acid strength handout
TRANSCRIPT
![Page 1: Acid Strength Handout](https://reader036.vdocuments.site/reader036/viewer/2022071602/613d74f3736caf36b75d8d64/html5/thumbnails/1.jpg)
Predicting Acid Strength
UCI Chem 51ADr. Link
![Page 2: Acid Strength Handout](https://reader036.vdocuments.site/reader036/viewer/2022071602/613d74f3736caf36b75d8d64/html5/thumbnails/2.jpg)
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
![Page 3: Acid Strength Handout](https://reader036.vdocuments.site/reader036/viewer/2022071602/613d74f3736caf36b75d8d64/html5/thumbnails/3.jpg)
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?
![Page 4: Acid Strength Handout](https://reader036.vdocuments.site/reader036/viewer/2022071602/613d74f3736caf36b75d8d64/html5/thumbnails/4.jpg)
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!
![Page 5: Acid Strength Handout](https://reader036.vdocuments.site/reader036/viewer/2022071602/613d74f3736caf36b75d8d64/html5/thumbnails/5.jpg)
Factors Affecting pKa
Element Effect
Inductive Effect
Resonance Effect
Hybridization Effect
![Page 6: Acid Strength Handout](https://reader036.vdocuments.site/reader036/viewer/2022071602/613d74f3736caf36b75d8d64/html5/thumbnails/6.jpg)
Quick Question!Choose the compound with the lowest pKa.
H2O CH4
HF NH3
![Page 7: Acid Strength Handout](https://reader036.vdocuments.site/reader036/viewer/2022071602/613d74f3736caf36b75d8d64/html5/thumbnails/7.jpg)
Element Effect: RowAcross a row, acidity of H-A increases.
Electronegativity
C ON F
![Page 8: Acid Strength Handout](https://reader036.vdocuments.site/reader036/viewer/2022071602/613d74f3736caf36b75d8d64/html5/thumbnails/8.jpg)
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
![Page 9: Acid Strength Handout](https://reader036.vdocuments.site/reader036/viewer/2022071602/613d74f3736caf36b75d8d64/html5/thumbnails/9.jpg)
Another Quick Question!Choose the most acidic compound.
CH3CH2OH ClCH2CH2OH
Cl2CHCH2OH F2CHCH2OH
![Page 10: Acid Strength Handout](https://reader036.vdocuments.site/reader036/viewer/2022071602/613d74f3736caf36b75d8d64/html5/thumbnails/10.jpg)
Inductive EffectInductive Effect: Electronegative atoms stabilize the conjugate base by pulling electron density toward themselves.
OF
![Page 11: Acid Strength Handout](https://reader036.vdocuments.site/reader036/viewer/2022071602/613d74f3736caf36b75d8d64/html5/thumbnails/11.jpg)
Inductive Effect: Anions
![Page 12: Acid Strength Handout](https://reader036.vdocuments.site/reader036/viewer/2022071602/613d74f3736caf36b75d8d64/html5/thumbnails/12.jpg)
Inductive Effect DetailsMore electron-withdrawing groups = greater inductive effect
Stronger electron-withdrawing groups = greater inductive effect
Inductive effects decrease dramatically with distance.
![Page 13: Acid Strength Handout](https://reader036.vdocuments.site/reader036/viewer/2022071602/613d74f3736caf36b75d8d64/html5/thumbnails/13.jpg)
A Familiar QuestionChoose the compound with the highest pKa.
OH
+ NaOHOH
+ NaOH
![Page 14: Acid Strength Handout](https://reader036.vdocuments.site/reader036/viewer/2022071602/613d74f3736caf36b75d8d64/html5/thumbnails/14.jpg)
Resonance EffectResonance Effect: Conjugate base is stabilized by resonance (delocalization of charge)
![Page 15: Acid Strength Handout](https://reader036.vdocuments.site/reader036/viewer/2022071602/613d74f3736caf36b75d8d64/html5/thumbnails/15.jpg)
One More Question!Which contains the most acidic proton?
CH3CH3$H
HH
H
HH
Hybridization matters!
![Page 16: Acid Strength Handout](https://reader036.vdocuments.site/reader036/viewer/2022071602/613d74f3736caf36b75d8d64/html5/thumbnails/16.jpg)
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.
![Page 17: Acid Strength Handout](https://reader036.vdocuments.site/reader036/viewer/2022071602/613d74f3736caf36b75d8d64/html5/thumbnails/17.jpg)
Priority of Effects
1.
2.
3.
![Page 18: Acid Strength Handout](https://reader036.vdocuments.site/reader036/viewer/2022071602/613d74f3736caf36b75d8d64/html5/thumbnails/18.jpg)
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
![Page 19: Acid Strength Handout](https://reader036.vdocuments.site/reader036/viewer/2022071602/613d74f3736caf36b75d8d64/html5/thumbnails/19.jpg)
Basicity ExamplesChoose the stronger base:
OH- NH2- CH3
-
CH3CH2NH2 CCl3CH2NH2 CF3CH2NH2
![Page 20: Acid Strength Handout](https://reader036.vdocuments.site/reader036/viewer/2022071602/613d74f3736caf36b75d8d64/html5/thumbnails/20.jpg)
Wrapping Up
Practice predicting relative acidities or basicities based on structure.
Provide rationale for your choices!