Teoria de Acidos y Bases Capitulo 6 Miessler and Tarr

Acid-Base and Donor-Acceptor
chemistry
Based on “Inorganic Chemistry”, Miessler and Tarr,
4th edition, 2011, Pearson Prentice Hall
Images from Miessler and Tarr “Inorganic Chemistry” 2011 obtained from Pearson Education, Inc.
Acid-base as organizing concepts
“…acid-base concepts occupy a somewhat nebulous position
in the logical structure of chemistry. They are, strictly
speaking, neither facts nor theories and are, therefore, never
really “right” or “wrong”. Rather they are classificatory
definitions or organizational analogies. They are useful or not
useful… acid-base definitions are always a reflection of the
facts and theories current in chemistry at the time of their
formulation and … they must, necessarily, evolve and change
as the facts and theories themselves evolve and change…”
Preface of W.B. Jensen “The Lewis Acid-Base Concepts”, Wiley InterScience,
New York, 1980, cited in Miessler and Tarr “Inorganic Chemistry”.
Arrhenius
Acids form H+ ions
Bases form OH- ions
acid + base salt + water
Bronsted-Lowry
acids are H+ donors and bases are H+ acceptors
Conjugate acids and bases – species that only differ by the
presence or absence of the H+ ion
Solvent-system
Arrhenius is a solvent system definition (in water)
Solvent-system (aprotic)
Lewis
Acids are electron-pair acceptors
Bases are electron-pair donors
In these reactions, the product is called an adduct
Frontier Orbitals and
Acid-Base Reactions
HOMO-LUMO
Interactions
Lewis definition reformulated…
A base has an electron pair in a HOMO of suitable symmetry
and energy to interact with the LUMO of the acid.
Hydrogen bonding
possibilities
HOMO of B well below
LUMO of HA – poor
match of energies
Reactants’ energy below
that of BHA
Hydrogen bonding
possibilities
Good match of energies
– good hydrogen
bonding occurs
HOMO of B at nearly
the same energy as
LUMO of HA
BHA energy lower than
reactants
Hydrogen bonding
possibilities
Very poor match of
energies
HOMO of B well below
both LUMO and
HOMO of HA
Transfer of H+ occurs
BH+ and A- energy is
lower than B + HA or
BHA
Hard and Soft acids and bases
Prior to explaining anything, let’s look at some values in
terms of relative solubilities
Do you see any trend?
Mercury(I) has a similar trend
Lithium(I) has the opposite trend
Coordination of thiocyanate to
metals
Thiocyanate (SCN-) can bind to metals via the sulfur atom
(highly polarizable metal ions such as Hg2+) or it can bind to
metals via the nitrogen atom (smaller, less polarizable metals
such as Zn2+)
How do you explain this?
Equilibrium constants of exchange
reactions
[CH3Hg(H2O)]+ + HCl ↔ CH3HgCl + H3O+
K = 1.8x1012
[CH3Hg(H2O)]+ + HF ↔ CH3HgF + H3O+
K = 4.5x10-2
HSABs (Hard and soft acids and bases)
Polarizable acids and bases are SOFT
Nonpolarizable acids and bases are HARD
“Hard acids prefer to bind to hard bases, and soft acids prefer
to bind to soft bases”
This DOES NOT means that no interaction will occur
between a soft acid and a hard base or viceversa
Relative solubilities
AgF > AgCl > AgBr > AgI
Iodide is the most polarizable (soft) anion and it interacts more strongly
than the others with Ag+ (a soft cation)
The color of these salts is also worth noticing…
AgI is yellow, AgBr is sligthly yellow and AgCl and AgF are white
Color depends on relative energy of occupied and unoccupied orbitals
Lithium halides have the inverse tendency – LiF is the least soluble,
because both Li+ and F- are hard ions
Hard and soft cations
Coordination of thiocyanate
Hg2+ ion is much larger and polarizable (soft), hence it
interacts with thiocyanate via the sulfur atom
Zn2+ ion interacts via the nitrogen (hard-hard interaction)
The notation used to write the formula of the complex ion is
as follows [Hg(SCN)4]2- and [Zn(NCS)4]2- to show that the
bound atom is S and N, respectively.
Equilibrium constants of exchange
reactions
Mercury ion will interact more strongly with the softer
halide ion
Hard and soft acids
Hard and soft bases
Inherent acid-base strength
Real acidity and alkalinity must be taken into consideration
If two bases are competing for an acid, the inherent base
strength should prevail instead of the softness considerations
Although Zn2+ and O2- are the strongest acid and base, the
reaction is favoring the soft-soft and hard-hard combination
Qualitative analysis
Relative acid-base strength
Changes in boiling or melting points can indicate the
presence of adducts
BF3•O(C2H5)2 has a boiling point of 125°C
Direct calorimetric methods or temperature dependence of
equilibrium constants can be used to measure enthalpies and
entropies of acid-base reactions
Gas phase measurements of the formation of protonated species
can provide similar thermodynamic data
IR spectra provides information on force constants
NMR coupling constants provides information upon adduct
formation
UV-Vis show changes in energy levels
Thermodynamic measurements (using
Hess Law)
Proton affinity
The purest measure of acid-base strength, but difficult to
relate to solution reactions
Defined as BH+(g) B(g) + H+(g) proton affinity = ∆H
In gas-phase, both pyridine and aniline are stronger bases
than ammonia, but not in solution
Acidity and Basicity of Binary
Hydrogen Compounds
Acidity and Basicity of Binary
Hydrogen Compounds
Inductive effects
Who is a strongest base? PH3 or PF3?
Strength of oxyacids
Acidity of cations in aqueous solutions
The higher the charge and the smaller the size, the stronger the acid
Acidity of metal cations
The stronger the cation acid, the less soluble the hydroxide
At the highly charged extreme, the free cation is no longer a
detectable species
Steric effects
When the following bases’ reaction with H+ are compared,
the usual order is followed (top), but when BF3 or BMe3 is
used the order is reversed
Solvation and acid-base strength
Gas-phase measurements show trisubstituted amines to be
more basis than the di-, mono-, and unsubstituted
In aqueous solution, the order is reversed, mostly because of
the solvation of the protonated cations
Non-aqueous solvents
Leveling effect – acids or bases are brought down to the limiting
conjugate acid or base of the solvent
Strongest acid possible in H2O is H3O+
Strongest base possible in H2O is OH-
Acidic solvents are used to measure the relative strength of strong
acids (basic solvents are used for bases)
Superacids – stronger than H2SO4
The stronger the acid, the more negative its H0 value
Fuming sulfuric acid is prepared by dissolving SO3 in H2SO4
Lewis superacids
Formed by the fluorides as a result of the transfer of anions
to form complex fluoro anions
They are strong Friedel-Crafts catalysts (like AlCl3)