HardChemistryHigh School (11–12)Organic Chemistry: Some Basic Principles and Techniques › Electronic Effects and Reaction IntermediatesConceptualA-LevelIBJEE Main
Phenol, C₆H₅OH, is a much stronger acid than ethanol, CH₃CH₂OH…
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Phenol, , is a much stronger acid than ethanol, . Use the ions that each one gives to explain why.
Need a reminder? The key ideaShow
The inductive effect is a permanent pull () or push () of electrons along sigma bonds. It fades with distance. Resonance spreads pi electrons and lone pairs over several atoms. The hybrid is more stable than any one structure, and its equal bonds share the bond order. The electromeric effect is a temporary shift of a pi pair under attack. Hyperconjugation spreads the sigma electrons of an alpha C-H bond, so more alpha hydrogens mean more stability.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The ions. Ethanol gives the ethoxide ion, . Phenol gives the phenoxide ion, .
Step 2. Ethoxide. The charge stays on oxygen. The ethyl group has a effect, which pushes more electrons onto that oxygen. That makes the ion less stable.
Step 3. Phenoxide. The lone pair on oxygen can spread into the ring by resonance. The negative charge is shared with carbon atoms of the ring. A spread-out charge is more stable.
Answer: the phenoxide ion is stabilised by resonance and the ethoxide ion is destabilised by the effect. A more stable ion means a stronger acid, so phenol is the stronger acid.
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