In 2,3-dimethylhex-3-ene, $(CH X_{3}) X_{2} CH - C (CH X_{3}) = CH - CH X_{2} CH X_{3}$ , the methyl group on C3 and the ethyl group on C4 lie on the same side of the double bond. What is its CIP descriptor?

Answer: C

On C3 the substituents are isopropyl and methyl; isopropyl

(C, C, H)

outranks methyl

(H, H, H)

. On C4 ethyl outranks

H

. The methyl is cis to the ethyl, so isopropyl (high priority) is trans to ethyl (high priority). The high priority groups lie on opposite sides:

E

. A common slip is to call it

Z

because the two methyl-type groups are cis.