Does resonance always affect acidity and basicity?

In summary, resonance can affect the acidity of an acid or the basicity of its conjugate base, even if it is not delocalizing the charge on the atom directly attached to the proton. In an example with a benzene ring and carboxylic acid group, the alcohol on the side can be deprotonated and the resonance structure of the benzene ring can help delocalize the negative charge onto the carbonyl. This is evident in the difference in pKa values between benzoic acid, with a pKa of 4.2, and cyclohexanecarboxylic acid, with a pKa of 4.9. This effect is not significant, as the resonance structures involving
  • #1
ngu9997
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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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  • #2
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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Related to Does resonance always affect acidity and basicity?

1. Does the presence of resonance always affect the acidity and basicity of a compound?

No, the presence of resonance does not always affect the acidity and basicity of a compound. Resonance refers to the delocalization of electrons within a molecule, which can impact the stability and reactivity of the compound. However, the acidity and basicity of a compound also depend on other factors such as the strength of the bond and the electronegativity of the atoms involved.

2. How does resonance affect the acidity and basicity of a compound?

Resonance can affect the acidity and basicity of a compound by stabilizing or destabilizing the resulting ions after proton transfer. For example, if resonance delocalizes the negative charge of an anion, it can make the compound more stable and therefore more acidic. On the other hand, if resonance delocalizes the positive charge of a cation, it can make the compound less stable and therefore less acidic.

3. Are all resonance structures equally important for determining the acidity and basicity of a compound?

No, not all resonance structures are equally important for determining the acidity and basicity of a compound. Some resonance structures may contribute more to the overall stability of the compound and therefore have a greater impact on its acidity and basicity. This is because the stability of a resonance structure depends on factors such as the number of covalent bonds, the electronegativity of the atoms, and the placement of charges.

4. Can resonance ever decrease the acidity or basicity of a compound?

Yes, resonance can sometimes decrease the acidity or basicity of a compound. This can occur when resonance structures with opposing effects cancel each other out, resulting in a less stable overall compound. Additionally, if a resonance structure decreases the electron density around the proton that is being transferred, it can make the compound less acidic.

5. Are there any exceptions to the general rule that resonance affects acidity and basicity?

Yes, there are exceptions to the general rule that resonance affects acidity and basicity. For example, in some cases, the presence of a highly electronegative atom near a proton can decrease the acidity of a compound, even with the presence of resonance. Additionally, the strength of the bond between the proton and the rest of the molecule can also play a role in determining the acidity and basicity, regardless of resonance effects.

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