Question

Which compound does not exist 1,2,3,4,5,6,7-octaheptaene, cis-2-butane, trans-3-hexene, 1-propene, or cis-4-decene?

Answer 1

Cis-2-butane doesn't exist.

Double bonds cannot freely rotate, they are fixed. The double bond cannot twist, so cis trans isomers can be formed. This is from the rotations of the bonds. Since a double bonded molecule can't be freely rotated, there can be two isomers formed.

In the picture, the chlorine atom is on either side of the rotation, forming two different molecules, because the bond cannot rotate.

However, the molecule is an alkane, it has no double bonds between carbons. Single bonds can rotate freely, so the molecule cannot form cis or trans isomers, since you could just rotate the bond to make either, so its just the same molecule.