Introduction
What is E-Z isomerism?
E-Zisomerism (also known as cis-trans isomerism or Geometric isomerism) is a typeof stereoisomerism in which the same groups are arranged differently.
Thegeneral approach of the E-Z system is to observe the two groups at the end ofeach double bond. Then, analyze that the higher priority group at the one endof the double bond and the higher priority group at the other end of the doublebond are on the same side (Z, from German zusammen = together) or the oppositesides (E, from German entgegen = opposite) of the double bond
A problem in naming Geometric E-Z isomers
Consider a simple example of 1,2-dichloroethene. The geometric isomerism is given below:
Onecan easily observe which one is cis and which is trans just in one glance. Youmust remember that trans means “across” and cis means“opposite”. This is a simple visual way of observing the two isomersthan why do we need another system?
The problem arises when the compound gets more complicated. For example, are you able to name the isomers given below by using cis and trans?
Thereason is that everything attached to the carbon-carbon double bond isdifferent, the way they look doesn’t make obvious that they are being cis ortrans to each other.
Working of E-Z system
We will use the second exampleto explain this system. Just look at each end of the double bond that is attachedto each of them and give these two groups a priority according to a set ofrules.
In the above example, there is bromine and fluorine on the left-hand side of the bond. It turns out that bromine has a higher priority than fluorine. At the right-hand side, chlorine turns out to be higher in priority than that of hydrogen.
If the two groups havinghigher priorities are on the same side of the double bond than termed it as theZ-isomer. And if the two groups having higher priorities are on the oppositesides of the double bond, then termed it as E-isomer.
Hence the two isomers are
Rules for determining priorities
After the development of thesystem. The people named it as Cahn-Ingold-Prelog (CIP).
- The first rule for a simple case
Justhave a look at the atoms that are attached directly to the carbon atoms at eachend of the double bond-assuming the two and separately.
Theatom has a higher atomic number will be given a high priority.
Let’s again have a look at the example that we have considered before too.
Now consider the first isomer and examine individually at the left-hand side and then the right-hand carbon atom. Now to declare the priorities just compare the atomic numbers of the atoms that are attached.
Noticethat the atoms having the higher priorities are on the same side of the doublebond. That will be declared as the Z-isomer.
Obviouslyat each end. the second isomer, however, it has the same atoms but this timethe higher priority atoms are on opposite sides of the double bond. That isknown as the E-isomer.
Consider another example of 1,2-Dichloroethene.
Nowobserve the priority of the two groups of the left-hand side isomer on thefirst carbon.
The atomicnumber of chlorine is higher than that of hydrogen and therefore has a highpriority. This goes the same for the carbon atom present in the other isomers.
The first isomer will be declared as the E-isomer as the higher priority groups are on opposite sides of the bond. The other one will be declared as Z-isomer as the higher priority groups are on the same side.
Nowconsider an example of but-2-ene.
Hereyou will encounter a slight complication because here we have not got a singleatom attached to the double bond but a group of atoms.
Thisis not a problem just focus on the atom that is directly attached to the doublebond here the carbon in the CH3 group.
Inthis case, you can ignore the hydrogen atom in the CH3 groupentirely. But in complicated groups, one just has to worry about the atoms thatare not directly attached to the double bond.
The given below is one of the isomers of the but-2-ene.
TheCH3 group is considered the high priority because the carbon atom here has anatomic number of six as compared with an atomic number of one for the hydrogenwhich is also attached to the carbon-carbon double bond.
The isomerthat is drawn above has the two higher priority groups on opposite sides of thedouble bond. hence, the compound is E-but-2-ene.
- Minor addition to the rule to allow forisotopes
Deuterium, an isotope of hydrogen has a relative atomic mass number of 2. It has one proton and therefore yet has an atomic number of one. You will see that it isn’t the same as that of the atom of an atom of ordinary hydrogen and hence these two compounds given below are geometric isomers.
Thedeuterium and hydrogen have the same atomic number, therefore, they have thesame priority on this base. In such a case the one having the higher relativeatomic mass has a higher priority. So in such isomers, the chlorine and thedeuterium are counted among the higher priority groups on each end of thedouble bond.
Hencethe left-hand isomer in the last diagram is the E-form and the right-hand oneis the Z-form.
- Extended rules to more complicatedmolecules
Consider the following complicated example to find out whether it is a Z or E isomer by applying some additional rules
Justobserve the left-hand end of the molecule. What is attached directly to thecarbon-carbon double bond?
Acarbon atom is attached directly to the bond in both of the attached groups.These two atoms will have the same atomic number and hence the same priority.So this will not help for sure.
Inthese types of cases, you now observe what is directly attached to those twocarbons but without counting the carbon of the double bond and then compare thepriorities of these next lot of atoms.
Insimple cases, you can do this in your head but sometimes it is necessary towrite the attached atoms down and enlist them having the high priority atomfirst. This will help in comparing. For example
In theCH3 group
Theatoms attached to the carbon are H H H.
In theCH3CH2 group
Theatoms which are attached directly to the carbon of the CH2 group areC H H.
In thesecond list, the C is written first because it has the highest atomic number
Nowcompare the two lists atom by atom. The first atom in each list is an H in theCH3 group and a C in the CH3CH2 group. Thecarbon will have a higher priority because of having a higher atomic number.Therefore, this gives the CH3CH2 group a higher prioritythan the CH3 group.
Now just notice the other end of the double bond. the extra thing that describes is that if you are having a double bond, just count the attached atom twice. The given below is the structure again.
Hereagain, the atoms which are attached directly to the carbon-carbon double bondare both carbons. Therefore, we need to look at what is attached to thosecarbons.
in theCH2OH group
theatoms attached directly to carbon are O H H
in theCHO group
theatoms attached directly to the carbon are O O H.
Don’tforget that the oxygen is counted twice because of the carbon-oxygen doublebond. in both of the lists, the first oxygen is written because of having ahigher atomic number than hydrogen.
Sowhat is the priority of the two groups? Oxygen is the first atom in both of thelists but that will not help. Now observe the next atom in both the lists. Inthe CH2OH group, it’s hydrogen and in the CHO list, it’s oxygen.
Theoxygen will have a high priority and this gives the CHO group a higher prioritythan the CH2OH group.
Theisomer is, therefore, a Z-form because the two higher priority groups i.e. CH3CH2group and the CHO group, are both on the same side of the bond.
Can cis- and trans- be easily translated into Z and E?
Consider the example of 1,2-dichloroethene and but-2-ene cases.
But this rule doesn’t work every time. Just look at this uncomplicated molecule.
This is a cis-isomer. Thereare two CH3 groups on the same side of the double bond. but also figure out thepriorities on the right-hand end of the double bond.
The two atoms that aredirectly attached are carbon and bromine. Bromine has a high atomic number soit will have a higher priority on that end. On the other end, the CH3 groupwill have a high priority.
This shows that the two highpriority groups are on the opposite sides of the double bond so this is anE-isomer and not a Z-. Note that you can never convert these systems into theother direction.
References
- https://www.chemguide.co.uk/basicorg/isomerism/ez.html
- https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Fundamentals/Structure_of_Organic_Molecules/The_E-Z_system_for_naming_alkenes
- https://en.wikipedia.org/wiki/E%E2%80%93Z_notation
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