E. D. GLORIAUST – FACULTY OF PHARMACYCHEM200 – ORGANIC CHEMISTRY

Unit II: Acids and Bases

1. Factors that affect the acidity/basicity of a compound2. Arrhenius Acids and Bases3. Bronsted-Lowry Acids and Bases4. Lewis Acids and Bases5. Ka, pKa, Kb, pKb, pH

Contents

Acids and Bases

Johannes Bronsted and Thomas Lowry- Independently proposed that:- An acid is a proton donor- A base is a proton acceptor- The acid-base reaction is a proton transfer

reaction.

1. Formal Charge2. Electronegativity3. Structural effects

1. Resonance2. Hyperconjugation3. Inductive effect4. Steric effect5. Hydrogen bonding

What are the factors that affect the acidity or basicity of a compound?

• The charge on an atom in a molecule or polyatomic ionFormal Charge

1. Write a correct Lewis structure for the molecule or ion.2. Assign to each atom all its unshared (nonbonding)

electrons and one-half its shared electrons.3. Compare the number arrived at in Step 2 with the

number of valence electrons in the neutral, unbonded atom.

To derive a Formal Charge:

Number of valence Number of One-halfFormal Charge = electrons in free -- nonbonding -- bonding

atom electrons electrons

Example

Summary of Common Formal Charges

•H3O+

•CH2O-

Example:

Electronegativity

H – F < H – Cl < H – Br < H – I

Variability in size within the Family

Resonance

Theory of Resonance – developed by Linus Pauling in the 1930s, states that many molecules and ions are best described by writing two or more Lewis structure and considering the real molecule or ion to be a composite of these structures.

What is Resonance?

Resonance Contributing Structures – representations of a molecule or ion that differ only in the distribution of valence electrons.Resonance hybrids – A molecule or ion that is best described as a composite of a number of contributing structures.Double-headed arrows – a symbol used to connect contributing structures.

1. Individual resonance forms are imaginary, not real.2. Resonance forms differ only in the placement of their

nonbonding electrons.3. Different resonance forms of a substance don’t have to

be equivalent.4. The resonance hybrid is more stable than any individual

resonance forms.5. Electron pairs can only move to adjacent positions.

Adjacent positions means neighboring atoms and/or bonds.

6. The Lewis structures that result from moving electrons must be valid and must contain the same net charges as all the other resonance structures.

Rules for Resonance Forms

1. Do not break a single bond.2. Do not exceed an octet for second row

elements.

The Two Commandments of Resonance

1. A lone pair next to a pi bond.2. A lone pair next to a positive charge.3. A pi bond next to a positive charge.4. A pi bond between two atoms, where one of those

atoms is electronegative.5. Pi bond going all the way around a ring.

Five patterns in drawing resonance structures

1. Minimize charges2. Electronegative atoms (N, O, Cl, etc) can bear a

positive charge, but only if they possess an octet of electrons.

3. Avoid drawing a resonance structure in which two carbons bear opposites charges.

3 rules in determining which resonance structures are significant

Inductive Effect

• the polarization of electron density transmitted through covalent bonds by a nearby atom of higher electronegativity.

Inductive Effect

Cl CH2 C OH

O

Cl CH C OH

OCl

MORE ACIDIC

MORE ACIDIC

H2CH3C CH C OH

OCl

HC CH2 C OH

OCl

H3C

Resonance and Inductive Effects of Various Functions

IE : EW IE: ED RE: EW RE : ED-- NR3

+ --NH3+ --O- --COO- --COOR --COOH --OH --OR

--COOH --COOR --CH2 --CHR2 --CH2R --CR3

--COR --CHO --NH2 -- NR2

--NO2 --CN --SO2R --SO2NHR --NHR2 --SH --CHO --COR --NO2 --CN--F --Cl --Br --I --CF3

--Ar

--OH --OR--NH2

--SH --SO2R

Hyperconjugation

• Any adjacent bonding electron pair will participate in carbocation hyperconjugation. However, only C–H and C–C bonds provide a significant level of increased stability.

Hyperconjugation

Inc. number of alkyl groups attached to carbocation

Inc. # of bonds adjacent to the

carbocation

Inc. # hyperconjugative interactions

Inc. electron density to the

electron-deficient carbon

Inc. StabilityInc. Acidity

Hyperconjugation: Carbocation stabilization

Hyperconjugation: Carbocation stabilization

Hyperconjugation

Steric Effect

CH3 C CH2

CH3

CH3

CH

CH3C OH

O

CH3 C CH2

CH3

CH3

CH

C(CH 3)3C OH

O

MORE ACIDIC

Hydrogen Bonding

• is a special type of dipole-dipole attraction between molecules, not a covalent bond to a hydrogen atom. It results from the attractive force between a hydrogen atom covalently bonded to a very electronegative atom such as a N, O, or F atom and another very electronegative atom.

Hydrogen Bonding

C OH

O H

O

FH OH OH

MORE ACIDIC

HOOC

OHHO

HOOCOH

m-nitrophenol p-nitrophenol o-nitrophenol

HO

NO2

HO

O2N

HONO2

• 2.10• 2.17• 2.19• 2.28• 2.29• 2.36• 2.39

Answer the following:

• Brown, W. H., Poon, T. (2014). Introduction to Organic Chemistry (5th ed.). USA: John Wiley & Sons, Inc.

References