covalent bonding what is covalent bonding?courses.chem.psu.edu/chem110spring/lecture...
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Mary J. Bojan Chem 110
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
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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
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Covalent Bonding = overlap of atomic orbitals to form a new orbital
H + H → H2
H2 molecule
Covalent bonding =
H (1s) H (1s)
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H + H → H2
Two forces operating: •
• balance of forces a bond length (0.74 Å for H2)
Why does a covalent
bond form?
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There are two types of bonds. σ-bond Ø results from Ø electron density is
π-bond Ø results from Ø electron density is
pp
Two p-orbitals
Examples:
s-s s-p
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Overlap of atomic orbitals does not adequately explain bonding in molecules.
Using only unpaired subshell electrons: Expect:
The molecule would not have an octet on carbon.
Carbon ground-state: (1s2)2s22p2 1s of H 2p of C
Imagine how bonding in CH4 might occur.
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Propose the following process for bonding in CH4:
1. Promote electrons on C
2. hybridization
Four atomic orbitals mix to form four hybrid orbitals 2s + 3 × 2p → 4 × sp3
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Propose the following process for bonding in CH4:
3. Bond formation: _______________________ Form 4 C—H bonds by overlapping each hybrid sp3 orbital with an 1s orbital of hydrogen.
σ-bond formation
The new bonds are 109o apart. One of the four bonds formed by overlap of an sp3 orbital with a hydrogen 1s orbital
NOTE: start with four atomic orbitals s px py pz
end up with four hybrid orbitals 4 sp3
Hybrid orbitals:
combinations of atomic orbitals (on one atom).
better for bonding (more directed)
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Four atomic orbitals mix to form
four hybrid orbitals
The notation means that each hybrid is composed of 1/4 s and 3/4 p orbitals. Orbitals on centered on the atom from which they originate
1 x s + 3 x p = 4 x sp3
Bond formation is the result of overlap of H (1s) orbitals with the 4 hybrid orbitals on C.
sp3 Hybrid Orbitals
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sp and sp2 Hybrid Orbitals
1 x s + 1 x p 2 x sp
Three atomic orbitals mix to form three hybrid orbitals
Two atomic orbitals mix to form two hybrid orbitals
1 x s + 2 x p 3 x sp2
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Summary
Two atomic orbitals mix to form two hybrid orbitals 1 x s + 1 x p 2 x sp Three atomic orbitals mix to form three hybrid orbitals 1 x s + 2 x p 3 x sp2 Four atomic orbitals mix to form four hybrid orbitals 1 x s + 3 x p 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
Problem: Can’t use atomic orbitals to describe bonding in molecules Solution: make molecular orbitals by mixing atomic orbitals
(call them hybrid orbitals)
Each hybrid orbital can accommodate 1 pair of electrons.
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Use VSEPR to determine shape of hybrid orbitals: the electron pairs will get as far from each other as
possible.
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The hybridization scheme can be deduced from the electron-pair geometry of the
molecule.
Number of electron
pairs
Atomic orbitals used
Hybrid type formed
Electron-pair geometry
Examples
2 s, p two sp linear BeF2, HgCl2
3 s, p, p three sp2 trigonal planar BF3, SO3, CO32−
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−
Summary of hybridization types
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Multiple Bonds: example ethylene shape about C: hybrid orbitals on C _____ bond angles _______
C CH
H
H
H
Only need three hybrid orbitals (3 electron domains)
Approach hybridization of C with the end in mind
We can use the concepts of hybrid orbitals to explain bonding situations like multiple bonds and resonance.
One s and two p atomic orbitals combine to form 3
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Multiple Bonds
One C—C and two C—H bonds (on each carbon) are
formed using sp2 orbitals on carbons. ( σ- bonds) C C
H
H
H
H
Note: there is an electron in the p-orbital
These p-orbitals can overlap, sideways: π-bond
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Orbital Theory of Bonding explains:
Why double bonds occur frequently with C, N, and O but not with larger molecules
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Orbital Theory of Bonding explains:
Why rotation about double bond does not occur
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N and O have singly occupied p-orbitals
Orbital Theory of Bonding explains resonance
e- pair geometry: hybrid orbitals on N and O are
Recall: resonance is a BLENDING of the resonance structures… REAL structure is a blend of the three Lewis
structures.
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All of the p-orbitals overlap at the same time: this is called delocalized π-bonding.
Molecules with resonance structures have delocalized π bonding
Delocalized bonding brings added stability to a molecule.
Difference between localized and delocalized π bonding
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Demonstration of stability due to resonance Same reaction: hydrocarbon + Br2
Colorless red
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ALKANE Heptane + Br2 —→ ALKENE (and ALKYNES) 2-pentene + Br2 —→ AROMATIC toluene + Br2 —→
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Delocalized π bonding explains the 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
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Summary of Covalent bonding 1. Draw Lewis Structure 2. Use VSEPR to determine shape
Ø e- pair geometry Ø 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: Ø Each single bond = covalent bond
= σbond Ø A covalent bond forms when orbitals overlap.
Ø σ-bond: head-on overlap Ø π-bond: sidewise overlap
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Determine the hybrid orbitals on the nitrogen in angle 1 and the C in angle 2.
Angle #1 Angle #2 A. sp3 sp2 B. sp sp2 C. sp2 sp3 D. sp2 sp E. sp3 sp3
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Other Molecules Method: use knowledge of molecular geometry to deduce the shapes (and names) of hybrid orbitals.
Hybridization scheme
Structure (Electron pairs)
deduce
four sp3 hybrids lone pair in an sp3 hybrid trigonal pyramidal
NH3 tetrahedral
NH H
H
N
H
HH
four sp3 hybrids two lone pairs in sp3 hybrids
bent H2O tetrahedral
OHH
O
H
H
hybridization? only three bonding pairs trigonal planar
BF3 Trigonal planar
BF
F
F
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Bonding in CH4
Expect: "CH4" molecule BUT two different kinds of bonding one s-s bond, and three s-p bonds 90o bond angles.
If all subshell electrons participated in bonding: Carbon "bonding-ready": (1s2) 2s12px
12py12pz
1
Such molecule would have an octet on carbon Real molecule is tetrahedral 109.5o bond angles, all bonds equal
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Addition Reaction
For Alkynes
CH3C C CH3 + 2HCl
Double and triple bonds are reactive: addition reactions typically occur at room temperature.
For Alkenes
CH3CH=CH2 + Br2
or CH3CH=CHCH3 + HX where X = Cl, Br, I, OH, etc.
C C C H
Br
H
Br
H
H
H
H
C C C C
H
H
X
H
H
H
H H
H
H
C C C C
H
H
Cl
Cl
H
H
H H
H
H
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Reactivity of Hydrocarbons • Same reaction: • hydrocarbon + Br2 • Colorless red
• ALKANE
• Heptane + Br2
• ALKENE (and ALKYNES)
• 2-pentene + Br2
• AROMATIC
• toluene + + Br2
CH3