[Delian]

Hello guys could someone explain me why 2-methyl-2-butene compund doesn't appear to be cis-trans isomer? Also why 1-butene is not cis-trans isomer either? Thanks for any explanation.

[Babcock_Hall]

It is a forum rule rule (see red link above) that you must provide your thoughts before we can help you.  Drawing structures might help.

[Delian]

I wrote both formulas on paper and I think i completely understand cis-trans but these 2 exceptions. Cis is when they are at the same line and trans when they are in opposite directions (My main language is not English so it's a little tough to explain chemistry problem). However, I know why 1,2-dichloroethylene is trans isomer. Because Cl's are in the opposite directions of double bond which can't be rotated cause of pi bond.

[Babcock_Hall]

Your explanation of the difference between cis and trans is difficult to follow.  May we see your drawing of 1-butene?

[Delian]

Sure.. it's in attachment.

[Delian]

I probably know why 1-butene is not cis-trans isomer. However I have no clue why second one isn't an isomer.

[Delian]

I'm sending also 2-methyl-2-butene formula in attachment.

[Enthalpy]

Hi Delian,

I suggest that you draw formulas with representative angles rather than vertical and horizontal. Here 2-methyl-2-butene using this forum's drawing tool:

Now, do you see any difference between a cis and a trans version?

1,1-dimethyl-1-propene would have made it clearer, but the naming rules tell "find the longest chain".

Beyond that, molecules are 3D objects. It is often necessary to imagine molecules in 3D to tell what isomers exist. Symbols exist to tell on the paper what bonds plunge behind the plane and what others emerge.

[Delian]

Thanks for reply, Enthalpy. Yes, I know the difference between cis and trans. As a prove I'm sending attachment where i clearly identify whether the formula is cis-trans or not. However, in more complicated formulas sometimes I can't tell whether it's cis-trans isomer or not. I watched lots of videos about that, asked friends but thing is they can't explain it to me in person right now because I'm far away from them.

[Babcock_Hall]

@OP, In 2-methyl-2-butene, what are the two groups attached to carbon-2?

[Delian]

2 methyl groups (CH3-).

[Babcock_Hall]

So if you switched the two groups, would the resulting molecule be identical to the one you started with, or would it be different?

[Delian]

It's gonna be the same formula so it's not cis-trans. But I still can't understand why are we rotating 2 groups attached to second Carbon instead of 1 group attached to second carbon and 1 group attached to 3rd carbon? Also from attachment I have already sent I came to conclusion that you are rotating 1 group from first carbon and 1 group from second carbon not both groups on 1 carbon if you understand what I mean?

[Delian]

I'm so dumb. You CAN'T rotate groups where double bond is. I finally understand this. Thanks a lot for help and sending snack for Mr. Babcock_Hall.