Download Acids and bases

Acid-base and donor-acceptor chemistry
Hard and soft acids and bases
Classical concepts
Arrhenius:
• acids form hydrogen ions H+ (hydronium, oxonium H3O+) in aqueous solution
• bases form hydroxide ions OH- in aqueous solution
• acid + base  salt + water
e.g. HNO3 + KOH  KNO3 + H2O
Brønsted-Lowry:
• acids tend to lose H+
• bases tend to gain H+
• acid 1 + base 1  base 1 + acid 2 (conjugate pairs)
H3O+ + NO2-  H2O + HNO2
NH4+ + NH2-  NH3 + NH3
In any solvent, the reaction always favors the formation of the weaker acids or bases
The Lewis concept is more general
and can be interpreted in terms of MO’s
Remember
that frontier orbitals
define the chemistry
of a molecule CO is a s-donor and
a p-acceptor
d+
C
dO
M
C
O
C
O
M
Acids and bases (the Lewis concept)
A base is an electron-pair donor
An acid is an electron-pair acceptor
acid
adduct
base
Lewis acid-base adducts involving metal ions
are called coordination compounds (or complexes)
Frontier orbitals and acid-base reactions
Remember the NH3 molecule
Frontier orbitals and acid-base reactions
The protonation of NH3
New LUMO
(non-bonding)
New HOMO
(bonding)
(Td)
(C3v)
In most acid-base reactions HOMO-LUMO combinations
lead to new HOMO-LUMO of the product
But remember that there must be useful overlap (same symmetry)
and similar energies to form new bonding and antibonding orbitals
What reactions take place if energies are very different?
Frontier orbitals and acid-base reactions
Even when symmetries match several reactions are possible,
depending on the relative energies
Frontier orbitals and acid-base reactions
Very different energies like A-B ó A-E
no adducts form
Similar energies like A-C ó A-D
adducts form
A base has an electron-pair
in a HOMO of suitable symmetry
to interact with the LUMO of the acid
The MO basis for hydrogen bonding
F-H-F-
MO diagram derived from atomic orbitals
(using F…….F group orbitals + H orbitals)
Bonding e
Non-bonding e
But it is also possible from HF + F-
HOMO-LUMO of HF for s interaction
Non-bonding
(no symmetry match)
Non-bonding
(no E match)
The MO basis for hydrogen bonding
F-H-F-
LUMO
HOMO
Formation of the orbitals
HOMO
HOMO
First take bonding and
antibonding combinations.
Similarly for unsymmetrical B-H-A
Total energy of B-H-A
lower than the sum of
the energies of reactants
Poor energy match,
little or no H-bonding
e.g. CH4 + H2O
Good energy match,
strong H-bonding
e.g. CH3COOH + H2O
Very poor energy match
no adduct formed
H+ transfer reaction
e.g. HCl + H2O
Hard and soft acids and bases
Hard acids or bases are small and non-polarizable
Soft acids and bases are larger and more polarizable
Halide ions increase in softness:
fluoride < chloride<bromide<iodide
Hard-hard or soft-soft interactions are stronger (with less soluble salts)
than hard-soft interactions (which tend to be more soluble).
Most metals are classified as Hard (Class a) acids or acceptors.
Exceptions shown below: acceptors metals in red box are always soft (Class b).
Other metals are soft in low oxidation states and are indicated by symbol.
Class (b) or soft always
Solubilities: AgF > AgCl > AgBr >AgI
But……
LiBr > LiCl > LiI > LiF
Chatt’s explanationClass (b) soft metals have d electrons available for p-bonding
Model: Base donates electron density to metal acceptor. Back donation, from acid to
base, may occur from the d electrons of the acid metal into vacant orbitals on the base.
Higher oxidation states of elements to the right of transition metals have more class b character
since there are electrons outside the d shell.
Ex. (Tl(III) > Tl(I), has two 6s electrons outside the 5d making them less available for π-bonding
For transition metals:
high oxidation states and position to the left of periodic table are hard
low oxidation states and position to the right of periodic table are soft
Soft donor molecules or ions that are readily polarizable and have vacant d or π* orbitals
available for π-bonding react best with class (b) soft metals
Tendency to complex with hard metal ions
N >> P > As > Sb
O >> S > Se > Te
F > Cl > Br > I
Tendency to complex with soft metal ions
N << P > As > Sb
O << S > Se ~ Te
F < Cl < Br < I
The hard-soft distinction is linked to polarizability, the degree to which a molecule
or ion may be easily distorted by interaction with other molecules or ions.
Hard acids or bases are small and non-polarizable
Soft acids and bases are larger and more polarizable
Hard acids are cations with high positive charge (3+ or greater),
or cations with d electrons not available for π-bonding
Soft acids are cations with a moderate positive charge (2+ or lower),
Or cations with d electrons readily availbale for π-bonding
The larger and more massive an ion, the softer (large number of internal electrons
Shield the outer ones making the atom or ion more polarizable)
For bases, a large number of electrons or a larger size are related to soft character
Hard acids tend to react better with hard bases and soft acids with soft
bases, in order to produce hard-hard or soft-soft combinations
In general, hard-hard combinations are energetically
more favorable than soft-soft
An acid or a base may be hard or soft
and at the same time it may be strong or weak
Both characteristics must always be taken into account
e.g. If two bases equally soft compete for the same acid,
the one with greater basicity will be preferred
but if they are not equally soft, the preference may be inverted
Fajans’ rules
1.
For a given cation, covalent character increases
with increasing anion size.
2. For a given anion, covalent character increases
with decreasing cation size.
3. The covalent character increases
with increasing charge on either ion.
4. Covalent character is greater for cations with non-noble gas
electronic configurations.
A greater covalent character resulting from a soft-soft interaction is related
With lower solubility, color and short interionic distances,
whereas hard-hard interactions result in colorless and highly soluble compounds
Quantitative measurements
IA

2
Absolute hardness
(Pearson)
s
1

Softness
IA

2
Mulliken’s absolute electronegativity
(Pearson)
EHOMO = -I
ELUMO = -A
Energy levels
for halogens
and relations between
,  and HOMOLUMO energies