Aromatic Chemistry (4) Phenol

Aromatic Chemistry (4) Phenol

Specification extracts:

OCR

Phenols

6.1.1.(h)  

the weak acidity of phenols shown by the neutralisation reaction with NaOH but absence of reaction with carbonates 

Aromatic Chemistry (4) Phenol as a weak acid

In this fourth post on aromatic chemistry, I’m looking at the acidic properties of phenol.

I’m going to discuss the fact that phenol is a weak acid and illustrate this looking at its reaction with sodium hydroxide solution and the reaction of carboxylic acids with carbonates.

Phenol as a weak acid

Phenol is an aromatic alcohol but it behaves very differently from aliphatic alcohols like ethanol.

The structure of phenol (C₆H₅OH) means that the OH group attached to the benzene ring can add extra electrons to the ring π system and as result lose the proton from the OH group.  (see below)

The resultant anion (C₆H₅O^–) is stabilised as the negative charge on the oxygen can be distributed around the ring π system. 

The image below shows how the phenoxide anion is stabilised.

As a result phenol is a weak acid.  It has a K_a of 1.6 × 10^—10. and a pK_a of 9.8

As a weak acid it displays some of the conventional properties of acids.  So it will neutralise sodium hydroxide solution forming sodium phenoxide. (see below)

However, if we compare the pKa of phenol (9.8) with those of carboxylic acids such as ethanoic acid (4.76) and with carbonic acid (dissolved CO₂ solution) (pK_a2 10.32) we note that the CO₂ solution has a final second pK_a greater than that of phenol in aqueous solution so addition of phenol to a carbonate does NOT result in the effervescence of CO₂ gas.

However, we can also see that the pK_a of ethanoic acid is less than that of phenol AND dissolved carbon dioxide solution.  Thus addition of carbonates to ethanoic acid and other carboxylic acids will result in the effervescence of CO₂ gas.

Thus we have a simple test to distinguish a carboxyl group from a phenolic group.  Add a few drops of sodium carbonate solution and the carboxyl group will cause the solution to fizz!!.

This is a useful test since many carboxylic acids have a pKa less than that of the carbonate ion as this table of selected acids shows: