Chem 203 pKas

pKa of HCl

-8

pKa of H2SO4 (both protons)

-3, 1.99

pKa of H3O+

-1.7

H2O

15.7

NH3

35

NH4+

9.2

pKa of a carboxylic acid

4.76

pKa of alcohol/hydroxyl

16

C-H pka in alkane

51

Vinyllic C-H pka in alkene

44

C-H pka in alkyne

24

Ketone H pKa

19

Allyic alkene C-H pKa

43

More s character means…

more electronegative carbon → better stabilization of negative charge (more stable base)→ more acidic H

Resonance vs Equilibrium

Resonance: same molecule, different valid Lewis structures.

Equilibrium: two or more different chemical species interconvert. (different atoms)

Types of isomers

• Constitutional isomers — same molecular formula, different connectivity.

  • Example: ethanol vs dimethyl ether

• Conformational isomers — same connectivity, different arrangement caused by rotation around single bonds.

  • Example: staggered vs eclipsed ethane

• Configurational isomers — same connectivity, but you must break bonds to convert between them.

  • Includes enantiomers and diastereomers such as cis/trans or E/Z.

What determines structural stability (ARIO)

ARIO determines stability of a negative charge:

• A – Atom: more electronegative/larger atom = more stable

• R – Resonance: more delocalization = more stable

• I – Induction: nearby electron-withdrawing (electronegative) groups = more stable

• O – Orbital: more s-character = more stable (sp > sp² > sp³)

More stable conjugate means…

lower pKa, stronger acid.

Why does more S character mean more stable?

More s-character = more stable because an s orbital holds electrons closer to the nucleus than a p orbital does.

electrons closer to nucleus → stronger attraction to positive nucleus → lower energy → more stable

Why does resonance make an structure more stable

For resonance, stability comes from spreading the charge out over multiple atoms.

localized negative charge → less stable
delocalized negative charge by resonance → more stable

Why does an electron-withdrawing group stabilize a negative charge? (inductive effects)

It pulls electron density away through σ bonds, reducing electron–electron repulsion and stabilizing the charge.