Now, there's two different naming conventions. One is the simpler naming convention, and it works when we have one functional group on each of the carbons in the double bond. And then in the next two videos, we'll talk about the slightly more advanced naming scheme that will work when we have more than one functional group.
So the first way to name it is if you have your functional groups. So let me circle the functional groups. So in this top one, our functional groups are on opposite sides. They're apart. They're on opposite sides of the carbon-carbon double bond. This is on the top, This is on the bottom. In this molecule right here, our functional groups are on the same side. So when our functional groups are on different sides, we could call it trans.
We could call it trans, literally, I believe, Latin for, apart, or Latin for opposite. I'm not a Latin scholar, so forgive me. But the functional groups are apart, so we call it trans-butene. The other convention to use comes from the German forepart and there we call it entgegen. And instead of writing out entgegen, we write E in parentheses. So we would call them-- let me just use different colors since this molecule down here is blue. So we would call this E -butene. These mean the same thing.
E -butene. But this is now kind of the standard convention. We'll see this notation where you use entgegen, or E , is actually more powerful. Specifically, the relationship between the two molecules above is that of diastereomers: stereoisomers which are not mirror images of each other. These two molecules have different physical properties — different boiling points, melting points, reactivities, spectral characteristics and so on. We apply this to two stereoisomers which are non-superimposable mirror images of each other.
We use the terms cis- and trans — to denote the relative configuration of two groups to each other in situations where there is restricted rotation. Bottom line: syn and anti forms can generally be interconverted through bond rotation: cis and trans forms cannot.
In 1,2-dichlorocyclopentane we saw that C-1 and C-2 each had non-identical substituents H and Cl above and below the ring, and they each had at least one substituent in common in fact they have two substituents in common: H and Cl. They also have at least one substituent in common H. So we can refer to cis ethylmethylcyclohexane as the isomer where the two hydrogens are pointing in the same direction, and trans where they point in opposite directions.
These are enantiomers , by the way. In that case, cis — and trans- is redundant. This stands in contrast to conventional sigma bonds single bonds in acyclic molecules, where free rotation is possible: witness 1,2-dichloroethane below left.
We can also use the cis — trans nomenclature to distinguish isomers such as 2-methylhexene above right. In the cis isomer, the two hydrogens are on the same side of the pi bond, and in the trans isomer, the two hydrogens are on the opposite side of the bond. As with rings, the minimum requirement for cis-trans isomerism in alkenes is that each carbon is bonded to two different groups, and that the two carbons have at least one substituent in common.
A quick digression: one consequence of our newfound appreciation of geometrical isomerism is that many simple-sounding molecule names are actually ambiguous.
To nail down the specific molecule, we need to specify cis — or trans — 3-hexene. Note that 1-hexene is still OK, since the 1-position of 1-hexene is attached to two identical groups hydrogens and thus no cis — trans isomers are possible. In reality, trans -cyclohexene is impossibly strained. Try kissing yourself on the tailbone. That will give you some idea of the strain involved in trying to accommodate a trans — double bond in a six membered ring.
At ring sizes of 8 and above, we do need to put a cis — or trans- in the name, because the trans — isomer becomes feasible. Imagine trying to kiss yourself on the tailbone if you had the neck of a giraffe: suddenly not impossible! We saw that cis and trans fails in rings when the two carbons lacked a common substituent. It also fails for alkenes under these circumstances.
On the left, the chlorine is cis to Br and trans to F. How do we decide? Note: they are not necessarily the same and do not always correlate: see footnote for an example of a cis alkene which is E. Whatever works for you.
As with chiral centers, ranking according to atomic number can result in ties if we restrict ourselves merely to the atoms directly attached to the pi bonds. For instance, the alkene below presents us with a dilemma: one of the carbons of the alkene is attached to two carbon atoms. So how do we determine priorities in this case.
How do we break ties? In the case of ties, we must apply the method of dots. Dots are handy placeholders which is why I like to use this method. In this case each pi bond is designated by a number with its own separate E or Z configuration.
Furthermore, it only gives relative configurations. Written by : Dr. Organic Chemistry 2nd Edition. General Chemistry. Sofia: Kliment Ohridski. Organic Chemistry. User assumes all risk of use, damage, or injury. You agree that we have no liability for any damages.
What is Cis? What is Trans? Polarity and boiling point of Cis and Trans Cis: The polarity causes increased intermolecular forces, which result in an increase of the boiling point. Symmetry and melting point of Cis and Trans Cis: The cis isomers are less symmetrical and have a lower melting point, compared to the trans isomers. Stability and solubility of Cis and Trans Cis: In acyclic systems, the cis isomers are more unstable than trans isomers. Summary Cis Vs. Trans: Cis-trans isomerism consists in the possibility of placing substituent groups on one or on different sides of a double bond plane or a non-aromatic cycle.
They are found both among organic and inorganic compounds. In the cis isomer, the substituent groups are placed on one side, while in the trans isomer, the substituent groups are placed on different sides of a double bond plane or a non-aromatic cycle.
The polarity causes increased intermolecular forces, which results in an increase of the boiling point. The cis isomers are more polar than the trans isomers and have a higher boiling point. The cis isomers are less symmetrical and have a lower melting point, compared to the trans isomers.
In acyclic systems, the cis isomers are more unstable than trans isomers. Author Recent Posts. Mariam has more than 10 years of professional experience in scientific research and environmental consultancy. She has worked within non-profit, profit, and academic environment, and consulted business clients and competent authorities. Her main professional interests are in the area of: Scientific research; Web content writing; Environmental consultancy.
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