Discussion Problem
Classify the Sigma Bonds in These Molecules
In organic molecules, 2p AOs are typically used to construct pi MOs while 1s orbitals
(from hydrogen) and hybrid orbitals are typically used to construct sigma MOs. Since
hybrid orbitals are identified by an atom’s EPDs, it is possible to determine the type of
hybrid atomic orbitals for each atom by inspection of a Lewis structure. For a pair of
atoms that form a covalent bond, the hybrid AOs describe the type of sigma MO
involved. This idea is illustrated by the examples provided below.
1s + sp3 sigma MO
H
C
H
C O
H
H2C C
H O
H3C
H
N C CH3
Discussion Problem
Using a pair of MO energy diagrams, show why the acetylide anion is more stable (i.e., less
basic) than the vinyl anion. Do not draw complete diagrams; only show the parts that are
essential to answering this question. Based on what you draw, briefly explain why acetylide
anion is more stable than vinyl anion.
H
H
C
C
H
H
vinyl anion
C– 2p energy level
C 2p energy level
H 1s energy level
C– 2s energy level
C 2s energy level
C
C
acetylide anion
Discussion Problem
Partially drawn MO diagrams of CH3-NH2 and CH3OH are given below. These diagrams were constructed using hybrid
AOs for carbon, nitrogen, and oxygen. Complete the MO diagram diagrams. Follow the steps below to add MO levels for the C-N, N-H, C-O and O-H bonds.
o
Place the nonbonded level centered on N in the column labeled “C-N MOs”.
o
Place the nonbonded levels centered on O in the column labeled “C-O MOs”.
o
Assume the LUMO for CH3-NH2 is the N-H σ* level.
o
Assume the LUMO for CH3OH is the O-H σ* level.
o
Be sure the HOMO and LUMO levels for CH3-NH2 are in their correct positions relative to
those in CH3-OH
o
Be sure the C-N σ* is correctly positioned relative to the C-O σ*
Label C, N and O AO levels (i.e., give the hybrid AO type)
Label all MO levels (i.e., indicate n, a, σ, σ*, π, π*)
Add electrons to all AO and MO levels
H AOs
C-H MOs
H
H
H C
N
H
H
C AOs
C-N MOs N AOs
N-H MOs H AOs
H AOs
C-H MOs
H
H C
O H
H
C AOs
C-O MOs O AOs
O-H MOs H AO