Page 61 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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π *
CHAPTER 1
Chemical Bonding
and Molecular Structure
C 2p
C 2p z z

π

The orbitals that remain are the four hydrogen 1s, two carbon 1s, and four carbon
2p orbitals. All lie in the molecular plane. The combinations using the carbon 2s
and hydrogen 1s orbitals can take only two forms that meet the molecular symmetry
requirements. One, , is bonding between all atoms, whereas is antibonding between
∗
all nearest-neighbor atoms. No other combination corresponds to the symmetry of the
ethene molecule.

σ σ ∗

Let us next consider the interaction of 2p with the four hydrogen 1s orbitals.
y
There are four possibilities that conform to the molecular symmetry.

A B C D

Orbital A is bonding between all nearest-neighbor atoms, whereas B is bonding within
the CH units but antibonding with respect to the two carbons. The orbital labeled
2
C is C−C bonding but antibonding with respect to the hydrogens. Finally, orbital D
is antibonding with respect to all nearest-neighbor atoms. Similarly, the 2p orbitals
x
must be considered. Again, four possible combinations arise. Note that the nature of
the overlap of the 2p orbitals is different from the 2p case, so the two sets of MOs
y
x
have different energies.

E F G H

The final problem in construction of a qualitative MO diagram for ethene
is the relative placement of the orbitals. There are some useful guidelines. The
relationship between the relative energy and the number of nodes has already
been mentioned. The more nodes, the higher the energy of the orbital. Since
-type interactions are normally weaker than -type, we expect the separation
between and ∗ to be greater than between and . Within the sets
∗