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

e. CH₃CH=C=CHCH₃

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

f. CH₃CH=NCH=C=O

In this, and previous, chapters, we have seen 1,2-alkyl and 1,2-hydride shifts. If both are possible, as in the carbocation shown, which would you expect to occur? Explain your answer.

Which type of molecular orbital is typically represented by a boundary-surface diagram showing two lobes with a nodal plane between them along the internuclear axis?