CIE AS Chemistry (9701) exams from 2022

Revision Notes

3.4.4 Alcohol Dissociation

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

 

Hydroxy Compounds Positive Inductive Effect in Alcohols, downloadable AS & A Level Chemistry revision notes

Alkyl groups in the alkoxide ion donate electron density to the negatively charged oxygen, causing it to more readily accept a proton and form the alcohol again

  • 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

 

Hydroxy Compounds Acid Behaviour of Water, downloadable AS & A Level Chemistry revision notes

Water is a stronger acid than alcohols as there are no electron-donating groups in the hydroxide ion, causing the O to be less likely to accept a proton and reform water

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