1.3 covalent bonding - electrons shared
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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1.3 Covalent Bonding - Electrons Shared
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
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 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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1.3 Covalent Bonding - Similar electronegativities
H . + H . H : H
Hydrogen atoms Hydrogen molecule
C + 4 H C
H
H
H
H
Lewis dot representations of molecules
B.D.E
+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
H C C HH : C : : : C : H
C : : C
H H
H H
C CH
H
H
H
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
LiLiHH
FF::........HH
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1.6 Structural Formula - Shorthand in Organic Chemistry
CH3CH2CH2CH3
H H
HH H
H HH
HH
CH3CH2CH2CH2OH OH
H ClH
H
HH
H HH
H
HH
Cl
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1.6 Constitutional Isomers
H
CH
H
O C H
H
H
H C C O H
H
H
H
H
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
O NO
OH
OO
O
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1.8 Resonance Structures - Electron Delocalization
OO
O OO
O
CH3C
O
O
CH3C
O
O
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 H2C=O
Linear geometry of carbon dioxide
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1.10 Molecular dipole moments
Figure 1.7
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• Curved arrows are used to track the flow of
electrons in chemical reactions.
• Consider the reaction shown below which shows
the dissociation of AB:
1.11 Curved Arrows – Extremely Important
A B A+ + B-
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Many reactions involve both bond breaking
and bond formation. More than one arrow
may be required.
Curved Arrows to Describe a Reaction
H O + C
H
H
H
Br C
H
O
H
HH + Br-
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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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H AB .. B H A–..
+
1.13 A Brønsted-Lowry Acid-Base Reaction
A proton is transferred from the acid to the base. + +
base acid conjugate
acid
conjugate
base
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hydronium ion (Hhydronium ion (H33OO++))
HH BrBrOO
HH
HH
.... ....
HH
HH
.... OO HH BrBr––
.... ........
........
........
++
Proton Transfer from HBr to Water
basebase acidacid conjugate conjugate conjugate conjugate
acid acid base base
++ ++
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[H3O+][Br–]
[HBr]Ka =
H BrO
H
H
.. ..
H
H
.. O H Br–
.. ....
....
....
++ +
pKa = – log10 Ka
Equilibrium Constant for Proton Transfer
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H O H + H Br H O H + Br
H?
Acids and Bases: Arrow Pushing
H O H + H Br H O H + Br
H
H O H + H Br H O H + Br
H
[H3O+][Br–]
[HBr]Ka = ~ 106 for HBr, pKa = - 5.8
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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.
C
H
H
H
H C
H
H
H
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 (HI>HBr>HCl>HF) down the periodic table as
H-X bond strength decreases and conjugate base (X:- anion) size increases (basic strength of anion decreases).
strongest H—X bond weakest H—X bond
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ElectronegativityAcidity increases across periodic table as the atom attached to H gets more electronegative (HF>H2O>H2N>CH4).
least electronegative most electronegative
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Inductive EffectsElectronegative groups/atoms remote from the acidic H can effect the pKa of the acid.
pKa = 16 pKa = 11.3
CH3CH2O H CF3CH2O H
• O – H bond in CF3CH2OH is more polarized
• CF3CH2O- is stabilized by EW fluorine atoms
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Resonance Stabilization in AnionDelocalization of charge in anion (resonance) makes the anion more stable and thus the conjugate acid more acidic e.g. (CH3CO2H > CH3CH2OH).
CH3C
O
O
CH3C
O
O
CH3C
OH
O
CH3 CH2 OH CH3 CH2 O
pKa ~16
pKa ~5
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1.16 Acid-base reactions - equilibria
H Cl NaOH NaCl + H2O+
H3C
O
OHNaOH
H3C
O
ONaH2O+ +
H2OCH3ONaNaOHCH3OH + +
The equilibrium will lie to the side of the
weaker conjugate base
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1.17 Lewis acids and Lewis bases
FF33BB ++ OO
CHCH22CHCH33
CHCH22CHCH33
•••• ••••–– ++
FF33BB OO
CHCH22CHCH33
CHCH22CHCH33
••••
Lewis acidLewis acid Lewis baseLewis base
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.