Predicting Acid Strength

UCI Chem 51ADr. Link

Goals

After this lesson you should be able to:

Predict relative strength of acids or bases using structure

Provide reasoning for acid or base strength predictions

Acid Strength

We use pKa to measure acid strength.

We can correlate acid strength to base strength.

What happens if we don’t have pKa information?

Golden Rule of Acid Strength

Anything that stabilizes the conjugate base increases the strength of the acid!

H A HO

HHO

H

H

A++

Stabilize me!

Factors Affecting pKa

Element Effect

Inductive Effect

Resonance Effect

Hybridization Effect

Quick Question!Choose the compound with the lowest pKa.

H2O CH4

HF NH3

Element Effect: RowAcross a row, acidity of H-A increases.

Electronegativity

C ON F

Element Effect: ColumnAcidity increases as size of conjugate base increases.

H"F,%pKa%=%3.2%%

H"Cl,%pKa%=%"7%

H"Br,%pKa%=%"9%

H"I,%pKa%=%"10%

increasing)acidity)

Spreading charge over larger volume stabilizes conjugate base

Another Quick Question!Choose the most acidic compound.

CH3CH2OH ClCH2CH2OH

Cl2CHCH2OH F2CHCH2OH

Inductive EffectInductive Effect: Electronegative atoms stabilize the conjugate base by pulling electron density toward themselves.

OF

Inductive Effect: Anions

Inductive Effect DetailsMore electron-withdrawing groups = greater inductive effect

Stronger electron-withdrawing groups = greater inductive effect

Inductive effects decrease dramatically with distance.

A Familiar QuestionChoose the compound with the highest pKa.

OH

+ NaOHOH

+ NaOH

Resonance EffectResonance Effect: Conjugate base is stabilized by resonance (delocalization of charge)

One More Question!Which contains the most acidic proton?

CH3CH3$H

HH

H

HH

Hybridization matters!

Hybridization EffectHybridization Effect: Increased percent s-character of orbital containing lone pair in conjugate base increases acidity of corresponding acid.

Why?

S-orbitals closer to nucleus than p-orbitals.

Priority of Effects

1.

2.

3.

Translating Acid and Base Strength

Two ways to think about basicity:

Stronger acid has weaker conjugate base and vice versa

Increased charge density on atom = more likely to share e- = stronger base

F- Br-

NH3

Basicity ExamplesChoose the stronger base:

OH- NH2- CH3

-

CH3CH2NH2 CCl3CH2NH2 CF3CH2NH2

Wrapping Up

Practice predicting relative acidities or basicities based on structure.

Provide rationale for your choices!