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pKa Values
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pKa value < 0 indicates a strong acid
pKa value > 0 indicates a weak acid
HB  H+ + B(equilibrium)
+
k = [H ][B ]/[HB]
if k > 1 favoured (more of the top present) and pKa is -ve
if k < 1 unfavoured (more of bottom present than the top present) and pKa is +ve
pKa = -log10Ka therefore pKa = 0 when there is a 50/50 equilibrium
Pauling’s rules:
o In oxy acids of formula (HO)xE(O)y, pKa = 8 – 5y
For subsequent deprotonation add 5 to original pKa value
o E
...
H2SO4 = (HO)2SO2 pKa = 8 – (2x5) = -2
This is H2SO4  H+ + HSO4o For HSO4-  H+ + SO42-, pKa = -2 + 5 = +3 (not so easily deprotonated)
o If there are no hydroxyl groups attached: pKa is 8
 Such as Si(OH)4, B(OH)3
o Any variation: could be due to many things
 Such as H2CO3 (carbonic acid): predict +3
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4
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Lewis acids and Lewis bases
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Acid – electron pair acceptor (electrophile)
 H+ (the Bronsted acid)
 M2+ - gaseous state, bind ligands (such as Co2+)
 Molecules with incomplete octets of electrons: such as BMe3, AlCl3 (both
have 6 electrons)
 Molecules with complete octets that can rearrange their electrons, such as
CO2 - oxygens withdraw electrons from carbon, leaving the carbon
electrophilic (an acidic electron acceptor)
 Molecules with complete octets that can accept further electron pairs, such
as SiF4 + 2F-  [SiF6]2- becomes octahedral; uses 3d orbitals to expand from
an octet
Base – electron pair donor (nucleophile)
o Together they can form an adduct: A + B  A-B
 BF3 + NH3  F3B-NH3

o


LUMO-HOMO interaction
 Forms a covalent (dative/coordinate) bond
Displacement reactions can also occur
 A stronger Lewis base can displace a weaker Lewis base from its adduct
 F3B-OEt2 + pyridine  F3B-py + Et2O

o

 Same for a stronger acid displacing a weaker acid
Metathesis reaction (double displacement reaction)
 ‘interchange reaction’ – swap partners
 Et3Si-I + AgF  Et3Si-F +AgI
 A-B
A’-B’ A-B’ A’-B
 Driving forces: very strong Si-F bond vs weak Si-I bond; lattice energy of AgI

Strength of acid-base interactions
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ΔG = -RTlnKf
ΔG = ΔH –TΔS
For a reaction to proceed, ΔG needs to be negative (ΔH needs to be more negative than ΔS)
So reaction has to be exothermic
Four factors affecting magnitude of Kf:
o Strength of A-B bond (to do with H, enthalpy)
 Ka = ability to give away a proton, Kf = ability to form an adduct
 Therefore is there a correlation between 1/Ka and Kf? Sometimes (not all
acids behave like H+ towards Lewis bases)
...
g
...
5(I-A)
o Pauling: Xp = (1
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37



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