Cis or trans in long chains

[Brooke VanSickle 1J]

I understand how to determine cis or trans in simple examples, but I don't understand how to do it with long chains such as the one shown on page 105 of the course reader, 3,6-dimethyl-3-octene. Is that cis or trans and why?

[Shannon Cavanaugh 1I]

Dr. Lavelle described it as: "are long Carbon chains on the opposite or same side (of the double bond)?"

3,6-dimethyl-3-octene is a TRANS geometric isomer because the "long Carbon chains" are the 3-Carbon chain left of the double bond and the 5-Carbon chain right of the double bond.

The double bond separates these chains on opposite sides so it is trans.

[LinaLi2E]

The 3,6-Dimethyl-3-Octene is trans. So look at the double bond, that is the middle. So the two branches from the first carbon of the double bond is CH₂CH₃ and CH₃. The two branches of the second carbon is H and CH₂CHCH₃CH₂CH₃. So we know that for cis, the priority groups are on the same side while for tans, the priority groups are on opposite sides. The priority group for the first carbon is CH₂CH₃ while the priority group for the second carbon is CH₂CHCH₃CH₂CH₃. If you notice, it is the longer chain that is attached to the carbon. These priority group are on opposite sides if you rotate the image so that the double bond is parallel to you. Thus, this molecule is a trans. Hope that makes sense!