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Assessing Acidity Qualitatively - Factor 3: Induction - Exercise 18

Exercise 3.18 - Assessing Acidity Qualitatively - Factor 3: Induction

Exercise 3.18 - Assessing Acidity Qualitatively - Factor 3: Induction

Introduction

This video introduces the third factor in assessing acidity qualitatively: induction. This follows the previous discussions on the atom and resonance factors. Induction helps determine which hydrogens in a molecule are the most acidic based on the presence and proximity of electronegative atoms.

Main Points

- After considering the atom and resonance factors, induction is used to further assess acidity.

- The hydrogen with the most electronegative atoms nearby is the most acidic.

- Electronegative atoms pull electron density toward themselves, stabilizing the negative charge left behind when the hydrogen ion (H⁺) leaves.

- Electronegativity increases as you move up and to the right on the periodic table.

- Atoms like nitrogen, oxygen, fluorine, and chlorine are highly electronegative.

- These atoms can pull electron density through sigma bonds, even if they are not directly bonded to the hydrogen in question.

- Example A:

    - Ethanol with a hydrogen on oxygen (alcohol).
    - Adding fluorine atoms increases the acidity by stabilizing the negative charge through induction.
    - Fluorines close to the acidic hydrogen have a stronger inductive effect.

- Example B:

    - Compare molecules with electronegative atoms at different distances.
    - Hydrogens closer to electronegative atoms are more acidic due to stronger inductive effects.
Examples/Evidence

- Molecule 1:

    - Hydrogens on oxygen versus hydrogens on carbon.
    - Hydrogens on oxygen are more acidic because oxygen is more electronegative than carbon.
    - No resonance difference between hydrogens on different oxygens.
    - Induction: Hydrogens closer to three fluorines are more acidic.

- Molecule 2:

    - Hydrogens on different oxygens.
    - Both hydrogens leave behind resonance-stabilized structures.
    - Induction: Hydrogen closer to two chlorines is more acidic due to stronger electron withdrawal.
Conclusion

Induction is a key factor in determining the acidity of hydrogens in a molecule. By assessing the presence and proximity of electronegative atoms, one can predict which hydrogens are more acidic. This qualitative approach is essential when numerical data like pKa values are unavailable.

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