Does resonance always affect acidity and basicity?

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SUMMARY

Resonance significantly influences the acidity of acids and the basicity of their conjugate bases, even when the resonance does not directly delocalize the charge on the atom attached to the proton. In the case of benzoic acid, which has a pKa of 4.2, resonance from the benzene ring contributes to the stability of the conjugate base, enhancing its acidity compared to cyclohexanecarboxylic acid, which has a pKa of 4.9. The resonance effect from the benzene ring, while minor, plays a role in the overall acidity and basicity of these compounds.

PREREQUISITES
  • Understanding of acid-base chemistry and pKa values
  • Familiarity with resonance structures and their implications
  • Knowledge of functional groups, specifically carboxylic acids
  • Basic grasp of molecular orbital theory
NEXT STEPS
  • Study the impact of resonance on acidity using various carboxylic acids
  • Explore the concept of resonance hybrid and its contribution to molecular stability
  • Investigate the relationship between pKa values and molecular structure
  • Learn about the effects of substituents on the acidity of aromatic compounds
USEFUL FOR

Chemistry students, organic chemists, and anyone interested in understanding the relationship between molecular structure and acidity/basicity in organic compounds.

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Does resonance affect the acidity of an acid (or the basicity of its conjugate base) if the resonance isn't delocalizing the charge on the atom that is directly attached to the atom with the proton, hydrogen itself.

For example a benzene ring with a carboxylic acid group attached to one side. The alcohol on the side is what would be deprotonated. So there's one resonance structure where the negative charge on the conjugate base's oxygen is delocalized by resonance onto the carbonyl. Does the benzene ring's resonance also help to delocalize this charge (or does the benzene ring's resonance not affect the basicity of the this conjugate base)?
 
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Apparently, the answer to your example is yes because benzoic acid has a pKa of 4.2 versus cyclohexanecarboxylic acid, which has a pKa of 4.9. The effect is fairly minor, which would be expected as resonance structures where the negative charge gets delocalized to the benzene ring would be expected to be fairly minor contributors to the resonance hybrid.
 
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