Lecture 29 (Slides) October 26

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Reaction of O, S and N with H Atoms
• The complete electron configurations for S could
be written as
• Sulfur 1s22s22p63px23py13pz1
• Again, we can “pair up” all electrons if S and two
H atoms combine to form H2S. The valence bond
picture suggests that all bond angles in H2O, NH3
and H2S should be 90o. This is close to the value
seen in H2S (92o) but significantly underestimates
bond angles in H2O (105o) and NH3 (107o).
FIGURE 11-3
Bonding in H2S represented by atomic orbital overlap
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The Methane “Problem”
• The ground state configurations for C can be
written as 1s22s22px12py1 Using the valence
bond picture and the concept of paired
electrons in molecular orbitals we might
expect C to react with H atoms to form CH2.
The CH2 molecule does form but is unstable (a
transient species). However, carbon “happily”
reacts with H to form the methane, CH4.
Methane and Hybridization
• By experiment, as previously discussed,
methane has a regular tetrahedral geometry –
four equal bond distances and all bond angles
of 109.5o. The regular geometry of methane
and its ability to form four bonds can be
explained using the concept of hybridization.
How have we explained carbon’s tendency to
form four bonds previously?
Methane and Hybridization – cont’d:
• In the hybridization picture we imagine
methane being formed from C and H atoms in
three steps. In the first step we take a ground
state C atom and excite one electron (from the
2s orbital) to form the lowest lying ( or first)
excited state.
• Carbon Ground State: 1s22s22px12py1
• Carbon Excited State: 1s22s12px12py12pz1
Methane and Hybridization – cont’d:
• In the second step we imagine “combining”
the single occupied 2s orbital and the three
occupied 3p orbitals in the excited to form four
equivalent sp3 hybrid orbitals (each containing
a single unpaired electron). In step three the
“hybridized C atom” reacts with four H atoms
to form a CH4 molecule. The process is
represented on the next few slides.
Hybridization of Atomic Orbitals
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FIGURE 11-6
The sp3 hybridization scheme
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FIGURE 11-7
Bonding and structure of CH4
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Organic Compounds and Structures:
An Overview
FIGURE 26-1
•Representations of the methane molecule
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Organic Chemistry
• The next two slides illustrate what starts to
happen when two or more sp3 hybridized
carbons are linked. The chemistry of carbon is
infinitely varied and organic compounds are
part of all of living things, important energy
sources, key pharmaceuticals and so on.
FIGURE 26-2
The ethane molecule C2H6
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FIGURE 26-3
The propane molecule, C3H8
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Alkanes from petroleum
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Hybridization in NH3 and H2O
• The sp3 hybridization picture can also be used
to discuss the bonding in NH3 and H2O. The
neutral N and O atoms have more valence
electrons than does C. We thus end up putting
either one lone pair of electrons (for N) or two
lone pairs of electrons (for O) into sp3 hybrid
orbitals. The following slides represent the
process for N (NH3).
Bonding in H2O and NH3
Notice that hybrid
orbitals can
accommodate lonepair electrons as well
as bonding electrons.
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FIGURE 11-8
sp3 hybrid orbitals and bonding in NH3
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Class Examples
• 1. Jumping the gun “just a bit” let’s draw
structural formulas for ethane (H3C-CH3),
methyl amine (H3C-NH2), methanol (CH3-OH)
and propane (H3C-CH2-CH3). What structural
features do these molecules have in common?
Does the “octet rule” still hold?
“Finding” sp3 Hybridized Atoms
• 2. Using the molecular structure of the
morphine molecule shown on the next slide,
find (a) an sp3 hybridized C atom, (b) an sp3
hybridized N atom and (c) an sp3 hybridized O
atom. There may be more than one example of
each. The example is a little unfair since many
“non-terminal” H atoms are not shown in this
structure.
Morphine , a very
powerful and
addictive painkiller,
can be isolated from
the opium poppy
(Papaver
somniferum).
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Hybridization in B and Be Compounds
• A hybridization scheme can be invoked for B
that involves exciting a B atom from its ground
electronic state 1s22s22px1 (say) to its first
excited state 1s22s12px12py1. The hybrid
orbitals formed here (from the combination of
a single s orbital and two p orbitals are called
sp2 hybrid orbitals. The sp2 hybridization
scheme is invoked for the BF3 molecule.
sp2 Hybrid Orbitals
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FIGURE 11-9
The sp2 hybridization scheme
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sp Hybridization
• sp hybridization is important for molecules
such as H-C≡N and H-C≡C-H. Acetylene
(ethyne) is the first member of an important
series of organic compounds, the alkynes.
sp Hybridization
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FIGURE 11-10
The sp hybridization scheme
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Class Example
• 3. The sp3 hybridization scheme can be
invoked to explain the bonding in the silane
(SiH4) molecule. What atomic orbitals on
silicon would be used to construct hybrid
orbitals?
Covalent Bonds – Orbital Overlap –
Sigma and Pi Bonds
• The formation of both sigma bonds (σ bonds)
and pi bonds (π bonds) is likely familiar.
• Sigma bonds are formed (for a pair of atoms)
by the overlap of atomic orbitals “pointing”
towards, in each case, the other bonded atom.
We can use s and p orbitals (etc) to form sigma
bonds.
Covalent Bonds – Orbital Overlap –
Sigma and Pi Bonds – cont’d:
• In sigma bonds the two atomic orbitals used to
“construct” the molecular orbital overlap in the
most spatially “direct” manner possible.
• The overlap of a H 1s orbital and S 3p orbitals
is shown on the next slide.
• As an aside, one can see that it difficult to
“point” an s orbital in any direction. Why?
FIGURE 11-3
Bonding in H2S represented by atomic orbital
overlap
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Pi Bonds
• Pi (π) bonds are formed when the bonding
electron pair is placed in a molecular orbital
formed (frequently) by p orbitals on adjacent
atoms overlapping. The p orbitals on the
bonded atoms are oriented perpendicular to the
internuclear axis (which makes orbital overlap
slightly less favourable).
Carbon-Carbon Double Bonds
• The carbon-carbon double bonds in common
organic molecules are comprised of one σ
bond and one π bond. The simplest molecule
of this type is ethylene, H2C=CH2. Ethylene is
a planar symmetric molecule (four identical CH bonds and all bond angles near 120o).
• Aside: Hydrocarbons containing C=C double
bonds are called unsaturated. They can react
with H2 to from saturated hydrocarbons.
Carbon-Carbon Double Bonds –
cont’d:
• In saturated hydrocarbons (such as propane
H3C-CH2-CH3 the bonding behaviour of all C
atoms is well explained using the sp3
hybridization scheme. In many unsaturated
hydrocarbons the bonding of C atoms joined
by double bonds is rationalized using an sp2
hybridization scheme.
Carbon-Carbon Double Bonds –
cont’d:
• For the sp3 hybridization scheme (for carbon!)
we imagined distributing four valence
electrons among a 2s and the three 2p atomic
orbitals and “scrambling” these orbitals
together to form four hybrid orbitals. For the
sp2 hybridization scheme we will “scramble”
the single 2s orbital and two 2p orbitals to
form three hybrid sp2 orbitals. A single 2p
orbital remaining is used to from a pi bond.
Multiple Covalent Bonds
Bonding in C2H4
H
• Ethylene has a double bond in its Lewis structure.
C
• VSEPR says trigonal planar at carbon.
H
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H
C
H
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Bonding in Ethylene – an Information
Packed Slide!
• The next slide contains lots of information.
• Upper left corner – a representation (for C)
showing the structure and disposition in space
of the three hybrid sp2 orbitals and the “left
over” carbon p atomic orbital.
• Upper right corner – picture showing how the
five sigma bonds in ethylene are formed using
four H 1s orbitals and six carbon sp2 orbitals.
Bonding in Ethylene – an Information
Packed Slide – cont’d!
• Lower left corner. The sigma bonds are in
place and the p orbitals on the two C atoms
have “not yet” overlapped to from a pi bond.
In fact the p orbitals have been drawn slightly
“smaller than life” for clarity.
• Bottom center and right. Two representations
of the pi bond in ethylene. Note the electron
density well away from the C-C internuclear
axis.
FIGURE 11-14
Sigma (s) and pi (π) bonding in C2H4
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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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