5.111 Lecture Summary #15 Readings for today: Sections 3.4, 3.5, 3.6 and 3.7 (Sections 3.4, 3.5, 3.6, 3.7, and 3.8 in

3rd ed) – Valence Bond Theory. Read for Lecture #16: Sections 6.13, 6.15, 6.16, 6.17, 6.18, and 6.20 (Sections 6.14, 6.16,

6.17, 6.18, 6.19, and 6.21 in 3rd ed) – The Enthalpy of Chemical Change. Assignment: Problem set #5 (due Friday, October 17th at noon) _______________________________________________________________________________ Topics: Valence bond theory and hybridization I. Sigma and pi bonds II. Hybridization of atomic orbitals A. sp3 hybridization B. sp2 hybridization C. sp hybridization ________________________________________________________________________________ VALENCE BOND THEORY AND HYBRIDIZATION

H

1s

H

1s In valence bond theory, bonds result from the pairing of unpaired electrons in atomic orbitals. I. SIGMA AND PI BONDS

σ (sigma) bond: cylindrically symmetric with ____ nodal plane across the bond axis.

π (pi) bond: a bond with e- density in two lobes, one on each side of the bond axis.

A pi bond has a _______________ nodal plane along the bond axis.

We can describe multiple bonds according to valence-bond theory.

• single bond: ____________________

• double bond: one σ-bond plus one ____________________

• triple bond: one σ-bond plus ___________ π-bonds

1

Applying simple VB theory results in the following prediction for methane bonding:

C2s

2px 2py

2pz

2px

++

--

H

H2pz

C

According to this model, the C is bonded to only two H-atoms with an H-C-H bond of _______°. This is NOT what is observed for methane! II. HYBRIDIZATION OF ATOMIC ORBITALS

A. sp3 hybridization

A carbon atom has four unpaired electrons available for bonding once a 2s-electron is

__________________ to an empty 2-p orbital.

2s

2px 2py2pz

C(4 valence e-s)

E

orbitals

2sp3 2sp3 2sp3 2sp3e- promotion

The sp3 hybrid orbitals are equivalent and degenerate. They differ only in their

___________________ in space.

2s 2pz 2px 2py

-+- +

-++ -

- -

+

+

+- +

four sp3 hybrid orbitals

2

In carbon, each sp3 orbital contains a single electron, allowing four bonds.

sp3

H

H

H

H

C 2sp3 2sp3 2sp3 2sp3

°

C

2sp3 1s

σ bond

HWhat provides the energy for the initial electron promotion?

_______________!

Each bond is labeled based on the bond type (σ or π) and atomic orbital composition. ____ (C______, H______) Consider ethane, C2H6.

Two bond types in ethane: ____________________ and ____________________ . Nitrogen: Electron promotion _____________ occur with nitrogen because promotion

would not increase the number of unpaired electrons available for bonding.

N-H bond description: ________________

N-atom geometry: ____________________

109.5°

C

H H

HC

H

H

Hσ bond

2sp3 2sp3 2sp3 2sp3 2sp32sp 2sp3 2sp33

2s

2px 2py2pz

E

hybrid orbitals

2sp3 2sp3 2sp3 2sp3

N(5 valence e-s)

sp3

°

H

H

H

N2sp3 2sp3 2sp3 2sp3

2sp3 1s

σ bond

N H

3

Oxygen: Electron promotion does not occur.

2s

2px 2py2pz

E

hybrid orbitals

2sp3 2sp3 2sp3 2sp3

O(6 valence e-s)

sp3

H

H

O2sp3 2sp3 2sp3 2sp3

O H

104.5°

H2O geometry: ________________

B. sp2 hybridization

sp2 hybrid orbitals form from the combination of one s-orbital and two p-orbitals.

2s 2pz 2px 2py

-+- +

-++

2py

+

-- +

+

-

+

-

3 hybrid orbitals 1 p-orbital

Boron: Boron has 3 unpaired electrons available for bonding once a 2s-electron is promoted to an empty 2-p orbital.

2s

2px 2py2pz

E

hybrid orbitals

2sp2 2sp2 2sp2

B(3 valence e-s)

electron promotion

The s-orbital and two of the p-orbitals hybridize to form ______________ sp2 orbitals. The three sp2-orbitals lie in a ________________ to minimize electron repulsion.

4

++

+

x

z

sp2

120°

B

H

H

H

2py2sp2 2sp2 2sp2

σ ( , H1s)

trigonal planar Carbon: Carbon can also form sp2 hybrid orbitals.

2s 2pz 2px 2py

-+- +

-

Ethylene (C2H4) has a C-C double bond, meaning one _______-bond and 1 _______-bond.

looking down the y-axis looking down the x-axis σ( _________, __________) π( _________, _________)

2s

2px 2py2pz

E

hybrid orbitals

2sp2 2sp2 2sp22py

C(4 valence e-s)

electron promotion

hybridize

++

2py

- +

+

+

-- -

+

++

+

x

z

sp2

120°

2py2sp2 2sp2 2sp2

trigonal planar

2py

++

y

z

+

2py

++++

+

+x

+

+z

+

2py 2py

5

++

y

z

+

2py

++ HH

++

+

+x

+

+z

+

H

H H

H

In addition to the C-C double bond, there are four C-H bonds: σ( _______, ______) Note: molecules cannot rotate around a double bond. Rotation would require breaking the pi bond. H H

H

H

Benzene (C6H6)

+

++

+

+

++

+

+

+

+

++

+

C

C

C+

+

+

C

2py

2sp2

C

C

H

H H

H

HH

+

++

+

+

+

++

+

+

+

++

+

C

C

C+

+

+

C

C

C

H

H H

H

HH

___________ σ(C2sp2, C2sp2) bonds ___________ π(C2py, C2py) bonds ___________ σ(C2sp2, H1s) bonds

C

C

C C

C

C

H H

H

HH

H

C

C

C C

C

C

H H

H

HH

H

In reality, the 6 pi-electrons are ________________ over all six carbon atoms in the benzene

molecule. Each C-C bond is a ________ bond.

6

C. sp hybridization

sp hybrid orbitals form from the combination of one s-orbital and 1 p-orbital.

2s

2px 2py2pz

E

hybrid orbitals

2sp2px 2py

C(4 valence e-s)

electron promotion

2sp

2py

+

--

+

+-

2px

-

+

2s 2pz 2px 2py

-+- +

-++

x

z + ++ +

2px

HH C C

π(2px)

x

z + ++

2px

HH C C

π(2px)

π(2py)

x

z

180�+ +2py

sp

C

+ +

2px

2py HH

σ( ______, _______ ) π( ______, _______ ) π( ______, _______ )

7