There is free rotation around the C―C bond in ethane. There is an extremely high barrier to rotation around the C=C bond in in ethene. Explain.

Draw orbital pictures of the pi bonding in the following compounds:

a. CH₃COCH₃

b. HCN

Indicate the kind of molecular orbital (σ, σ*, π, or π*) that results when the two atomic orbitals are combined:

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Draw the following orbitals:

a. 3s orbital

b. 4s orbital

c. 3p orbital

A molecular orbital diagram is shown for the C―Cl bond in chloromethane. If two more electrons were added to chloromethane, where would the electrons go?

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Looking ahead in Chapter 4, we explain that molecules like CH₃⁺ are Lewis acids or electron pair acceptors. Into which orbital would the new electron pair go?

Draw the molecular orbital picture of trans-but-2-ene. Be sure to label all σ and π bonds. Is there free rotation around the C₂― C₃ bond? Why or why not?

What frequency of light would be required to excite an electron if the HOMO–LUMO energy gap was 33.9 kcal/mol (142 kJ/mol)?

How might electrons be excited from π to π* based on the molecular orbital diagram shown? [This will be relevant in Chapter 21.]

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