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 1

Transcript of Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture...

Page 1: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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Page 2: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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

Page 3: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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)

Page 4: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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?

Page 5: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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

Page 6: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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.

Page 7: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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

Page 8: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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)

Page 9: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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

Page 10: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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

Page 11: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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.

Page 12: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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Use VSEPR to determine shape of hybrid orbitals: the electron pairs will get as far from each other as

possible.

Page 13: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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

Page 14: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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

Page 15: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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

Page 17: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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Orbital Theory of Bonding explains:

Why rotation about double bond does not occur

Page 18: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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.

Page 19: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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

Page 20: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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 —→

Page 21: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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

Page 22: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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

Page 23: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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

Page 24: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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

Page 25: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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

Page 26: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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

Page 27: Covalent Bonding What is covalent bonding?courses.chem.psu.edu/chem110spring/lecture notes/pdfs/8_Ch7_3.pdfValence Bond Theory ... this overlap of orbitals is a covalent bond. Molecular

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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