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 donorA Brønsted base is a proton acceptor

acidbase acid base

acidconjugate

basebase conjugate

acid

O

H

H + O

H

H O

H

H H OH-+[ ] +

Acid-Base Properties of Water

H2O (l) H+ (aq) + OH- (aq)

H2O + H2O H3O+ + OH-

acid conjugate

base

base conjugate

acid

autoionization of water

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 O 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

H O Cl O

O••

••••••

••

••••

••••

H O Br O

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

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

Definition of An Acid

H+ H O H••••

+ OH-••••••

acid base

N H••

H

H

H+ + N H

H

H

H

acid base

Lewis Acids and Bases

N H••

H

H

acid(electrophile)

base(nucleophile)

F B

F

F

+ F B

F

F

N H

H

H

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+ NH3NH4

+:

1A1

2 E

3 A1

2 A1

1 E

1 A1

1 A1

2 A1

2 T2

1 T2

En

erg

y

LUMO

HOMO

H+ NH3NH4

+:

1A1

2 E

3 A1

2 A1

1 E

1 A1

1 A1

2 A1

2 T2

1 T2

En

erg

y

LUMO

HOMO

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?

What molecular orbitals are involved?

How do the electrons flow?

NH

HH

H F+ NH

HH

H F

Follow the Electrons. Push your arrows.

What orbitals are involved?

What are the important MOs of HF and NH3?

How do you analyze a mechanism?

F 2p

H 1s

σ* Looks most like H

σLooks mostlike F

Lowest UnoccupiedMolecularOrbital

Highest OoccupiedMolecularOrbital

MO diagram of HF

acid – LUMOmost important

H F

NH3

HF

acid – LUMOmost important

base – HOMOmost important

Wrong SymmetryNo Good Interaction

Does not work side on

Same Symmetry End OnGood Interaction

NH3 HOMO HF LUMO

Symmetric-Symmetric S-S

Reaction is Symmetry Allowed

S (symmetric)No nodes.

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

2) In fact, C is always the donora) Formal Charge

b) MO Frontier Orbitalsi. HOMO that is involved in bonding

is mostly on C

ii. C-like HOMO donated to the M Lewis acid

C O C O M+

C O-1 +1

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)

CH3

H H

H CH3

H

H

H

H

H

H2

CH3

H

H

H

H

Br

HBr

CH3

H

H

H

H

OH

H2O

CH3

H

H

H

HO

H

H2O

Regiochemistry

Alkene Additions

Regiochemistry –

At which atom center does a reaction take place

H2 + CH2=CH2 CH3CH3

Reaction MechanismsHow do the atoms of the reactant molecules

rearrange to form the product molecules?

?

Reaction MechanismsBut first. Do we expect the reaction to occur?Look at the thermodynamics.

?ΔGf°=0.0

ΔGf°=+68 kJ/mol ΔGf°=-32 kJ/mol

ΔGr°= Σproducts - Σreactants

ΔGr°= -32 – 68 = -100 kJ/mol

Energy

Reaction Coordinate

reactants

products

ΔGr

Ea

Energy of activation

ΔG‡

Transition State

Energy

Reaction Coordinate

reactants

products

ΔG‡

H2 + C2H4

0 + 68 kJ

C2H6

-32 kJ

-100 kJ

ΔGr = -100 kJ/mol

ΔG‡ = ?

exergonic

Energy of activation

Reaction MechanismsHow do the atoms of the reactant molecules

rearrange to form the product molecules?

H2 + CH2=CH2 CH3CH3

H2 + CH2=CH2 CH3CH3

C CH H

HH

H H

It looks good. What is wrong with this mechanism?

Examinethe M.O.s

C C

HH

HHHH

π

π*

HOMO

LUMO

LUMO

HOMOσ

σ*

SS

A

A

H2C2H4

π*LUMO

σS

A

HOMO

A-S Wrong SymmetryNo Interaction

S (symmetric with respect to reflection)

A (antisymmetric with

respect to reflection)

It has a node.

πHOMO

LUMOσ*

S

A

A-S Wrong SymmetryNo Interaction

This reaction pathis forbidden by orbital symmetry

π

π*

HOMO

LUMO

LUMO

HOMOσ

σ*

SS

A

AThe direct reactionof H2 and C2H4

is forbidden by orbital symmetry

Energy

Reaction Coordinate

reactants

products

ΔG‡

H2 + C2H4

0 + 68 kJ

C2H6

-32 kJ

-100 kJ

ΔGr = -100 kJ/mol

Forbidden ReactionΔG‡ is too high

We need a catalyst!

What is a catalyst?

An added component that changes thereaction mechanism to one with a lower energy pathway. A lower ΔG‡

The catalyst is neither produced ordestroyed during the reaction. It doesnot change ΔG of the reaction. It doesnot 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 catalysisThey 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

N

Ni

N

Ni

O

O

N N NN

N

NN

H2NNH2

H2N NH2H2N

NNH2

H

H2NN

NNH2

H

H

N

NH2NH2

H2N

Structures of Common Chelating Ligands

2,2’-bipyridine (bpy) 1,10-phenathroline (phen)

terpyridine (terpy)

ethylenediamine (en) propylenediamine (prn) diethylenetriamine (dien)

triethylenetetramine (trien)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 Cryptate Effect

NH3

H2NNH2

N

H

NH2H2N

N

H

NH2N

H

NH2

N

N N

N

H

H

H

H

N

N N

N

N

N

[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 n = 8.12 13.5 13.8 14.6

NN

NN

NN

H2NNH2

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

2,3,2

Teta=5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane

NNH

N HN

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

Nature’s Macrocycle

Porphyrin

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.

Vitamin B12 contains a porphyrin-like unit called corrin

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.