Acidity and Basicity in Benzene based compounds

Whilst trawling through your notes you may have noticed a similarity between the explanation why phenol is a little bit acidic and why phenylamine is a little bit basic.

The explanation is effectively the same but the effect it almost the opposite.

Let's first compare the acidity and basicity of their alkyl chained compatriots

• Butanol is neutral, phenol is slightly acidic.

• Butylamine is basic, phenylamine is much less basic

So the presence of a benzene ring as opposed to an alkyl chain makes...

• a hydroxyl functional more acidic 

• an amine functional group less basic 

Before we go any further, remind yourself as to the definitions of an acid and a base.

• Acids are proton donors, so anything that makes a molecule more likely to give away a proton makes it more acidic. 

• Bases are proton acceptors, and to do this they need a lone pair, so anything that makes a molecule less likely to accept a proton would make it less basic.

So. let's answer the questions

• Why is phenol acidic when butanol isn't?

          and

• Why is phenylamine less basic than butylamine?

The answer to both questions comes down to the lone pairs.

If an atom with a lone pair on it is attached to a benzene ring then this lone pair becomes part of benzene's delocalised system. This delocalisation means the negative charge doesn't just sit there on the atom, it moves around the molecule. This fact can be used to explain both the questions above.

So, to explain phenol's slight acidity (compared to butanol's neutrality)...

Phenol is acidic because the phenoxide ion formed is stabilised to some extent. The negative charge on the oxygen atom is delocalised around the ring. The more stable the ion is, the more likely it is to form. So Phenol is acidic because the ring draws the lone pair away from the ion

and, to explain phenylamine's reduced alkalinity (compared to butylamine's alkalinity)...

Phenylamine is not that basic because the lone pair is no longer fully available to join to hydrogen ions. Nitrogen is still the most electronegative atom in the molecule so the lone pairs of electrons will still be attracted towards it, but the intensity of charge around the nitrogen is nothing like what it is in butylamine. So phenylamine is a not so good base because the ring draws the lone pair away from the nitrogen.

Notice how the two diagrams are almost identical

Make sense? 

Two effects, one reason.