![\"\"](\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/11\/image-63.png\")
<\/figure><\/div>\n\n\n\nOne\ncan easily observe which one is cis and which is trans just in one glance. You\nmust remember that trans means “across” and cis means\n“opposite”. This is a simple visual way of observing the two isomers\nthan why do we need another system?<\/p>\n\n\n\n
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?<\/p>\n\n\n\n
![\"\"](\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/11\/image-64.png\")
<\/figure><\/div>\n\n\n\nThe\nreason is that everything attached to the carbon-carbon double bond is\ndifferent, the way they look doesn\u2019t make obvious that they are being cis or\ntrans to each other. <\/p>\n\n\n\n
Working of E-Z system<\/h2>\n\n\n\n
We will use the second example\nto explain this system. Just look at each end of the double bond that is attached\nto each of them and give these two groups a priority according to a set of\nrules.<\/p>\n\n\n\n
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.<\/p>\n\n\n\n
![\"\"](\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/11\/image-65.png\")
<\/figure><\/div>\n\n\n\nIf the two groups having\nhigher priorities are on the same side of the double bond than termed it as the\nZ-isomer. And if the two groups having higher priorities are on the opposite\nsides of the double bond, then termed it as E-isomer. <\/p>\n\n\n\n
Hence the two isomers are<\/p>\n\n\n\n
![\"\"](\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/11\/image-66.png\")
<\/figure><\/div>\n\n\n\nRules for determining priorities<\/h2>\n\n\n\n
After the development of the\nsystem. The people named it as Cahn-Ingold-Prelog (CIP).<\/p>\n\n\n\n
- The first rule for a simple case<\/strong><\/li><\/ul>\n\n\n\n
Just\nhave a look at the atoms that are attached directly to the carbon atoms at each\nend of the double bond-assuming the two and separately.<\/p>\n\n\n\n
The\natom has a higher atomic number will be given a high priority.<\/p>\n\n\n\n
Let\u2019s again have a look at the example that we have considered before too.<\/p>\n\n\n\n
![\"\"](\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/11\/image-67.png\")
<\/figure><\/div>\n\n\n\nNow 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.<\/p>\n\n\n\n
![\"\"](\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/11\/image-68.png\")
<\/figure><\/div>\n\n\n\nNotice\nthat the atoms having the higher priorities are on the same side of the double\nbond. That will be declared as the Z-isomer.<\/p>\n\n\n\n
Obviously\nat each end. the second isomer, however, it has the same atoms but this time\nthe higher priority atoms are on opposite sides of the double bond. That is\nknown as the E-isomer.<\/p>\n\n\n\n
Consider another example of 1,2-Dichloroethene.<\/p>\n\n\n\n
![\"\"](\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/11\/image-69.png\")
<\/figure><\/div>\n\n\n\nNow\nobserve the priority of the two groups of the left-hand side isomer on the\nfirst carbon.<\/p>\n\n\n\n
The atomic\nnumber of chlorine is higher than that of hydrogen and therefore has a high\npriority. This goes the same for the carbon atom present in the other isomers. <\/p>\n\n\n\n
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.<\/p>\n\n\n\n
![\"\"](\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/11\/image-70.png\")
<\/figure><\/div>\n\n\n\nNow\nconsider an example of but-2-ene.<\/p>\n\n\n\n
Here\nyou will encounter a slight complication because here we have not got a single\natom attached to the double bond but a group of atoms.<\/p>\n\n\n\n
This\nis not a problem just focus on the atom that is directly attached to the double\nbond here the carbon in the CH3<\/sub> group.<\/p>\n\n\n\n
In\nthis case, you can ignore the hydrogen atom in the CH3 <\/sub>group\nentirely. But in complicated groups, one just has to worry about the atoms that\nare not directly attached to the double bond.<\/p>\n\n\n\n
The given below is one of the isomers of the but-2-ene.<\/p>\n\n\n\n
![\"\"](\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/11\/image-71.png\")
<\/figcaption><\/figure><\/div>\n\n\n\n The\nCH3 group is considered the high priority because the carbon atom here has an\natomic number of six as compared with an atomic number of one for the hydrogen\nwhich is also attached to the carbon-carbon double bond. <\/p>\n\n\n\n
The isomer\nthat is drawn above has the two higher priority groups on opposite sides of the\ndouble bond. hence, the compound is E-but-2-ene.<\/p>\n\n\n\n
- Minor addition to the rule to allow for\nisotopes<\/strong><\/li><\/ul>\n\n\n\n
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.<\/p>\n\n\n\n
![\"\"](\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/11\/image-73.png\")
<\/figure><\/div>\n\n\n\nThe\ndeuterium and hydrogen have the same atomic number, therefore, they have the\nsame priority on this base. In such a case the one having the higher relative\natomic mass has a higher priority. So in such isomers, the chlorine and the\ndeuterium are counted among the higher priority groups on each end of the\ndouble bond.<\/p>\n\n\n\n
Hence\nthe left-hand isomer in the last diagram is the E-form and the right-hand one\nis the Z-form.<\/p>\n\n\n\n
- Extended rules to more complicated\nmolecules<\/strong><\/li><\/ul>\n\n\n\n
Consider the following complicated example to find out whether it is a Z or E isomer by applying some additional rules<\/p>\n\n\n\n
![\"\"](\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/11\/image-72.png\")
<\/figure><\/div>\n\n\n\nJust\nobserve the left-hand end of the molecule. What is attached directly to the\ncarbon-carbon double bond?<\/p>\n\n\n\n
A\ncarbon atom is attached directly to the bond in both of the attached groups.\nThese two atoms will have the same atomic number and hence the same priority.\nSo this will not help for sure.<\/p>\n\n\n\n
In\nthese types of cases, you now observe what is directly attached to those two\ncarbons but without counting the carbon of the double bond and then compare the\npriorities of these next lot of atoms.<\/p>\n\n\n\n
In\nsimple cases, you can do this in your head but sometimes it is necessary to\nwrite the attached atoms down and enlist them having the high priority atom\nfirst. This will help in comparing. For example<\/p>\n\n\n\n
In the\nCH3<\/sub> group<\/p>\n\n\n\n
The\natoms attached to the carbon are H H H.<\/p>\n\n\n\n
In the\nCH3<\/sub>CH2<\/sub> group<\/p>\n\n\n\n
The\natoms which are attached directly to the carbon of the CH2<\/sub> group are\nC H H.<\/p>\n\n\n\n
In the\nsecond list, the C is written first because it has the highest atomic number<\/p>\n\n\n\n
Now\ncompare the two lists atom by atom. The first atom in each list is an H in the\nCH3<\/sub> group and a C in the CH3<\/sub>CH2<\/sub> group. The\ncarbon will have a higher priority because of having a higher atomic number.\nTherefore, this gives the CH3<\/sub>CH2<\/sub> group a higher priority\nthan the CH3<\/sub> group.<\/p>\n\n\n\n
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.<\/p>\n\n\n\n
![\"\"](\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/11\/image-74.png\")
<\/figure><\/div>\n\n\n\nHere\nagain, the atoms which are attached directly to the carbon-carbon double bond\nare both carbons. Therefore, we need to look at what is attached to those\ncarbons.<\/p>\n\n\n\n
in the\nCH2OH group<\/p>\n\n\n\n
the\natoms attached directly to carbon are O H H <\/p>\n\n\n\n
in the\nCHO group<\/p>\n\n\n\n
the\natoms attached directly to the carbon are O O H.<\/p>\n\n\n\n
Don\u2019t\nforget that the oxygen is counted twice because of the carbon-oxygen double\nbond. in both of the lists, the first oxygen is written because of having a\nhigher atomic number than hydrogen. <\/p>\n\n\n\n
So\nwhat is the priority of the two groups? Oxygen is the first atom in both of the\nlists but that will not help. Now observe the next atom in both the lists. In\nthe CH2<\/sub>OH group, it’s hydrogen and in the CHO list, it’s oxygen.<\/p>\n\n\n\n
The\noxygen will have a high priority and this gives the CHO group a higher priority\nthan the CH2<\/sub>OH group.<\/p>\n\n\n\n
The\nisomer is, therefore, a Z-form because the two higher priority groups i.e. CH3<\/sub>CH2<\/sub>\ngroup and the CHO group, are both on the same side of the bond.<\/p>\n\n\n\n
Can cis- and trans- be easily translated into Z and E?<\/h2>\n\n\n\n
Consider the example of 1,2-dichloroethene and but-2-ene cases.<\/p>\n\n\n\n
![\"\"](\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/11\/image-75.png\")
<\/figure><\/div>\n\n\n\nBut this rule doesn\u2019t work every time. Just look at this uncomplicated molecule.<\/p>\n\n\n\n
![\"\"](\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/11\/image-76.png\")
<\/figure><\/div>\n\n\n\nThis is a cis-isomer. There\nare two CH3 groups on the same side of the double bond. but also figure out the\npriorities on the right-hand end of the double bond.<\/p>\n\n\n\n
The two atoms that are\ndirectly attached are carbon and bromine. Bromine has a high atomic number so\nit will have a higher priority on that end. On the other end, the CH3 group\nwill have a high priority.<\/p>\n\n\n\n
This shows that the two high\npriority groups are on the opposite sides of the double bond so this is an\nE-isomer and not a Z-. Note that you can never convert these systems into the\nother direction.<\/p>\n\n\n\n
References<\/h4>\n\n\n\n
- https:\/\/www.chemguide.co.uk\/basicorg\/isomerism\/ez.html<\/a><\/li>
- https:\/\/chem.libretexts.org\/Bookshelves\/Organic_Chemistry\/Supplemental_Modules_(Organic_Chemistry)\/Fundamentals\/Structure_of_Organic_Molecules\/The_E-Z_system_for_naming_alkenes<\/a><\/li>
- https:\/\/en.wikipedia.org\/wiki\/E%E2%80%93Z_notation<\/a><\/li><\/ul>\n","protected":false},"excerpt":{"rendered":"
Introduction What is E-Z isomerism? E-Z isomerism (also known as cis-trans isomerism or Geometric isomerism) is a type of stereoisomerism in which the same groups are arranged differently. The general approach of the E-Z system is to observe the two groups at the end of each double bond. Then, analyze that the higher priority group … Read more<\/a><\/p>\n","protected":false},"author":6,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"rating_form_position":"","rating_results_position":"","mr_structured_data_type":""},"categories":[1],"tags":[],"multi-rating":{"mr_rating_results":[{"adjusted_star_result":5,"star_result":5,"total_max_option_value":5,"adjusted_score_result":5,"score_result":5,"percentage_result":100,"adjusted_percentage_result":100,"count":3,"post_id":1219}]},"_links":{"self":[{"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/posts\/1219"}],"collection":[{"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/users\/6"}],"replies":[{"embeddable":true,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/comments?post=1219"}],"version-history":[{"count":3,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/posts\/1219\/revisions"}],"predecessor-version":[{"id":1332,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/posts\/1219\/revisions\/1332"}],"wp:attachment":[{"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/media?parent=1219"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/categories?post=1219"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/tags?post=1219"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
- https:\/\/chem.libretexts.org\/Bookshelves\/Organic_Chemistry\/Supplemental_Modules_(Organic_Chemistry)\/Fundamentals\/Structure_of_Organic_Molecules\/The_E-Z_system_for_naming_alkenes<\/a><\/li>
- https:\/\/www.chemguide.co.uk\/basicorg\/isomerism\/ez.html<\/a><\/li>
- Extended rules to more complicated\nmolecules<\/strong><\/li><\/ul>\n\n\n\n
- Minor addition to the rule to allow for\nisotopes<\/strong><\/li><\/ul>\n\n\n\n