Chemistry 3719 - Organic Chemistry I

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Chapter 1 1.2-1.3 Bonding Atoms trying to
attain the stable configuration of a noble
(inert) gas - often referred to as the octet rule
1.2 Ionic Bonding - Electrons Transferred
1.3 Covalent Bonding - Electrons Shared
type of bond that is formed is dictated by the
relative electronegativities of the elements
involved
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Electronegativity the attraction of an atom for
electrons
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Important Electronegativity Values
H 2.1 Li Be B C N O F 1.0 2.0 2.5 3.0 3.5 4.0
Cl 3.0 Br 2.8 I
2.5
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Important Electronegativity Values
H 2.1 Li Be B C N O F 1.0 2.0 2.5 3.0 3.5 4.0
Cl 3.0 Br 2.8 I
2.5
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1.2 Ionic bonding
Electrons Transferred Big differences in E.N.
values Metals reacting with non-metals
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1.3 Covalent Bonding - Similar electronegativities
Lewis dot representations of molecules
H . H .
H H
B.D.E
Hydrogen atoms
Hydrogen molecule
104 kcal/mol
B.D.E
104 kcal/mol
B.D.E. bond dissociation energy
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1.3 Lewis Dot Structures of Molecules
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1.4 Double bonds and triple bonds
Double bonds - alkenes
Triple bonds - alkynes
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1.5 Polar covalent bonds and electronegativity
H2 HF H2O CH4 CH3Cl
Based on electronegativity
?
?-
Li
H
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1.6 Structural Formula in Organic Chemistry
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1.6 Constitutional Isomers
Same molecular formula, completely different
chemical and physical properties
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1.7 Formal Charge
Formal charge group number - number
of bonds - number of unshared
electrons
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1.8 Resonance Structures - Electron Delocalization
Table 1.6 formal rules for resonance
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1.9 Shapes of Molecules
Shapes of molecules are predicted using VSEPR
theory
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1.9 Shape of a molecule in terms of its
atoms Figure 1.9
Table 1.7 VSEPR and molecular geometry
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Trigonal planar geometry of bonds to carbon in
H2CO
Linear geometry of carbon dioxide
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1.10 Molecular dipole moments
Figure 1.7
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1.11 Curved Arrows Extremely Important

• Curved arrows are used to track the flow of
• electrons in chemical reactions.
• Consider the reaction shown below which shows
• the dissociation of AB

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Curved Arrows to Describe a Reaction

• Many reactions involve both bond breaking
• and bond formation. More than one arrow
• may be required.

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1.12 Acids and Bases - Definitions
Arrhenius An acid ionizes in water to give
protons. A base ionizes in water to give
hydroxide ions. Brønsted-Lowry An acid is a
proton donor. A base is a proton
acceptor. Lewis An acid is an electron pair
acceptor. A base is an electron pair donor.
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1.13 A Brønsted-Lowry Acid-Base Reaction
A proton is transferred from the acid to the base.


.
.
B

H
A
B
A
H

base
acid
conjugate acid
conjugate base
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Proton Transfer from HBr to Water
hydronium ion (H3O)
H
H
..


..
.
.
.
.
O
Br
H
H
Br
O


..
..
H
H
base acid conjugate conjugate acid base
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Equilibrium Constant for Proton Transfer
..
.

.

O

H
Br
H
O
..
H3OBr
Ka
HBr
pKa log10 Ka
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Acids and Bases Arrow Pushing
H3OBr
Ka
106 for HBr, pKa - 5.8
HBr
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Need to know by next class pKa
-log10Ka STRONG ACID LOW pKa WEAK ACID
HIGH pKa HI, HCl, HNO3, H3PO4 pKa -10 to -5
Super strong acids H3O pKa
1.7 RCO2H pKa 5 acids PhOH pKa
10 get H2O, ROH pKa 16 weaker RCCH
(alkynes) pKa 26 RNH2 pKa 36 Extremely
weak acid RCH3 pKa 60 Not acidic at all
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1.14 What happened to pKb?

• A separate basicity constant Kb is not
  necessary.
• Because of the conjugate relationships in the
  Brønsted-Lowry approach, we can examine acid-base
  reactions by relying exclusively on pKa values.

pKa 60 Essentially not acidic
Corresponding base Extremely strong
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1.15 How Structure Affects Acid/Base Strength

• Bond Strength
• Acidity of HX increases (HIgtHBrgtHClgtHF) down the
  periodic table as H-X bond strength decreases and
  conjugate base (X- anion) size increases (basic
  strength of anion decreases).

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Electronegativity Acidity increases across
periodic table as the atom attached to H gets
more electronegative (HFgtH2OgtH2NgtCH4).
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Inductive Effects Electronegative groups/atoms
remote from the acidic H can effect the pKa of
the acid.
pKa 16 pKa 11.3

• O H bond in CF3CH2OH is more polarized
• CF3CH2O- is stabilized by EW fluorine atoms

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Resonance Stabilization in Anion Delocalization
of charge in anion (resonance) makes the anion
more stable and thus the conjugate acid more
acidic e.g. (CH3CO2H gt CH3CH2OH).
pKa 16
pKa 5
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1.16 Acid-base reactions - equilibria
The equilibrium will lie to the side of the
weaker conjugate base
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1.17 Lewis acids and Lewis bases
F3B

Lewis acid
Lewis base
Product is a stable substance. It is a liquid
with a boiling point of 126C. Of the two
reactants, BF3 is a gas and CH3CH2OCH2CH3 has a
boiling point of 34C.