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

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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