Which one of the following compounds will readily react with dilute NaOH?

Step 1: Key principle — acidity determines reaction with NaOH

For a compound to react with $\ce{NaOH}$ (a base), it must be acidic enough to donate a proton to $\ce{OH^-}$.

$\ce{ROH + NaOH -> RO^-Na^+ + H2O}$ (acid-base reaction)

This only occurs if $\text{pK}_a$(ROH) < $\text{pK}_a$(H2O) ≈ 15.7

Step 2: Compare pKa values

| Compound | pKa | React with NaOH? | |---|---|---| | Benzyl alcohol ($\ce{C6H5CH2OH}$) | ~15.4 | borderline (barely, not readily) | | Ethanol | ~15.9 | No (weaker acid than water) | | tert-Butanol | ~18 | No (very weak acid) | | Phenol | ~10.0 | YES (much more acidic than water) |

Step 3: Why phenol reacts readily

Phenol (pKa ≈ 10) is significantly more acidic than water (pKa 15.7). The phenoxide ion ($\ce{C6H5O^-}$) is stabilized by resonance with the aromatic ring. Therefore, phenol readily donates its proton to $\ce{OH^-}$:

$\ce{C6H5OH + NaOH -> C6H5O^-Na^+ + H2O}$

Aliphatic alcohols (pKa ~ 16-18) are weaker acids than water, so they do NOT react with dilute NaOH.

Answer: Phenol ✓

Key Points to Remember:

• Phenol reacts with NaOH; aliphatic alcohols do NOT.
• Phenol (pKa 10) is more acidic than water (pKa 15.7).
• This is a key chemical distinction between phenols and alcohols.
• Phenol dissolves in NaOH solution; aliphatic alcohols do not.