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1.6 sp3 Hybrid Orbitals and the Structure
of Methane
Carbon has four valence electrons (2s22p2) that form four
bonds
Methane CH4
•
All four carbon-hydrogen bonds in methane are identical and are spatially
oriented toward the corners of a regular tetrahedron
sp3 hybrid orbitals
•
•
A hybrid orbital derived from the combination of an s atomic orbital with
three p atomic orbitals
Linus Pauling (1931) showed mathematically how s orbitals and p
orbitals on an atom can combine, hybridize, to form four equivalent
atomic orbitals with tetrahedral orientation called sp3 hybrids
sp3 Hybrid Orbitals and the Structure of
Methane
Two lobes of a p orbital have different algebraic signs (+ and -)
in corresponding wave functions
•
When a p orbital hybridizes with an s orbital
• Positive p lobe adds to s orbital generating larger lobe of sp3 hybrid
• Negative p lobe subtracts from s orbital generating smaller lobe of
sp3 hybrid
• Resulting asymmetrical sp3 hybrid is strongly oriented in one
direction
• Larger lobe of sp3 hybrid can overlap more effectively with an orbital
from another atom forming much stronger bonds than s or p orbitals
sp3 Hybrid Orbitals and the Structure of
Methane
When each of the four identical sp3 hybrid orbitals of carbon
overlaps with the 1s orbital of a hydrogen atom, four
identical C-H bonds are formed and methane results
• Each C-H bond in methane has a strength of 436 kJ/mol and a length of
109 pm
•
The four C-H bonds of methane have a specific geometry
•
The angle formed between two adjacent bonds describes a
characteristic bond angle
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1.7 sp3 Hybrid Orbitals and the Structure
of Ethane
Orbital hybridization accounts for the bonding together of carbon
atoms into chains and rings
Ethane C2H6
• Tetrahedral
• Bond angles are near 109.5º
• Carbon-carbon single bond
• Formed by s overlap of sp3
hybrids from each carbon
• The remaining sp3 hybrids of
each carbon overlap with 1s
orbitals of three hydrogen
atoms to form six carbonhydrogen bonds
1.8 sp2 Hybrid Orbitals and the Structure
of Ethylene
Ethylene C2H4
• Carbon-carbon double bond
•
Four shared electrons
• Planar (flat)
• Bond angles 120º
sp2 hybrid orbitals
• A hybrid orbital derived by
combination of an s atomic
orbital with two 2p atomic
orbitals
• One p orbital remains nonhybridized
sp2 Hybrid Orbitals and the Structure of
Ethylene
Bonding in Ethylene
• s bond in ethylene formed by head-on overlap of two sp2 hybrid
orbitals
Two non-hybridized 2p orbitals overlap sideways forming a p
bond
• Carbon-carbon double bond is shorter and stronger than carboncarbon single bond
•
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Worked Example 1.2
Predicting the Structures of Simple Molecules
from Their Formulas
Commonly used in biology as a tissue preservative,
formaldehyde, CH2O, contains a carbon-oxygen
double bond. Draw the line-bond structure of
formaldehyde, and indicate the hybridization of the
carbon atom.
1.9 sp Hybrid Orbitals and the Structure
of Acetylene
Acetylene C2H2
•
•
Linear
Carbon-carbon triple bond
•
Bond angles are 180º
•
Six shared electrons
sp hybrid orbital
•
A hybrid orbital derived from
the combination of one s and
one p atomic orbital
• The two sp hybrids are
separated by an angle of 180º
• Two 2p orbitals remain nonhybridized
sp Hybrid Orbitals and the Structure of
Acetylene
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1.10 Hybridization of Nitrogen, Oxygen,
Phosphorus, and Sulfur
Elements other than carbon form covalent bonds using hybrid
orbitals
Nitrogen
•
Methylamine CH3NH2
•
•
•
Organic derivative of ammonia and the substance responsible for the
odor of rotting fish
Bond angles are close to the 109.5º tetrahedral angle found in
methane
Nitrogen hybridizes to form four sp3 orbitals
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• One of the four sp orbitals is occupied by two nonbonding
electrons
Hybridization of Nitrogen, Oxygen,
Phosphorus, and Sulfur
Oxygen
• Methanol CH3OH
•
•
•
Methyl alcohol
Bonds are close to the109.5º tetrahedral angle
Two of the four sp3 hybrid orbitals on oxygen are occupied by
nonbonding electron lone pairs
Hybridization of Nitrogen, Oxygen,
Phosphorus, and Sulfur
Phosphorus
• Most commonly encountered in biological molecules in
organophosphates
• compounds that contain a phosphorus atom bonded to four oxygens
with one of the oxygens also bonded to carbon
• Methyl phosphate CH3OPO32• sp3 hybrid orbitals on phosphorus
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Hybridization of Nitrogen, Oxygen,
Phosphorus, and Sulfur
Sulfur
• Commonly encountered in biological molecules
• Thiols
• Have a sulfur atom bonded to one hydrogen and one
carbon
• Sulfides
• Have a sulfur atom bonded to
two carbons
• Methanethiol CH3SH
• Produced by some bacteria
• Simplest example of a thiol
• sp3 hybridization
• Dimethyl Sulfide (CH3)2S
• Simplest example of a sulfide
• sp3 hybridization
1.11 The Nature of Chemical Bonds:
Molecular Orbital Theory
Molecular Orbital Theory (MO)
• A description of covalent bond formation as resulting from a
mathematical combination of atomic orbitals (wave functions) to
form molecular orbitals
•
Additive combination of two 1s orbitals
•
•
Leads to formation of a low energy s bonding MO
• Egg shaped
Subtractive combination of two 1s orbitals
•
Leads to formation of a high energy s* antibonding MO
•
Elongated dumbbell shaped with a node between the nuclei
MO diagram
for molecular
hydrogen
The Nature of Chemical Bonds: Molecular
Orbital Theory
•
•
•
Additive combination of two 2p orbitals
• Leads to formation of a low energy p bonding MO
• The p bonding MO is formed by combining p orbital lobes with the
same algebraic sign
• No node between nuclei
Subtractive combination of two 2p orbitals
• Leads to formation of a high energy p* antibonding MO
• The p* antibonding MO is formed by combining p orbital lobes with
different algebraic signs
• Node between nuclei
Only the bonding p MO is occupied in ethylene
MO diagram
of C-C p bond
in ethylene
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1.12 Drawing Chemical Structures
Condensed Structures
• A shorthand way of writing structures in which carbon-
hydrogen and carbon-carbon bonds are understood rather
than explicitly shown
• 2-Methylbutane
Drawing Chemical Structures
Skeletal Structure (Line-Bond Structure)
•
A shorthand way of writing structures in which carbon atoms
are assumed to be at each intersection of two lines (bonds)
and at the end of each line
Three rules:
1. Carbon atoms not usually
shown, they are assumed
2. Hydrogen atoms bonded
to carbon are assumed
3. Atoms other than carbon
and hydrogen are shown
Note: Sometimes the writing order of
atoms is inverted to make bonding
connections clearer
Drawing Chemical Structures
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