Chapter D, Problem D.13YT

Interpretation Introduction

Interpretation:

The atomic orbital overlap and MO energy diagrams for ${\text{CH}}_{\text{3}}{\text{CH=CHCH}}_{\text{3}}$ (Figs. 3-13 through 3-25 to be reviewed) are to be constructed, and these diagrams are to be used to determine its HOMO. Whether the answer agrees with the one shown in Figure D-11 is to be explained.

Concept introduction:

Molecular orbitals are constructed from the AOs of different atoms. When two atoms are brought close enough together (i.e., about a bond length apart), the AOs of one atom significantly overlap the AOs of the other atom, enabling them to undergo constructive and destructive interference, or mix, to produce new orbitals. The two central $\text{C}$ atoms are both ${\text{sp}}^{\text{2}}$ hybridized and each has an unhybridized $\text{p}$ orbital. The ${\text{CH}}_{\text{3}}$ carbons are ${\text{sp}}^{\text{3}}$ hybridized, and the $\text{H}$ atoms each contribute a $\text{1s}$ orbital. End-on overlap occurs in the $\text{C-H}$ and $\text{C-C}$ bonding and, therefore, results in $\text{11}$ $\text{σ}$ bonds. Side-on overlap occurs between the two $\text{p}$ orbitals of the two ${\text{sp}}^{\text{2}}$- hybridized $\text{C}$ atoms. This results in $\text{π}$ bonding and $\text{π*}$ antibonding. When the $\text{24}$ AOs overlap, $\text{24}$ MOs are produced: $\text{11}$$\text{σ}$ bonding, $\text{11}$ $\text{σ*}$ antibonding, one $\text{π}$ bonding, and one $\text{π*}$ antibonding. When the $\text{24}$ total valence electrons fill the MOs, the $\text{π}$ MO is the HOMO, and the $\text{π*}$ MO is the LUMO.