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Predicting Acidity

-The smaller the pKa, the stronger the acid

IMPORTANT: The more stable its conjugate base, the stronger the acid

The Strength of an acid depends on 5 factors

1. Size

The bigger the atom bonded to the hydrogen, the more acidic the acid.

Example: HI is a stronger acid than HF. The conjugate base of HI is I- , which is more stable than F- because the charge is spread out (it is more polarizable).

Note: size "overrides" electronegativity

2.Electronegativity

When the atom bonded to the hydrogen increases in electronegativity (down a row), the strength of the acid increases.

Example: CH₃OH is more acidic than NH₃ because Oxygen is more electronegative than Nitrogen, and they are in the same row.

3. Hybridization

Hybridization affects electronegativity of an atom.

Electonegativity of the atom attached to hydrogen:

 (most "s" character)   sp > sp2 >  sp3  (least "s" character)

Example: HCCH is more acidic than H2C=CH2, which is more acidic than CH3CH3.

4. Inductive Effect

Induction or electron withdrawal refers to the shifting of electrons due to the presence of electronegative atoms nearby, causing a decrease in charge density. The more electronegative the atom and the closer it is to the acidic H, the stronger the acid.

Example: ClCH₂COOH is a stronger base than CH₃COOH. The conjugate bases of both compounds has resonance and delocalization of electrons. The main difference here is the presence of the Cl which "pulls" some of the electron density away from the negative charge.

5. Electron Delocalization/Resonance

Delocalization of electrons decreases charge density, increasing stability. An acid with a conjugate base that has delocalized electrons due to resonance is more acidic than an acid with a conjugate base with localized electrons.

Example: CH₃COOH is a stronger acid than CH₃OH

Stronger acids have lower pKa's An acid with a pKa less than the pH of the solvent it is in, will be in it's conjugate base form

Strong acids may have a combination of these factors (for example, both inductive withdrawal and resonance)

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