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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1.2 Ionic bonding
Electrons Transferred
Big differences in E.N. values
Metals reacting with non-metals
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Important Electronegativity Values
H
2.1
Li
1.0
Be
B
2.0
C
2.5
N
3.0
O
3.5
F
4.0
Cl
3.0
Br
2.8
I
2.5
1.3 Covalent Bonding - Similar electronegativities
H.
+
H.
H:H
Hydrogen atoms
B.D.E
Hydrogen molecule
+104 kcal/mol
C
+4H
H
H
B.D.E
+104 kcal/mol
B.D.E. = bond dissociation energy
C
H
H
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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
H H
C::C
H
H
H
H
C
H
C
H
Triple bonds - alkynes
H:C:::C:H
H C
C H
1.5 Polar covalent bonds and electronegativity
H2
HF
CH4
H 2O
CH3Cl
Based on electronegativity
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1.6 Structural Formula - Shorthand 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
O
O
O
O
O
Table 1.6 – formal rules for resonance
O
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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 H2C=O
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 used to track flow of electrons in chemical reactions.
•
Consider reaction shown below which shows the dissociation of AB:
A
B
+
A
+
-
B
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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.
H
H
O
+
H
C
H
H
Br
H
O
C
H
+
Br
-
H
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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: +
Base
H
acid
A
+
B
H
conjugate
acid
+
–
:A
conjugate
base
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Proton Transfer from HBr to Water
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Equilibrium Constant for Proton Transfer
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Acids and Bases: Arrow Pushing
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Need to know by next class:
pKa = -log10Ka
STRONG ACID = LOW pKa
HI, HCl, HNO3, H3PO4
H3O+
RCO2H
PhOH
H2O, ROH
RCCH (alkynes)
RNH2
RCH3
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WEAK ACID = HIGH pKa
pKa -10 to -5
pKa – 1.7
pKa ~ 5
pKa ~ 10
pKa ~ 16
pKa ~ 26
pKa ~ 36
pKa ~ 60
Super strong acids
acids
get
weaker
Extremely weak acid
Not acidic at all
1.14 What happened to pKb?
• A separate “basicity constant” Kb is not necessary.
• Because of the conjugate relationships in the BrønstedLowry approach, we can examine acid-base reactions by
relying exclusively on pKa values.
H
H C H
H
H
H C
H
pKa ~60
Corresponding base
Essentially not acidic
Extremely strong
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1.15 How Structure Affects Acid/Base Strength
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Bond Strength
• Acidity of HX increases (HI>HBr>HCl>HF) down the periodic table as H-X bond
strength decreases and conjugate base (X:- anion) size increases.
pKa
HF
3.1
strongest H—X bond
weakest acid
HCl
-3.9
HBr
-5.8
HI
-10.4
weakest H—X bond
strongest acid
Electronegativity
Acidity increases across periodic table as the atom attached to H gets
more electronegative (HF>H2O>H2N>CH4).
pKa
CH4
6
0
NH3
36
weakest acid
least electronegative
H 2O
16
HF
3.1
strongest acid
most electronegative
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Inductive Effects
Electronegative groups/atoms remote from the acidic H can effect the
pKa of the acid.
CH3CH2O H
pKa = 16
CF3CH2O H
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 > 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
Product is a stable substance. It is a liquid with a boiling point of 126°C. Of the
two reactants, BF3 is a gas and CH3CH2OCH2CH3 has a boiling point of 34°C.
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