Topc 4: Acidity and Charge Stabilisation Flashcards
HA + H2O —>
A- + H3O+
Ka =
[A-][H3O+]/[HA] = k[H2O]
Brønsted acid is
A proton donor
pKa =
-log10(Ka)
Do strong and week acids have high or low positive or negative Ka and pKa values
Strong acid
High positive Ka
High negative pKa
Weak acid
Low positive Ka
High positive pKa
Henderson-Hasselbalch equation
pH = pKa + log[[A-]/[HA]]
[A-]/[HA] = 10^[pH-pKa]
How does low charge density affect stability
Increases
How does high electronegativity effect stability
Increases
How does increased solvation effect stability
Increases
Why is trifluoromethanesulfonic acid so much more acidic than methanesulfonic acid
Talk about stability
Equilibrium lies further to right to stabilise conjugate base
Fluoro substituents have strong stabilising I- effect that strongly withdraw electron density from the negatively charged sulfonate group
Electron withdrawing effect distributes charge over larger area of molecule
Reduces charge density on trifilate anion increasing stability
Electron withdrawing substituents exert an …effect so they … negative charge and … positive charge
-I inductive
Stabilise
Destabilse
Electron donating substituents exert a … effect so they … negative charge and … positive charge
+I inductive
Destabilise
Stabilise
The pi resonance stabilisation (or mesomeric stabilisation). … on … atoms … . … is analogous to how … form … . Ability of aligned … to … means … . … of charge reduces its …, adding … to molecule.
p AOs
adjacent
overlap
Orbital overlap
2 p AOs
pi bond
p AOs
overlap
charge distrusted over greater area
Delocalisation
density
stability
Increased resonance forms often means
Greater stabilisation due to distribution of charge
Reduced charge density
Are resonance effects or inductive more important in atom stability
Usually resonance
