CIE A Level Chemistry复习笔记3.4.4 Alcohol Dissociation

• 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

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

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