Electronegativity

1
Chapter 2

• Electronegativity
• Dipole Moment
• Resonance Structure
• Acids and Bases
• Drawing Chemical Structures

2
Electronegativity
Page 37
3
Chapter 2

• Electronegativity
• Dipole Moment
• Resonance Structure
• Acids and Bases
• Drawing Chemical Structures

4
Polar covalent bond
Inductive effect - shifting of electron density
in the ? bond in response to electronegativity of
nearby atoms.
5
Dipole moment - ?(Debye)
-

2.5
2.1
Table 2.1 - Dipole Moments of Some Compounds
6
?(Debye)
Bond dipole
Resultant dipole
Bond dipole
7
Bond vs. Molecular dipole
? gt 0
? 0
8
A dipole problem
Suppose you are given
? 0
What can you say about the structure of the
molecule?
9
Other dipole examples
Polarity of the molecule.
Geometry of the molecule.
10
Dipole Examples
11
Chapter 2

• Electronegativity
• Dipole Moment
• Resonance Structure
• Acids and Bases
• Drawing Chemical Structures

12
Resonance Structures

• Used to describe the stability of a molecule, an
  anion, or a cation.
• The more resonance structure one can draw, the
  more stable is the molecule.
• If, for example, three resonance structures can
  be drawn for a molecule, say A, B, and C. The
  true structure of the molecule is a hybrid of all
  three.

13
Resonance Structure
Resonance Hybrid
Resonance form B
Resonance form A
Rule 1. Individual resonance forms are imaginary,
not real.
Rule 2. Resonance forms differ only in the
placement of their ? or non-bonding electrons.
Rule 4. Resonance forms must be valid Lewis
structures and obey normal rules of valency.
Rule 5. Resonance hybrid is more stable than any
individual resonance form.
14
Resonance Structure
Resonance form B
Resonance form A
Rule 3. Different resonance forms of a substance
dont have to be equivalent.
Form A is more stable than B.
15
Resonance Examples

A)
B)
C)
D)
16
Resonance Example (A)
Hybrid
17
Resonance Example (B)
Hybrid
18
Resonance Example (C)
Hybrid
19
Resonance Example (D)
major
major
minor
20
Chapter 2

• Electronegativity
• Dipole Moment
• Resonance Structure
• Acids and Bases
• Drawing Chemical Structures

21
Definition of Acids Bases
Svante Arrhenius (1859-1927)
Acid - Proton (H) donor Base - Hydroxide (OH-)
donor.
Johannes Brönsted (1879-1947) Thomas Lowry
(1874-1936)
Acid - Proton (H) donor Base - Proton (H)
acceptor.
G. N. Lewis (1875-1946)
Acid - e- pair acceptor Base - e- pair donor.
nucleophile
electrophile
22
Definition of Acids Bases
A, BL, L
BL, L
L
23
Acid Base Reaction
HA B ? A- HB
Conjugate Base
Conjugate Acid
Base
Acid
24
Acid Base Reaction
Conjugate Base
Conjugate Acid
Base
Acid
25
Strong vs. Weak Acid
If HA is a strong acid
If HA is a weak acid
26
Acid Base Strength
The stronger the acid, the smaller the pKa
The stronger the acid, the weaker the conjugate
base.
27
Bases
28
Predicting Acid Base Reaction
Conjugate Base
Conjugate Acid
Base
Acid
-419.2
-607.2
-237.1
-389.9
Data taken from Handbook of Chemistry and
Physics
pKa
4.76
15.74
Stronger acid
Weaker acid
Active
Stable
Higher E
Lower E
29
Predicting Acid Base Reaction
Will the following reaction take place?
pKa
4.76
15.54
30
Chapter 2

• Electronegativity
• Dipole Moment
• Resonance Structure
• Acids and Bases
• Drawing Chemical Structures

31
Condensed, Kekulé, Skeletal Structures
Compound B
Compound A
Skeletal
Kekulé
Compound C
Condensed
32
Condensed, Kekulé, Skeletal Structures
Determine the number of hydrogens bonded to
each carbon atom, show all the nonbonding
electrons, and give the molecular formula of each.