E and Z Notation For Alkenes (+ Cis/Trans)
Last updated: November 15th, 2022 |
E and Z Notation For Alkenes
- Unlike C–C single bonds, C–C double bonds can’t undergo rotation without breaking the pi bond
- One consequence of this is geometric isomerism – the existence of alkene stereoisomers that differ solely in how their substituents are arranged in space about the double bond
- In simple cases where there are two identical substituents on each carbon of the alkene, we can use cis– and trans– to designate the isomers where those substituents are on the same and opposite sides of the double bond, respectively.
- For geometric isomers that lack two identical substituents, we rank the two substituents on each end of the double bond according to the Cahn-Ingold-Prelog (CIP) rules.
- The Z isomer (“zusammen“, same) is the geometric isomer where the #1 ranked substituents are on the same side of the double bond. Mnemonic: “zee zame zide“
- E isomer (“entgegen“) is the geometric isomer where the #1 ranked substituents are on the opposite side of the double bond,
Table of Contents
- When do we use cis– and trans– Notation In Rings?
- cis– and trans– Isomerism In Alkenes
- Watch out for ambiguous names when geometrical isomerism is possible!
- cis– and trans– isomerism in cyclic alkenes
- When “cis“- and “trans‘” fails: E and Z Notation
- E and Z Notation For Alkenes
- Breaking Ties: The Method of Dots
- Conclusion: E and Z Notation For Alkenes
- Quiz Yourself!
This post was co-authored with Matt Pierce of Organic Chemistry Solutions. Ask Matt about scheduling an online tutoring session here.
Quick Review: cis– And trans- Isomerism (“Geometrical Isomerism”) In Rings
Earlier on our MOC series on cycloalkanes, we saw that a key feature of small rings is that they can’t be turned “inside out” without breaking bonds.(See post: Cycloalkanes – Dashes and Wedges)
One of the most important consequence of this is that it can lead to the existence of stereoisomers – molecules which share the same molecular formula and the same connectivity but have a different arrangement of atoms in space.
These two versions of 1,2 dichlorocyclopentane (below) are an example. They have the same connectivity – both are 1,2-dichlorocyclopentane – but have different arrangements of their atoms in space. The chlorines are on the same side of the ring in the left-hand isomer (both “wedges”, coming out of the page) and on the opposite sides (one wedged, one dashed) on the right-hand isomer.
These two molecules cannot be interconverted through rotation of the C-C bond without rupturing the ring (use a model kit and try, if you like). They are therefore isomers.
Molecules which have the same connectivity but different arrangement in space are known as stereoisomers.
Specifically, the relationship between the two molecules above is that of diastereomers: stereoisomers which are not mirror images of each other. (See post: Types of Isomers)
These two molecules have different physical properties – different boiling points, melting points, reactivities, spectral characteristics and so on.
[Just to note, the other subclass of stereoisomer is “enantiomers”. We apply this to two stereoisomers which are (non-superimposable) mirror images of each other. Also: keep in mind that the terms “diastereomer” and “enantiomer” denote comparative relationships, like the terms “brother” or “cousin”. ]
1. When Do We Use cis- And trans- Notation In Rings?
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.
[Side note: the “restricted rotation” is how cis- and trans- subtly differs from syn and anti, which we use in cases where there is free rotation, such as the orientation of methyl groups in “eclipsed” and “staggered” butane. Bottom line: syn and anti forms can generally be interconverted through bond rotation: cis and trans forms cannot. ]
In nomenclature, “cis” is used to distinguish the isomer where two identical groups (e.g. the two chlorines in 1,2-dichlorocyclopentane) are pointing in the same direction from the plane of the ring, and trans to distinguish the isomer where they point in opposite directions. [You might also hear organic chemists say, “the chlorines are cis to each other” or “the hydrogens are trans to one another”.]
A common name for these so-called “cis-trans” isomers is “geometric isomers”. Those scolds at IUPAC actually discourage the term “geometric isomers”, and for once, I agree: the term is somewhat redundant and can cause confusion. In the rest of this post I’ll just use the term “cis-trans” isomers.
In order for cis- trans- isomerism to exist in rings, we need two conditions:
- two (and only two) carbons each bearing non-identical substituents above and below the ring
- the two carbons have at least one of those substituents in common
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 ).
Here’s another example: cis- and trans– 1-ethyl-2-methylcyclobutane. Note that they each have two carbons which each bear non-identical substituents above and below the ring (H and CH3; H and CH2CH3). They also have at least one substituent in common (H). So we can refer to cis-1-ethyl-2-methylcyclohexane as the isomer where the two hydrogens are pointing in the same direction, and trans where they point in opposite directions.
If you’ve covered chirality, you might also note an interesting fact: there are two ways to draw each of the cis- and trans– isomers, and they can’t be superimposed on each other. These are enantiomers, by the way. (See post: Enantiomers, Diastereomers or the Same)
So cis- and trans- doesn’t specify which enantiomer (it can be applied to either). It’s just describing the relative configuration of the two groups (H in this case). If we want to specify a particular enantiomer, we need to use the Cahn-Ingold-Prelog (CIP) system of assigning R and S configurations, which provides us with the “absolute” configuration. In that case, cis– and trans- is redundant. (See post: Cahn-Ingold-Prelog System)
Because cis– and trans– is relative, it doesn’t work if the two carbons don’t share a common substituent. In that case you also have to use (R)/(S) .
We’re taking too long to go through rings here, so let’s just illustrate 2 examples where “cis” and trans” doesn’t work in rings and leave it there.
2. cis– and trans- Isomerism (Geometric Isomerism) In Alkenes
cis-trans isomerism is also possible for alkenes. As in small rings, rotation about pi bonds is also constrained: due to the “side-on” overlap of pi bonds, one can’t rotate a pi bond without breaking it. This stands in contrast to conventional sigma bonds (single bonds) in acyclic molecules, where free rotation is possible: witness 1,2-dichloroethane (below left).
Hence we can have molecules such as cis-1,2-dichloroethene [boiling point 60°C] and trans-1,2-dichloroethene [boiling point: 48°C] which can be separated from each other due to their differing physical properties.
We can also use the cis–trans nomenclature to distinguish isomers such as 2-methyl-3-hexene (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. [Note: this risks a “tsk-tsk” with accompanying finger-wag from IUPAC , but it nevertheless gets the right structure: see the Note 1 below for a digression as to why]
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.
As with rings, cis-trans isomerism isn’t possible if one of the carbons of the double bond is attached to two identical groups, as with 1,1-dibromo-1-propene, below. Try it for yourself if you’re not convinced.
3. Watch Out For Ambiguous Names Where Cis/Trans Isomerism Is Possible
A quick digression: one consequence of our newfound appreciation of geometrical isomerism is that many simple-sounding molecule names are actually ambiguous.
For instance, the descriptor “3-hexene” does not unambiguously describe a specific molecule. [The same is true for 2-butene: try it! ]. 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.
4. Cis– Trans- Isomerism For Cyclic Alkenes
cis- and trans can also be applied to alkenes in rings. For example, on paper it’s possible to draw cis– and trans– cyclohexene, since the pi bond fulfills the requirements for cis- trans- isomerism. 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 . [Note 2]
For this reason, for ring sizes 7 and below, it’s safe to ignore writing “cis” : the configuration is assumed.
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!)
5. A Solution For When “Cis” and “Trans” Fails: The E/Z System
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.
Case in point: try to apply cis and trans to the alkene below:
See the problem?
In the absence of two identical groups, we have no reference point!
On the left, the chlorine is cis to Br and trans to F. But does that really justify calling the isomer “cis” ? How do we decide?
What we need is some way to determine priorities in these situations.
[note: some textbooks may still refer to this alkene as exhibiting “cis-trans isomerism” even though we must use E and Z]
6. The E and Z Notation For Alkenes
Thankfully, we can apply the ranking system developed by Cahn, Ingold, and Prelog for chiral centers (as touched on in this earlier post on (R)/(S) nomenclature) for this purpose.
The protocol is as follows:
- Each carbon in the pi bond is attached to two substituents. For each carbon, these two substituents are ranked (1 or 2) according to the atomic numbers of the atom directly attached to the carbon. (e.g. Cl > F )
- If both substituents ranked 1 are on the same side of the pi bond, the bond is given the descriptor Z (short for German Zusammen, which means “together”).
- If both substituents ranked 1 are on the opposite side of the pi bond, the bond is given the descriptor E (short for German Entgegen, which means “opposite”).
So Z resembles “cis” and E resembles “trans” . (Note: they are not necessarily the same and do not always correlate: see Note 2 for an example of a cis alkene which is E . The E/Z system is comprehensive for all alkenes capable of geometric isomerism, including the cis/trans alkene examples above. We often use cis/trans for convenience, but E/Z is the “official”, IUPAC approved way to name alkene stereoisomers].
One easy way to remember Z is to say “Zee Zame Zide” in a German accent. My way of doing it was pretending that the Z stands for “zis”. Whatever works for you.
Here’s a practical example:
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.
7. Breaking Ties: The Method of Dots
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.
- Place a dot on each of the two atoms you are comparing.
- List the 3 atoms each atom is attached to, in order of atomic number.
- Compare the lists, much like you would compare a set of three playing cards. Just as a hand of (8, 8, 7) would beat (8, 7, 7), so would (C, C, H) beat (C, H, H).
- If the lists are identical, move the dots outward to the highest priority atom on the list.
- At the first point of difference, assign (E or Z).
- If there is no difference… then the groups are identical, and E / Z does not apply.
Here’s a practical example of the “method of dots”.
Here’s a more complex example with multiple alkenes. In this case each pi bond is designated by a number with its own separate E or Z configuration.
OK, this was long. But hopefully useful.
Watch out for a future post in which we go into more detail on the “method of dots”.
8. Conclusion: E and Z Notation For Alkenes
cis-trans- is OK for describing simple alkene stereoisomers, but only works in certain cases. Furthermore, it only gives relative configurations. The E/Z system is comprehensive and describes the absolute configuration of the molecule.
See below for an example of an E alkene which is “cis” and a Z alkene which is “trans”.
Just a reminder: this post was co-authored by Matt Pierce of Organic Chemistry Solutions. Ask Matt about scheduling an online tutoring session here.
Note 1: It’s possible to have an alkene we’d describe as ‘cis’ be E and vice versa.
E/Z is the preferred, more comprehensive nomenclature since it describes absolute configuration, whereas cis- trans- merely describes relative configuration.
Note 2: trans-cyclopropene, trans-cyclobutene, and trans-cyclopentene have never been synthesized or observed. trans-cyclohexene is a laboratory curiosity, stable at a few degrees above absolute zero. trans-cycloheptene has an extremely short half-life at room temperature. trans-cyclooctene is a stable molecule [it also exhibits axial chirality, which is interesting! ].
35 thoughts on “E and Z Notation For Alkenes (+ Cis/Trans)”
Greetings…58 years old here, applying to a graduate program, am reviewing my organic chemistry and found your website helpful. Thank you
Glad you find it helpful, Tom. Best wishes on your graduate program. James
Hi James, I have a question want to ask you, does cycloalkene have a geometrical isomer, for cyclohexene or pentene? I think is no, I can’t change the substituent at the double bond to get another isomer, can you tell me am I wrong?
I really want to know.
It’s not energetically possible for 5- or 6- membered rings. Seven membered rings can be trans but have a short lifetime. Only at 8-membered rings do they start to be reasonably stable.
Can you please say which is (cis-trans or E-Z) is accepted in IUPAC nomenclature
E-Z is preferred and is non-ambiguous, but in simple cases cis-trans- will do.
but some molcules
we take the longest chain of carbon to determine if it is cis or trans
rather than take the similar group as a point reference
does a cis and E configuration of a molecule make it less polar?
If we’re just talking about alkenes, then yes, very slightly.
Carbon is more electronegative than H. So there is some polarization along the C-H (but not that much).
The boiling point of a molecule is often used as an indicator for how polar it is.
Given this, for which molecule would you expect the boiling point to be higher: cis-2-butene or trans-2-butene? Hint: think about the dipole moments
If a molecule only has one double bond that has EZ stereochemistry do we signify that with either an E or Z without putting a locant in front? Does this idea apply with molecules that have multiple double bonds where only one has EZ stereochemistry?I am curious because I have seen both ways.I have put chemical names into molecule generators online that give the same molecule with or without the locant, so I assume it does not matter.
I do not always set the best example for nomenclature. From a practical standpoint the important thing is that the answer refer to a unique compound. If there is only one double bond capable of E/Z then it is not absolutely necessary to put it in, IMO.
Sometimes E or Z does not need to be mentioned at all. IUPAC gives the example of cyclohexene, where Z is assumed since the E form is extremely unstable.
very well explained, I loved the quiz. Thanks
You said geometric isomers (cis/trans) are a subset of diastereoisomers but aren’t they a subset of stereoisomers instead?
They are a subset of diastereomers, and diastereomers are a subset of stereoisomers.
Can you tell me whether the following compounds have E or Z configuration?
1) CH3 and H on one side of C-C double bond and CHO and Br on the other
2)CH3 and H on one side of C-N double bond and OH on the other
3)CH3 and H on one side of the C-C double bond and Br and OH on the other
Do any of them have identical substituents on one side of the double bond? If not, then they are capable of E/Z configurations.
which is more stable? like which will be the natural product of a reaction? will cis ever form and be more stable than trans?
Which is more stable, cis or trans?
– trans is more stable (less steric hindrance between alkyl groups). I don’t know how to answer the question “which will be the natural product of a reaction” because it depends on multiple factors. In the E2 for example there can be cases where only the cis can form and the trans is not possible due to the requirement that the C-H bond and C-leaving group bond are “anti”.
Cis can form in the reduction of alkynes with Lindlar and H2, for example.
When both cis and trans are possible trans is generally preferred. For example in the elimination (E2) reaction of say, 3-chloropentane, the trans will be the dominant product. I can’t give you an exact proportion at this moment but a good estimate would be at least 4:1.
Can we apply the Z, E nomenclature to orotic acid? What is the Z isomer? How do the rules apply?
I had to look up the structure. E and Z descriptors are not required for cycloalkenes with ring sizes less than 8, since the trans- double bond isomers are extremely unstable.
Top notch explanation!!!!
Thank you for the helpful description, but what do we do when we have a RACEMIC substituted-ring that does not have 2 identical substituents. Example a cyclopentane substituted with an amino acid at C1 (2 substituents are amine and carboxcylic acid) and a alkyl sidechanie at C2 (2 substituents are hydrogen and ethyl)?
From your description the way to do it would be to employ the CIP system and assign R and S configurations to the chiral centers. That will unambiguously assign the configuration of each molecule.
Amazing explanations as always man, you’re a life saver
Glad you find it helpful, DL!
I love the way you explain the concepts. It’s quite hard to find someone teaching organic chemistry in such a simple way. Thank you for creating the materials, these are helpful indeed.
Thank you Kripi!
When a molecule has both a chiral center (R/S) and an alkene functional group (E/Z), which comes first in its name? The R/S before E/Z, other way around, or do we just go by the locant? Thanks!
You just go by the locant. The attached example is (2Z, 7R, 11E)-trideca-2,11-dien-7-ol from the IUPAC Blue book, Page 1190 (P-188.8.131.52). https://imgur.com/XnFQkg7
Is the E/Z protocol appilcable to allene systems?
No it is not. Allenes can have a chiral axis. For chiral allenes, enantiomers are designated by the terms M and P .
So if you have a molecule that is Z konfigurated and another molecule that is E konfigurated, they are diastereomers? What if you have a molecule that is Z konfigurated and the other is Z konfigurated as well? Would they still be diastereomers?
-If the IUPAC descriptor for two molecules is identical except for the fact that they ONLY differ in E and Z then they are diastereomers. e.g. (E)-4-Methyl-3-octene and (Z)-4-Methyl-3-octene.
-If they have different IUPAC names then they are not stereoisomers. (E)-4-Methyl-3-octene and (Z)-4-Methyl-3-nonene
-If the IUPAC name is the same and the E/Z descriptor are the same, they are the same molecule.