Question

How many straight-chain isomers are there of hexene, `C_6H_12`?

Answer 1

Five isomers

The double bond may be between carbon 1 and 2, or 2 and 3, or 3 and 4. If the double bond is between carbon 2 and 3 or 3 and 4 there are 2 geometric isomers for each case (cis- and trans)

Answer 2

There are 5

Explanation:

Of course, there are no straight chain answers, because most bonds rotate around the `"C"`-atoms, except for the double bonds, and even those are not straight.

These bonds can be between `"C"`1=`"C"`2, `"C"`2=`"C"`3 and `"C"`3=`"C"`4.
(The others are mirror-images if you count from the other side of the chain.)

Now, with every double bond, there are two possibilities: cis or trans

If the rest of the molecule is on the same side it's called cis:

if not then it's called trans:

For the double bond between `"C"`1=`"C"`2 this doesn't make a difference, but for the other two, there are two possibilities, bringing the total to five.

• hex-`1`-ene
• cis-hex-`2`-ene
• trans-hex-`2`-ene
• cis-hex-`3`-ene
• trans-hex-`3`-ene

Another way of showing if a molecule is cis or trans is using *E-Z notation*.

(Pictures from Wiki of the two possibilities of but-2-ene)