Factors that affect acidity (5) Electron withdrawing groups

Factors that Affect Acidity (5) - Electron Withdrawing Groups

Introduction

This video explains how electron withdrawing groups (EWGs) affect acidity. It focuses on the inductive effects of EWGs and how they stabilize the conjugate base, making the original acid more acidic.

Main Points/Arguments

• Electron Withdrawing Groups and Acidity:

- EWGs, like fluorine, pull electrons away from the atom they are attached to, increasing the effective electronegativity of that atom.

- Increased electronegativity stabilizes the negative charge on the conjugate base, making the original species more acidic.

• Comparison of Methane and Fluorinated Methane:

- Methane (CH4) vs. Fluoromethane (CF3H):

    - In methane, the carbon is bonded to hydrogen, which does not significantly affect electronegativity.
    - In fluoromethane, the carbon is bonded to fluorine, a highly electronegative atom, which pulls electrons away from carbon, stabilizing the negative charge on the conjugate base (CF3-).

• Effect of Multiple EWGs:

- The more EWGs attached to the carbon, the more stable the conjugate base.

- Example: CF3- (most stable) > CF2H- > CFH2- > CH3- (least stable).

• Application to Carboxylic Acids:

- Comparing trifluoroacetic acid (CF3COOH) and acetic acid (CH3COOH):

    - Trifluoroacetic acid has three fluorine atoms pulling electrons away, making its conjugate base (CF3COO-) more stable and the acid stronger.
    - Acetic acid has hydrogen atoms, which do not have the same electron withdrawing effect, making its conjugate base (CH3COO-) less stable and the acid weaker.

Examples or Evidence

• Stability of Conjugate Bases:

- Fluorine atoms stabilize the negative charge by withdrawing electrons, making the conjugate base more stable.

- Trifluoromethane (CF3-) is more stable than methyl (CH3-) due to the inductive effect of fluorine.

• Acidity of Carboxylic Acids:

- Trifluoroacetic acid (CF3COOH) is about 10⁵ times more acidic than acetic acid (CH3COOH) due to the stabilizing effect of the three fluorine atoms.

Conclusion

The presence of electron withdrawing groups significantly increases the acidity of a molecule by stabilizing the conjugate base through inductive effects. This principle applies not only to hydrocarbons but also to other functional groups, such as carboxylic acids.