrophenoxide much more stable, so ortho-nitrophenol releases \( H^+ \) more readily and is more acidic than phenol.
Step 4: Resonance structures of the phenoxide ion. In the phenoxide ion \( (C_6H_5O^-) \) the negative charge on oxygen is delocalised into the benzene ring. The main resonance (canonical) structures are:
(1) negative charge on the oxygen atom;
(2) negative charge on the ortho carbon;
(3) negative charge on the para carbon;
(4) negative charge on the other ortho carbon.
\[ C_6H_5O^{-} \leftrightarrow \text{(charge at O)} \leftrightarrow \text{(charge at ortho-C)} \leftrightarrow \text{(charge at para-C)} \leftrightarrow \text{(charge at other ortho-C)} \]
For ortho-nitrophenoxide two further structures exist in which the negative charge sits on the oxygen atoms of the \( -NO_2 \) group, and these extra structures are the reason for its greater stability and acidity.
\[\boxed{-NO_2 \text{ (}-I,-M\text{) stabilises the phenoxide by extra delocalisation, so ortho-nitrophenol is more acidic}}\]