6.1.1 (h) The acidity of Phenols

(h) the weak acidity of Phenols shown by the neutralisation reaction with NaOH but absence of reaction with Carbonates

We expect alcohols to be soluble due to their OH group and the Hydrogen bonds it can make with water.

But this is only true with relatively short alcohols and the solubility tails off quickly after butanol due to the presence of an ever-lengthening, non-polar, hydrophobic alkyl group.

Phenols have a large, non-polar and therefore hydrophobic benzene ring - so they are not very soluble.

However, the ability to dissociate in water to form ions allows phenols to be at least somewhat soluble.

A phenoxide ion forms and an H⁺ ion (or, more accurately, an H₃O⁺ ion).

Being able to produce H⁺ ions in water makes Phenols acidic.

But only weakly so since they do not fully dissociate.

And this is a quick test to distinguish between an alcohol, a phenol and a carboxylic acid.

Alcohols really don't dissociate at all, they produce neutral solutions and so don't react with NaOH or Na₂CO₃.

Phenols dissociate very poorly, they produce weakly acidic solutions that react with NaOH (strong base) but are not strong enough to react with a weak base like Na₂CO₃ (so no bubbles would be seen).

Carboxyllic acids also dissociate poorly but better than phenols, they produce less weakly acidic solutions that react with both strong bases and weak bases like Na₂CO₃ (so bubbles would be seen)

pKa is a method of measuring strength of acids. The lower the pKa the more strongly acidic.

This follows the general equation Acid + Alkali --> Salt + Water that is so beloved by Year 9 students

You'll see this on old exam papers but it seems to have dropped off the syllabus.

However, it is just Acid + Metal --> Salt + Hydrogen which Year 9 are even keener on.