Which of the following can exist as cis, trans isomers?

Question

2-butene
1-butene
2-methyl-2-butene
3-hexene

also, how can i draw the structural formulas?

Answer 1

2-butene:
CH3-CH=CH-CH3

1-butene:
CH2=CH-CH2-CH3

2-methyl-2-butene:
CH3C(CH3)=CHCH3, or
(CH3)2C=CHCH3

3-hexene:
CH3CH2CH=CHCH2CH3

Only 1-butene cannot exist as cis/trans isomers.

Answer 2

Start with the second question first: The number in front of the alkene is the position at which the double bond "starts".

The principle is that if the cis form is different from the trans form, you have both. They are isomers. You can't have only one.

Then for 2-butene
CH3 CH3
>C=C<
H H
This is "cis", so switching the positions of the H and CH3 on one side, we get "trans"

For 1-butene
H CH3
> C=C <
H H
switching the CH3 and H on the right doesn't give us a different compound. We can flip the molecule over to give us the same compound as shown above.

You can do the others the same way. 3-hexene is cis trans, but not 2-methyl-2-butene. Note that you can draw structures here, but it takes some doing. Some word processors have better ways of showing bonds.