Question 1

What makes phenol acidic?

Accepted Answer

Phenol contains a hydroxyl group directly attached to an aromatic ring. The hydrogen of this –OH group can be removed by a base, forming a phenoxide ion. The phenoxide ion is resonance-stabilised because its negative charge is spread over oxygen and ring positions.

Question 2

What proton is removed when phenol is ionised?

Accepted Answer

The proton removed is the hydrogen attached to the phenolic –OH group. A base abstracts this proton as H+; the process is not removal of a hydride ion.

Question 3

Why is the phenoxide ion resonance-stabilised?

Accepted Answer

The oxygen of the phenoxide ion can share its electron pair with the aromatic ring. The commonly drawn resonance contributors place the negative charge on oxygen, the two ortho carbon positions, and the para carbon position, giving five contributors in total.

Question 4

Why is phenol more acidic than an alcohol?

Accepted Answer

The phenoxide ion formed from phenol is stabilised by resonance with the aromatic ring. The corresponding alkoxide from an alcohol does not receive the same ring resonance stabilisation, so phenol is comparatively more acidic.

Question 5

Are the resonance contributors separate phenol molecules?

Accepted Answer

No. The resonance contributors are different drawings used to represent one phenoxide ion. Curved arrows indicate electron-pair delocalisation; they do not show separate molecules or literal movement of atoms between structures.

Phenol Acidity: Resonance on the Ring Sets the pKa

About this lab

Practice