Transcript Lecture 30 (Slides Microsoft 97-2003)

FIGURE 11-14
Sigma (s) and pi (π) bonding in C2H4
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General Chemistry: Chapter 11
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Class examples
• 4. The ethanoic acid (“vinegar”) moleculae
and the methyl ethanoate molecule (an ester)
shown on the next slide contain C=O double
bonds. What is the hybridization of the C and
O atoms in these double bonds? (Mention
acetone, acetaldehyde, formaldehye?)
Esters
The distinctive aroma and flavor of oranges
are due in part to the ester octyl acetate,
CH3CO2CH2(CH2)6CH3
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General Chemistry: Chapter 26
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Carbon-Carbon Triple Bonds
• The C≡C triple bond is explained using an sp
hybridization scheme for C. We imagine
distributing the 4 valence electrons (again
singly) over the 2s and three 2p orbitals. One s
orbital and one p orbital are combined to form
two hybrid sp orbitals. Two p orbitals on C
(each containing a single electron) can be used
to form two pi bonds.
FIGURE 11-16
s and π bonding in C2H2
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Other Molecules with Triple Bonds
•
•
•
•
•
Carbon monoxide: C≡O
Hydrogen cyanide: H-C≡N
Methyl cyanide:
H3C-C≡N
Cyanoacetylene: H-C≡C-C≡N
Aside: The organic cyanides (or nitriles) are
found wherever people are smoking tobacco.
Many cyanoacetylenes are found in dusty
interstellar clouds (so is ethanol!).
Hybridization Summary for C Atoms
Hybridization
Scheme
sp3
sp2
sp
Types and
Number of
Covalent Bonds
4 sigma bonds
Example
Molecules
CH4, C2H6,
H3C-O-CH3
3 sigma bonds, 1 H2C=O, H2C=CH2
pi bond
2 sigma bonds, 2 H-C≡C-H, O=C=O
pi bonds
Alkenes and Alkynes
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Hybridization – Class Examples
• We will draw structures for a range of organic
molecules and determine which hybridization
scheme can be used to describe the bonding for
each C atom. These molecules will include
saturated hydrocarbons, unsaturated
hydrocarbons, alcohols, carboxylic acids,
amines, aromatic compounds…….
Molecular Orbitals and Wave
Properties of Electrons
• We’ve mentioned that atomic orbitals can
combine constructively to form a bonding
molecular orbital. In the simplest case two H
atoms are joined using a bonding molecular
orbital. The H2 molecule has lower potential
energy (or, is more stable) than the two
isolated H atoms. “Destructive” combinations
of atomic orbitals are also possible.
Molecular Orbital Theory
• Atomic orbitals are isolated on atoms.
• Molecular orbitals span two or more
atoms.
• LCAO
– Linear combination of atomic orbitals.
Ψ1 = φ1 + φ2
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Ψ2 = φ1 - φ2
General Chemistry: Chapter 11
Slide 11 of 57
Electron Density in Bonding and
Antibonding Orbitals
• Bonding orbitals – considerable electron
density between the bonded atoms.
• Non-bonding orbitals – very little electron
density between the bonded atoms
(energetically unfavourable result).
• Bonding and antibonding sigma orbitals are
represented on the next slide.
FIGURE 11-20
Formation of bonding and antibonding orbitals
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General Chemistry: Chapter 11
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FIGURE 11-21
The interaction of two hydrogen atoms according to molecular
theory
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General Chemistry: Chapter 11
Slide 14 of 57
Basic Ideas Concerning MOs
1. Number of MOs = Number of AOs.
2. Bonding (lower energy) and antibonding (higher
energy) MOs formed from AOs.
3.
e- fill the lowest energy MO first (aufbau process)
4. Maximum 2 e- per orbital (Pauli Exclusion
Principle)
5. Degenerate orbitals fill singly before they pair up
(Hund’s Rule).
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General Chemistry: Chapter 11
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Molecular Orbitals – Learning
Objectives
• 1. Construct molecular orbital diagrams for
diatomic molecules composed of elements
from the first period elements (H and He) and
the second period elements (Li, Be, B, C, N, O
F and Ne). This includes species with +ve and
-ve charges. (Eg. O2+ and CN-).
• 2. Label MOs in the MO diagram and show
their relative energies. Indicate whether MOs
are bonding or anti-bonding.
Molecular Orbitals – Learning
Objectives
• 3. Use the molecular formula (for neutral
molecules and diatomic ions) and charge to
determine the total number of electrons that we
must accommodate using the MO picture.
• 4. Distribute all of the electrons among the
available MOs – starting with the lowest
energy MOs (sound familiar?).
Molecular Orbitals – Learning
Objectives
• 5. After counting the number of electrons in
both bonding and anti-bonding orbitals
determine the bond order.
• 6. Use the MO diagram (and the number of
electrons in the various molecular orbitals) to
determine whether a molecule is diamagnetic
or paramagnetic.
Molecular Orbitals – Learning
Objectives
• 7. Understand a surprising feature of molecular
orbital theory. We can accommodate all of the
valence electrons in various molecular orbitals
for a diatomic species and end up with a bond
order of zero!
Bond Order
• Stable species have more electrons in
bonding orbitals than antibonding.
No. e- in bonding MOs - No. e- in antibonding MOs
Bond Order =
2
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Molecular Orbitals – Nomenclature:
• For the simplest atoms (H, He, Li, Be) only 1s
and 2s orbitals are occupied in the ground
electronic state. The overlap of two 1s orbitals
can only produce a sigma (σ) bond. In the H2
molecule, for example, two 1s atomic orbitals
can combine to form a σ1s bonding molecular
orbital and a σ1s* anti-bonding molecular
orbital. When 2p orbitals come into play we
can form both σ and π molecular orbitals.
Simplest Diatomics – MO Diagrams
• MO diagrams are initially a bit confusing
because they represent the formation of
chemical bonds using both a “before picture”
(showing the relative energies of the various
atomic orbitals) and an “after picture”
(showing the relative energies of the molecular
orbitals). We’ll illustrate this with the
molecules H2, He2, H2+ and He2+.
Diatomic Molecules of the First-Period
BO = (e-bond - e-antibond )/2
BOH + = (1-0)/2 = ½
2
BOH = (2-0)/2 = 1
2
BOHe + = (2-1)/2 = ½
2
FIGURE 11-22
BOHe = (2-2)/2 = 0
2
•Molecular orbital diagrams for the diatomic molecules and ions of the firstperiod elements
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General Chemistry: Chapter 11
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Class Examples
• Draw molecular orbital diagrams for Li2 and
Be2. Using the MO diagrams determine the
bond order for both molecules and, as well,
indicate from the MO diagrams whether the
molecules are diamagnetic or paramagnetic.
Molecular Orbitals of the Second
Period Elements
• First period use only 1s orbitals.
• Second period have 2s and 2p orbitals
available.
• p orbital overlap:
– End-on overlap is best – sigma bond (σ).
– Side-on overlap is good – pi bond (π).
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General Chemistry: Chapter 11
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Molecules with 2nd Period Atoms
• The simplest possible molecular orbital
diagram that one could imagine for second row
elements having 2p electrons is shown on the
next slide. This slide would necessarily apply
only to homonuclear diatomics. Note the
“symmetrical disposition” of bonding and
nonbonding orbitals.
FIGURE 11-25 (PART A)
Possible molecular orbital energy-level scheme for diatomic
molecules of the second-period elements
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MO Diagrams - Surprises
• The MO diagram presented on the previous
slide does not adequately explain all properties
of diatomic molecules formed from second
period elements. Overlap of 2p atomic orbitals
produces six MOs whose order energy order
can vary with atomic number of the bonded
atoms.
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MO Diagrams – Surprises – C2:
• Two possible MO diagrams are illustrated for
the C2 molecule on the next slide. The
presentation of MOs here is similar to that
used in drawing orbital diagrams for atoms. By
experiment we know that the C2 molecule (4
valence electrons contributed by each C atom
for a total of 8) is diamagnetic. Which of the
MO diagrams accounts for this diamagnetism?
•Experiment shows C2 to be diamagnetic,
supporting a modified energy-level diagram
Expected MO Diagram for C2
Modified MO Diagram for C2
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General Chemistry: Chapter 11
Slide 30 of 57