3-Isopropylpenta-1,3-diene, $CH X_{2} = CH - C (CH (CH X_{3}) X_{2}) = CH - CH X_{3}$ , is drawn with the methyl group on C4 and the isopropyl group on C3 on the same side of the C3=C4 bond. Choose the correct descriptor and reason.

Answer: A

On C3 compare vinyl and isopropyl. Both are

(C, C, H)

at the first sphere (vinyl has a duplicate carbon). At the next sphere the vinyl branches are

{C, H, H}

(the

CH X_{2}

carbon, with a duplicate carbon) and

{0, 0, 0}

(the duplicate), whereas the isopropyl branches are

{H, H, H}

twice. The first-ranked branch decides:

{C, H, H} > {H, H, H}

, so vinyl wins. The C4 methyl is cis to isopropyl, hence trans to vinyl:

E