Question

The considerably greater acid strength of $PhOH(p{K}_a=10)$ than that of $ROH(p{K}_a=18)$ is due to the fact that:

• A. $PhO$ (phenoxide ion) is a stronger base than $R{O}^{-}$ (alkoxide ion)
• B. $PhO$ (phenoxide ion) is a weaker base than $R{O}^{-}$ (alkoxide ion)
• C. $ROH$ is soluble in water
• D. $PhOH$ is aromatic in nature

Answer 1

Answer: (B)

$PhOH\leftrightarrow \underset{\text{phenoxide ion}}{Ph{O}^{-}}+H^{+}$
$R-OH\leftrightarrow \underset{\text{alkoxide ion}}{R{O}^{-}+H^{+}}$
Phenoxide ion is stabilized by resonance. Hence, it is a weak base while alkoxide ion is destabilised by $+I$ effect of alkyl group. Hence, it is stronger base than phenoxide ion. Further strongest the acid, weakest its conjugate base. As phenol is stronger acid than alcohol. Hence, its conjugate base, (i.e., phenoxide ion) is weaker than conjugate base (i.e., alkoxide) of alcohol.