Download Covalent Bonding What is covalent bonding? Hybrid Orbital Formation

Covalent Bonding
What is covalent bonding?
Covalent Bonds: overlap of orbitals
σ-bond
π-bond
Molecular Orbitals
Hybrid Orbital Formation
Shapes of Hybrid Orbitals
Hybrid orbitals and Multiple Bonds
resonance structures
Mary J. Bojan
Chem 110
1
Molecular Orbitals
• Lewis structures: accounting for bonding and lone-pair electrons
(where are the electrons?)
• VSEPR: Electron-pair structure, spatial distribution of electrons (3D)
How are bonds made?
We know electron distribution in atoms: atomic orbitals: (s, p, d …)
What is the electron distribution in molecules?
Two models:
Valence Bond Theory
Valence orbitals on one atom overlap with valence orbitals on
another atom: this overlap of orbitals is a covalent bond.
Molecular Orbital Theory Covered in Chem 112
Mary J. Bojan
Chem 110
2
Covalent Bonding
H + H → H2
H (1s)
H (1s)
H2 molecule
Covalent bonding:
Mary J. Bojan
Chem 110
3
H + H → H2
Two forces operating:
• 
• 
balance of forces  bond length (0.74 Å for H2)
Mary J. Bojan
Chem 110
4
Bond Types
σ-bond
•  results from
•  electron density is
Examples:
s-s
s-p
pp
π-bond
• 
• 
• 
results from
electron density is
Two p-orbitals
Mary J. Bojan
Chem 110
5
Bonding in CH4
Carbon ground-state: (1s2)2s22p2
1s of H
2p of C
Using only unpaired subshell electrons:
Expect:
The molecule would not have an octet on carbon.
Mary J. Bojan
Chem 110
6
Orbital Hybridization
1. 
Promote
electrons on C
1s
2.
2s
2p
1s
2s
2p
hybridization
Four atomic orbitals (2s + 3 × 2p) mix to form
four hybrid orbitals (4 × sp3)
shake well
1s
3. 
2s
2p
1s
sp3
Bond formation: _______________________
Form 4 C⎯H bonds by overlapping each hybrid
sp3 orbital with an 1s orbital of hydrogen.
σ-bond formation
One of the four bonds formed
by overlap of an sp3 orbital
with a hydrogen 1s orbital
The new bonds are 109o apart.
Mary J. Bojan
Chem 110
7
Orbital Hybridization
NOTE: start with four atomic orbitals
s
px
py
pz
end up with four hybrid orbitals
4 sp3
The notation means that each hybrid is composed of
1/4 s and 3/4 p orbitals.
Hybrid orbitals:
combinations of atomic orbitals (on one atom).
better for bonding (more directed)
Mary J. Bojan
Chem 110
8
sp3 Hybrid Orbitals
1xs+3xp
= 4 x sp3
Four atomic orbitals
mix to form
four hybrid orbitals
Mary J. Bojan
Chem 110
9
sp and sp2 Hybrid Orbitals
1xs+1xp
2 x sp
Two atomic orbitals
mix to form two
hybrid orbitals
1xs+2xp
3 x sp2
Three atomic orbitals
mix to form three
hybrid orbitals
Mary J. Bojan
Chem 110
10
Summary
Problem: Can’t use atomic orbitals to describe bonding in molecules
Solution: make molecular orbitals by mixing atomic orbitals (call them
hybrid orbitals)
Two atomic orbitals mix to form two hybrid orbitals
1xs+1xp
2 x sp
Three atomic orbitals mix to form three hybrid orbitals
1xs+2xp
3 x sp2
Four atomic orbitals mix to form four hybrid orbitals
1xs+3xp
4 x sp3
Five atomic orbitals mix to form five hybrid orbitals
1 x s + 3 x p +1 x d
5 x sp3d
Six atomic orbitals mix to form six hybrid orbitals
1 x s + 3 x p +2 x d
6 x sp3d2
Each hybrid
orbital can
accommodate 1
pair of electrons.
Use VSEPR to determine shape of hybrid orbitals: the electron pairs will get
as far from each other as possible.
Mary J. Bojan
Chem 110
11
Summary of hybridization types
The hybridization scheme can be deduced from the
electron-pair geometry of the molecule.
Number of
electron
pairs
Atomic
orbitals used
Hybrid type
formed
2
s, p
two sp
linear
3
s, p, p
three sp2
trigonal planar
4
s, p, p, p
four sp3
tetrahedral
CH4, NH3, H2O, NH4+
5
s, p, p, p, d
five sp3d
trigonal
bipyramidal
PF5, SF4, BrF3
6
s, p, p, p, d, d
six sp3d2
octahedral
SF6, ClF5, XeF4, PF6−
Mary J. Bojan
Chem 110
Electron-pair
geometry
Examples
BeF2, HgCl2
BF3, SO3, CO32−
12
Multiple Bonds
H
ethylene: shape about C:
hybrid orbitals on C _____
bond angles _______
H
C
C
H
H
One s and two p atomic orbitals combine to form 3 sp2 hybrid orbitals.
One C⎯C and two C⎯H bonds (on each carbon) are formed using sp2
orbitals on carbons. ( σ- bonds)
Mary J. Bojan
Chem 110
13
Multiple bonds
These p-orbitals can overlap, sideways: π-bond
Mary J. Bojan
Chem 110
14
Orbital Theory of Bonding explains:
•  Why rotation about double
bond does not occur
• 
Why double bonds occur
frequently with C, N, and O
but not with larger molecules
Mary J. Bojan
Chem 110
15
Delocalized Orbitals
e- pair geometry:
hybrid orbitals
on N and O are
N and O have singly
occupied p-orbitals
Difference between localized and
delocalized π bonding
Delocalized bonding brings added
stability to a molecule.
Molecules with resonance
structures have delocalized π
bonding
Mary J. Bojan
Chem 110
16
Reactivity of Hydrocarbons
Same reaction:
hydrocarbon +
Colorless
Br2
red
ALKANE
Heptane + Br2 ⎯→
ALKENE (and ALKYNES)
2-pentene + Br2 ⎯→
AROMATIC
toluene + Br2 ⎯→
CH3
Mary J. Bojan
Chem 110
17
Stability of aromatic hydrocarbons
Alkene + Br2 reacts readily
Aromatic + Br2: no reaction
•  π bonds of alkenes are very reactive toward addition.
(π bonds in alkynes even more so.)
•  π-bonds in benzene are NOT reactive due to the extra
stability of delocalized π system
Mary J. Bojan
Chem 110
18
Summary of Covalent bonding
1.  Draw Lewis Structure
2.  Use VSEPR to determine shape
  e- pair geometry
  molecular geometry
3.  What hybrid orbitals are involved in bonding?
Determined by electron pair geometry.
(Know the shapes of the hybrid orbitals.)
4. Is the molecule polar?
Determined by molecular geometry.
Remember:
  Each single bond
= covalent bond
= σ bond
  A covalent bond forms when orbitals overlap.
  σ-bond: head-on overlap
  π-bond: sidewise overlap
Mary J. Bojan
Chem 110
19
Determine the hybrid orbitals on the nitrogen in
angle 1 and the C in angle 2.
A.
B.
C.
D.
E.
Mary J. Bojan
Angle #1
sp3
sp
sp2
sp2
sp3
Angle #2
sp2
sp2
sp3
sp
sp3
Chem 110
20