Question

Alcohols and Phenols are acidic in nature. Electron withdrawing groups in phenol increase its acidic strength and electron releasing groups decrease it. Alcohols undergo nucleophilic substitution with hydrogen halides to yield alkyl halides. Dehydration of alcohols gives alkenes. On oxidation, primary alcohols yield aldehydes with mild oxidising agents and carboxylic acids with strong oxidising agents, while secondary alcohols yield ketones. Tertiary alcohols are resistant to oxidation. The presence of - OH group in phenols activates the aromatic ring towards electrophilic substitution and directs the incoming group to ortho and para positions due to resonance effect.

Answer the following questions :

Why is ortho-nitrophenol more acidic than ortho-methoxyphenol ?

Hint:

The nitro group withdraws electrons and stabilises the phenoxide ion, but the methoxy group donates electrons and destabilises it.

Answer 1

Step 1: Understanding the Concept:
A phenol is acidic because it loses \(\mathrm{H^+}\) and forms a phenoxide ion. The more stable the phenoxide ion, the stronger the acid.
The passage tells us that electron withdrawing groups raise acidity and electron releasing groups lower it.

Step 2: The nitro group in o-nitrophenol:
The \(\mathrm{-NO_2}\) group withdraws electrons by the -I effect and by the -R (resonance) effect.
The negative charge on the phenoxide oxygen spreads to the nitro group through resonance.
The spread of charge makes the o-nitrophenoxide ion more stable, so the phenol loses \(\mathrm{H^+}\) easily.

Step 3: The methoxy group in o-methoxyphenol:
The \(\mathrm{-OCH_3}\) group releases electrons into the ring by the +R effect.
This increases the negative charge density on the phenoxide oxygen and makes the ion less stable.
So loss of \(\mathrm{H^+}\) is harder.

Final Answer:
The electron withdrawing \(\mathrm{-NO_2}\) group stabilises the phenoxide ion, while the electron releasing \(\mathrm{-OCH_3}\) group destabilises it, so o-nitrophenol is the stronger acid. \[ \boxed{\text{o-nitrophenol}\ (pK_a \approx 7.2) > \text{o-methoxyphenol}\ (pK_a \approx 10)} \]