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When to use -cis and -trans?

Posted: Sat Mar 18, 2017 2:12 pm
by 104822659
When do you use cis and trans?
Is it only when there is a double bond or can a triple bonded molecule have cis/trans?
Also, why is this molecule a trans?
Image

Thanks!

Re: When to use -cis and -trans?

Posted: Sat Mar 18, 2017 2:51 pm
by Chem_Mod
Cis and trans are used to designate the orientation around a double bond (or, around a plane sometimes). A triple bond cannot be designated cis/trans because a triple bond is linear - there are only two substituents around a triple bond and those are both linear since the triple bond itself is linear. In comparison, a double bond can have 4 substituents around it. The cis/trans designation indicates the orientation of the two groups with the highest priority.

For that molecule, the CH2CH3 and the triple bond substituent are higher in priority than the Cl & F (see priority rules). Since they're on opposite sides of the bond, the orientation of the groups around the bond is trans.

Re: When to use -cis and -trans?

Posted: Sat Mar 18, 2017 5:59 pm
by Chem_Mod
Chem_Mod wrote:Cis and trans are used to designate the orientation around a double bond (or, around a plane sometimes). A triple bond cannot be designated cis/trans because a triple bond is linear - there are only two substituents around a triple bond and those are both linear since the triple bond itself is linear. In comparison, a double bond can have 4 substituents around it. The cis/trans designation indicates the orientation of the two groups with the highest priority.

For that molecule, the CH2CH3 and the triple bond substituent are higher in priority than the Cl & F (see priority rules). Since they're on opposite sides of the bond, the orientation of the groups around the bond is trans.


Just to clarify, the priority rules for E/Z are completely different than cis/trans. Please see the post I made regarding cis/trans and E/Z for details. The "groups of interest" for cis/trans will follow the parent chain. For E/Z, it will follow the priority rules.