CHEM 141 – Organic Chemistry I
Th, 2-12-09
1) Hybridation of Orbitals
• sp3, sp2, sp hybrid orbitals
• Geometries & bond angles of hybridized bonds
• Sigma (σ) and pi (π) bonds
• Relative bond lengths & strengths of single, double,
triple bonds
The Nature of Chemical Bonds:
Valence Bond Theory
Valence bond theory
• Bonding theory that describes a
covalent bond as resulting from
the overlap of two atomic
orbitals
• Electrons are paired in the
overlapping orbitals and are
attracted to nuclei of both
atoms, thus bonding the two
atoms together
•
•
•
H–H bond results from the
overlap of two singly occupied
hydrogen 1s orbitals
H-H bond is cylindrically
symmetrical
Bonds formed by head-on
overlap of two atomic orbitals
along a line drawn between the
nuclei are sigma (σ) bonds
The Nature of Chemical Bonds: Valence
Bond Theory
Bond strength
• H2 molecule has 436 kJ/mol less energy than the
starting 2 H atoms, the product is more stable than
the reactant and the H-H bond has a strength of 436
kJ/mol
• Conversely, the bond dissociation energy of H2 is 436
kJ/mol because it requires 436 kJ/mol of energy to
break the H2 bond
The Nature of Chemical Bonds: Valence
Bond Theory
There is an optimum distance between nuclei that leads to
maximum stability called the bond length
Bond length
• The distance between
nuclei at the minimum
energy point
• Because a covalent bond
is dynamic, like a spring,
the characteristic bond
length is the equilibrium
distance between the
nuclei of two atoms that
are bonded to each other
Hybrid Orbitals
• The Problem:
•
•
bonding by 2s and 2p atomic orbitals would
give bond angles of approximately 90°
instead we observe bond angles of
approximately 109.5°, 120°, and 180°
• A Solution
•
•
hybridization of atomic orbitals
2nd row elements use sp3, sp2, and sp hybrid
orbitals for bonding
Hybrid Orbitals
• Hybridization of orbitals (L. Pauling)
• the combination of two or more atomic orbitals forms
a new set of atomic orbitals, called hybrid orbitals
• We deal with three types of hybrid orbitals
sp3 (one s orbital + three p orbitals)
sp2 (one s orbital + two p orbitals)
sp (one s orbital + one p orbital)
• Overlap of hybrid orbitals can form two types of
bonds depending on the geometry of overlap
σ bonds are formed by “direct” overlap
π bonds are formed by “parallel” overlap
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 Hybridization of Carbon
C
2p
2s
promote
2p
2s
hybridize
C
2 sp3
H
H
C
H
4 sp3 hybrid orbitals; tetrahedral geometry
All bonds have same angle (109.5 0) & energy
H
sp3 Hybrid Orbitals
•
•
•
each sp3 hybrid orbital
has two lobes of
unequal size
the sign of the wave
function is positive in
one lobe, negative in
the other, and zero at
the nucleus
the four sp3 hybrid
orbitals are directed
toward the corners of a
regular tetrahedron at
angles of 109.5°
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
•
orbital overlap pictures of methane, ammonia,
and water
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 σ 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 carbon-hydrogen
bonds
sp2 Hybridization of Carbon
C
2p
2s
promote
2p
2s
hybridize
H
C
H
2p
H
2 sp2
C
H
3 sp2 hybrid orbitals; bond angles 120 0; trigonal planar
one remaining unhybridized p orbital
sp2 Hybrid Orbitals
•
•
the axes of the three sp2 hybrid orbitals lie in a
plane and are directed toward the corners of
an equilateral triangle
the unhybridized 2p orbital lies perpendicular
to the plane of the three hybrid orbitals
Bonding in Ethylene
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 2p atomic orbitals
• One p orbital remains nonhybridized
sp2 Hybrid Orbitals and the Structure of
Ethylene
•
σ bond in ethylene formed by head-on overlap of two sp2
hybrid orbitals
• Two non-hybridized 2p orbitals overlap sideways forming a
π bond
• Carbon-carbon double bond is shorter and stronger than
carbon-carbon single bond
Bonding in Formaldehyde
sp Hybridization of Carbon
C
2p
2s
promote
2p
2s
hybridize
2p
H
C
C
H
2 sp
2 sp hybrid orbitals; bond angle 180 0; linear
two remaining unhybridized p orbitals
sp Hybrid Orbitals
•
•
two lobes of unequal size at an angle of 180°
the unhybridized 2p orbitals are perpendicular
to each other and to the line created by the
axes of the two sp hybrid orbitals
sp Hybrid Orbitals and the
Structure of Acetylene
Acetylene
Linear
Carbon-carbon triple bond
• Six shared electrons
• Bond angles are 180º
•
•
sp hybridized orbital
A hybrid orbital derived
from the combination of
one s and one p atomic
orbital
• The two sp hybrids are
oriented at an angle of
180º to each other
• Two 2p orbitals remain
non-hybridized
•
Bonding in Acetylene, C2H2
Hybrid Orbitals
Group s Orb ital Predicted
Bond ed Hyb rid Bond
to Carb on ization
A ngles
4
sp 3
109.5°
Types of
Bond s
to Carbon
Examp le
N ame
4 sigma b on ds
HH
H-C-C-H
Ethan e
HH
2
2
sp
sp
2
120°
180°
3 sigma b on ds
and 1 p i bond
2 sigma b on ds
and 2 p i bonds
H
H
C
H
Eth ylen e
C
H
H-C C-H
Acetylene
Bond Lengths and Bond Strengths
Name
Ethane
Bond Length Bond Strength
[kJ (kcal)/mol]
(pm)
Formula
Bond
HH
C-C
sp3 -sp 3
153.2
376 (90)
C-H
sp3 -1s
111.4
422 (101)
C-C
sp2-sp2, 2p-2p
133.9
727 (174)
C-H
sp 2-1s
110.0
464(111)
C-C
C-H
sp-sp, tw o 2p-2p
121.2
109.0
966 (231)
556 (133)
H-C-C-H
HH
Ethylene
Orbital
Overlap
H
H
C C
H
H
Acetylene H-C C-H
sp-1s
Hybridization of Phosphorus
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