Acidity of Alcohols (Cambridge (CIE) A Level Chemistry): Revision Note

Acidity of Alcohols

• Alcohols have a low degree of dissociation

  • This means, that when dissolved in water, alcohol molecules do not dissociate (split up) to a great extent

ROH (aq)                   RO^- (aq) + H⁺ (aq)

^{alcohol   alkoxide ion}

• The position of the equilibrium lies to the left, meaning that there are far more alcohol molecules than RO^- and H⁺ ions

• When water dissociates, the position of the equilibrium still lies to the left, but there are more H⁺ ions compared to the dissociation of alcohols

H₂O (l) OH^– (aq) + H⁺ (aq)

• As alcohols have a lower [H⁺ (aq)] in solution compared to water, alcohols are weaker acids than water

The inductive effect in alcohols

• Electron-donating species such as alkyl groups push electrons into a covalent bond and are said to have a positive inductive effect

• In alcohols, the oxygen atom in the alkoxide ion is bonded to an electron-donating alkyl group

• This means that there is more electron density on the O^- atom

• The alkoxide ion is, therefore, more likely to accept an H⁺ ion and form the alcohol again

The inductive effect in alcohols

• When water dissociates, the hydroxide ion only has one other hydrogen atom

• There is no extra electron density on the oxygen which is less likely to accept an H⁺ ion

• Water is therefore a stronger acid than alcohols

The equilibrium of water and hydroxide ions