Download Lecture Notes 7 - La Salle University

Acids and Bases
or general chemistry, analytical
and a whole lot more!
Arrhenius acid is a substance that produces H+ (H3O+) in water
Arrhenius base is a substance that produces OH- in water
A Brønsted acid is a proton donor
A Brønsted base is a proton acceptor
base
base
acid
acid
base
acid
conjugate
acid
conjugate
base
Acid-Base Properties of Water
H+ (aq) + OH- (aq)
H2O (l)
autoionization of water
H
O
H
+ H
[H
O
H
]
H
+ H
H
base
H2O + H2O
acid
O
+
conjugate
acid
H3O+ + OHconjugate
base
O
-
Molecular Structure and Acid Strength
H X
H+ + X-
The
stronger
the bond
The
weaker
the acid
HF << HCl < HBr < HI
Molecular Structure and Acid Strength
Z
dO
d+
H
Z
O- + H+
The O-H bond will be more polar and easier to break if:
•
Z is very electronegative or
•
Z is in a high oxidation state
Molecular Structure and Acid Strength
1. Oxoacids having different central atoms (Z) that are from
the same group and that have the same oxidation number.
••
••
••
••
••
••
Acid strength increases with increasing electronegativity of Z
••
••
O
O
••
••
••
••
H O Cl O
H O Br O
•• •• • •
•• •• • •
Cl is more electronegative than Br
HClO3 > HBrO3
Molecular Structure and Acid Strength
2. Oxoacids having the same central atom (Z) but different
numbers of attached groups.
Acid strength increases as the oxidation number of Z increases.
HClO4 > HClO3 > HClO2 > HClO
Definition of An Acid
Arrhenius acid is a substance that produces H+ (H3O+) in water
A Brønsted acid is a proton donor
A Lewis acid is a substance that can accept a pair of electrons
A Lewis base is a substance that can donate a pair of electrons
••
••
H+ + OH••
acid base
H+ +
acid
••
H
N H
H
base
••
H O H
••
H
H N H
H
Lewis Acids and Bases
F B
F
+
••
H
F
N H
H
F
F B
F
H
N H
H
acid
base
(electrophile) (nucleophile)
No protons donated or accepted!
Let’s look at this reaction, and others like it more closely.
To see what’s really going on we need to look at the MOs.
H+
NH4+
: NH3
2E
Energy
3 A1
2 A1
2 T2
HOMO
2 A1
1E
1A1
1 T2
LUMO
1 A1
1 A1
H+
NH4+
: NH3
2E
Energy
3 A1
2 A1
2 T2
HOMO
2 A1
1E
1A1
1 T2
LUMO
1 A1
1 A1
Reaction Mechanisms
How do the atoms of the reactant molecules
rearrange to form the product molecules?
What is the sequence of bond breaking and
bond making?
What are the energetics of the process?
How do the electrons flow?
What molecular orbitals are involved?
How do you analyze a mechanism?
Follow the Electrons.
Push your arrows.
H
+
N
H
H
H
H
F
F
N
H
H
H
What orbitals are involved?
What are the important MOs
of HF and NH3?
σ*
Looks most
like H
Lowest
Unoccupied
Molecular
Orbital
H F
H 1s
MO diagram
of HF
σ
Looks most
like F
acid – LUMO
most important
F 2p
Highest
Ooccupied
Molecular
Orbital
HF
NH3
base – HOMO
most important
acid – LUMO
most important
Same Symmetry End On
Good Interaction
NH3 HOMO
Symmetric-Symmetric
S-S
Reaction is
Symmetry Allowed
S (symmetric)
No nodes.
HF LUMO
Wrong Symmetry
No Good Interaction
Does not work side on
Restating the Lewis Acid-Base Definition:
a)Base: has e- pair in HOMO of
correct energy and symmetry
b)Acid: has LUMO of correct energy
and symmetry
•
Carbon Monoxide as a Lewis
Base
1) Electronegativity suggests
O is the e- pair donor
C O
M+
C O
2) In fact, C is always the
donor
a) Formal Charge
-1
+1
C O
b) MO Frontier Orbitals
i.
HOMO that is involved in bonding
is mostly on C
ii. C-like HOMO donated to the M
Lewis acid
O C
M+
While we’re on the subject of
directionality in reactions
A little organic chemistry
(not my favorite, but it does have
MO’s)
Alkene Additions
H
H
H
CH3
H
H
H
CH3
H
CH3
H
H
Br
OH
HBr
H2O
Regiochemistry
HO
H
H
H2O
H
Regiochemistry –
At which atom
center does a
reaction take place
H
CH3
H
H
H2
H
H
CH3
H
H
H
Reaction Mechanisms
How do the atoms of the reactant molecules
rearrange to form the product molecules?
H2 + CH2=CH2
?
CH3CH3
Reaction Mechanisms
But first. Do we expect the reaction to occur?
Look at the thermodynamics.
ΔGr°= Σproducts - Σreactants
ΔGr°= -32 – 68 = -100 kJ/mol
ΔGf°=0.0
ΔGf°=+68 kJ/mol
?
ΔGf°=-32 kJ/mol
Energy
Transition State
ΔG‡
reactants
Ea
Energy of
activation
ΔGr
products
Reaction Coordinate
Energy
ΔGr = -100 kJ/mol
exergonic
ΔG‡
reactants
H2 + C2H4
0 + 68 kJ
ΔG‡ = ?
Energy of activation
-100 kJ
Reaction Coordinate
C2H6
-32 kJ
products
Reaction Mechanisms
How do the atoms of the reactant molecules
rearrange to form the product molecules?
H2 + CH2=CH2
CH3CH3
It looks good. What is wrong with this mechanism?
H
H
H
H
C
C
H
H
H
H
C
H
H
C
H
H
H2 + CH2=CH2
Examine
the M.O.s
CH3CH3
π*
A
LUMO
S
A
LUMO
H2
C2H4
π
σ*
HOMO
HOMO
σ
S
S (symmetric with
respect to reflection)
π*
A
LUMO
A (antisymmetric with
respect to reflection)
It has a node.
A-S Wrong Symmetry
No Interaction
HOMO
σ
S
A-S Wrong Symmetry
No Interaction
π
S
LUMO
σ*
A
This reaction path
is forbidden by orbital
symmetry
HOMO
π*
A
π
S
LUMO
LUMO
σ*
A
The direct reaction
of H2 and C2H4
is forbidden by orbital
symmetry
HOMO
HOMO
σ
S
Energy
ΔGr = -100 kJ/mol
ΔG‡
reactants
H2 + C2H4
0 + 68 kJ
Forbidden Reaction
ΔG‡ is too high
We need a
catalyst!
-100 kJ
Reaction Coordinate
C2H6
-32 kJ
products
What is a catalyst?
An added component that changes the
reaction mechanism to one with a
lower energy pathway. A lower ΔG‡
The catalyst is neither produced or
destroyed during the reaction. It does
not change ΔG of the reaction. It does
not change Keq of the reaction.
A catalyst can be simple like H+ or a metal
ion or it can be complex like an enzyme.
The platinum metals are often used in catalysis
They have filled d orbitals and empty s or p orbitals.
This means they can act as either an acid or a base.
Classes of Lewis bases
(Ligands)
• Monodentate Ligands - A ligand that donates only one
electron pair to a single metal
– One of the best examples is NH3.
• Bridging Ligands - A ligand that donates one or more
electron pairs to two or more metals
– Halides and hydroxide are good examples, since each
possesses two or more electron pairs on the donor atom
– As we considered before, one of the steps to forming insoluble
hydroxide and oxo compounds is the bridging of two metals in
the course of hydroysis.
Classes of Lewis bases – con’t
• Ambidentate Ligands - Ligands that
possess two or more donor atoms can act
as a monodentate ligand through either
donor atom, or bridge two metals.
• The pseudohalides such as cyanide, azide
and thiocyanate are good examples.
Denticity
• The number of potential donor interactions
is called the denticity
• ligands are classified as bidentate
chelating ligands (as for ethylenediamine),
hexadentate chelating ligands (as for
ethylenediaminetetraacetic acid; EDTA)
and so forth.
Chelating ligands
• ethylenediamine (on the left)
yields a five-membered ring
• acetylacetonate (on the right)
yields a six-membered ring
O
N
Ni
N
Ni
O
Structures of Common Chelating Ligands
N
N
N
N
2,2’-bipyridine (bpy)
N
1,10-phenathroline (phen)
N
N
terpyridine (terpy)
H2N
H2N
NH2
ethylenediamine (en)
H2N
N
H
H
N
triethylenetetramine (trien)
H2N
NH2
propylenediamine (prn)
NH2
NH2
diethylenetriamine (dien)
N
NH2
N
H
NH2
H2N
tri(ethylenediamine)amine (tren)
Macrocyclic ligands
• Macrocyclic Ligands - A special class of
generally tetradentate chelating ligands, where
the four donor atoms are arranged in a circular
ring array, as in the porphyrins and anes.
14
Increasing Topological Constraint
Coordination
Chelation
Macrocycle Effect
NH2
H2N
H
H
N
N
NH2
H
N
H
H
N
N
N
N
N
N
NH3
H2N
N N
N
H
H2N
Cryptate Effect
NH2
N
H
[Ni(H2O)6]2+ + n L ¾ [Ni(L)n(H2O)2]2+ + 4 H2O
L=
NH3
en
trien
2,3,2-
n=
4
2
1
1
log bn =
8.12
13.5
13.8
14.6
Tetradentate Tetraaza Macrocycles
[Cu(teta)]2+  Cu2+ + (teta)Hn n+
kd = 3.6 x 10-7 sec-1 in 6.1 M HCl at 25 oC
[Cu(2,3,2)]2+  Cu2+ + (2,3,2)Hn n+
kd = 4.1 sec-1 in 6.1 M HCl at 25 oC
N
NH2
N
N
N
N
N
H2N
2,3,2
Teta=5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane
Nature’s Macrocycle
NH
N
N
HN
Porphyrin
A porphyrin is “any of a
group of compounds
containing the porphin
structure of four pyrrole rings
connected by methine
bridges in a cyclic
configuration, to which a
variety of side chains are
attached; usually metalled,
e.g., with iron to form heme.”
Definition from Academic
Press Dictionary of Science
Technology
The iron(II) protoporphyrin-IX complex is the prosthetic group in hemoglobins
and myoglobins, which are responsible for oxygen transport and storage in
living tissues. Heme can also be found in the enzyme peroxidase, which
catalyzes the oxidation of substrates with hydrogen peroxide. The related
enzyme catalase, also containing heme, catalyzes the breakdown of hydrogen
peroxide to water and oxygen. Other heme-containing proteins include the
cytochromes, which serve as one-electron carriers in the electron transport
chain.
Reduction of one of the pyrrole units on the
porphyrin ring leads to a class of porphyrin
derivatives called chlorins. Chlorophylls (e.g.
chlorophyll-a), found abundantly in green plants,
belong to this category. They play very important
roles in the process of photosynthesis.
Porphyrins are also found in
other systems such as the wing
feathers of Turacus indicus , the
oxygen-carrying pigment
chlorocruorin from Sabella starte
indica , oil shale, and some
marine sponges.
Vitamin B12 contains a porphyrin-like unit called corrin