{"id":10224,"date":"2016-11-03T12:40:02","date_gmt":"2016-11-03T17:40:02","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=10224"},"modified":"2025-02-28T11:32:53","modified_gmt":"2025-02-28T17:32:53","slug":"alkene-nomenclature-cis-and-trans-and-e-and-z","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2016\/11\/03\/alkene-nomenclature-cis-and-trans-and-e-and-z\/","title":{"rendered":"E and Z Notation For Alkenes (+ Cis\/Trans)"},"content":{"rendered":"<p><strong>E and Z Notation For Alkenes<\/strong><\/p>\n<ul>\n<li>Unlike C\u2013C single bonds, C\u2013C double bonds can&#8217;t undergo rotation without breaking the pi bond<\/li>\n<li>One consequence of this is <em>geometric isomerism &#8211;\u00a0<\/em>the existence of alkene <strong>stereoisomers<\/strong> that differ solely in how their substituents are arranged in space about the double bond<\/li>\n<li>In simple cases where there are two identical substituents on each carbon of the alkene, we can use\u00a0<em><strong>cis<\/strong>&#8211;<\/em> and\u00a0<strong><em>trans<\/em><\/strong>&#8211; to designate the isomers where those substituents are on the\u00a0<em><strong>same<\/strong>\u00a0<\/em>and\u00a0<strong><em>opposite<\/em><\/strong> sides of the double bond, respectively.<\/li>\n<li>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.<\/li>\n<li>The\u00a0<strong><em>Z<\/em> isomer<\/strong> (&#8220;<em>zusammen<\/em>&#8220;, same) is the geometric isomer where the <strong>#1 ranked substituents<\/strong> are on the <strong>same side<\/strong> of the double bond. Mnemonic: &#8220;<strong>zee zame zide<\/strong>&#8220;<\/li>\n<li><em>E<\/em> isomer (&#8220;<em>entgegen<\/em>&#8220;) is the geometric isomer where the #1 ranked substituents are on the opposite side of the double bond,<\/li>\n<\/ul>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-37380\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/09\/0-Summary-of-cis-trans-e-and-z-entgegen-zusammen-nomenclature-zee-zame-zide-mnemonic-for-alkene-and-cycloalkane-stereoisomers.gif\" alt=\"Summary of cis trans e and z entgegen zusammen nomenclature zee zame zide mnemonic for alkene and cycloalkane stereoisomers\" width=\"800\" height=\"753\" \/><\/a><\/p>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li><a href=\"#one\">When do we use <em>cis<\/em>&#8211; and\u00a0<em>trans<\/em>&#8211; Notation In Rings?<\/a><\/li>\n<li><a href=\"#two\"><em>cis<\/em>&#8211; and <em>trans<\/em>&#8211; Isomerism In Alkenes<\/a><\/li>\n<li><a href=\"#three\">Watch out for ambiguous names when geometrical isomerism is possible!<\/a><\/li>\n<li><a href=\"#four\"><em>cis<\/em>&#8211; and <em>trans<\/em>&#8211; isomerism in cyclic alkenes<\/a><\/li>\n<li><a href=\"#five\">When &#8220;<em>cis<\/em>&#8220;- and &#8220;<em>trans<\/em>&#8216;&#8221; fails: <em>E<\/em> and <em>Z<\/em> Notation<\/a><\/li>\n<li><a href=\"#six\"><em>E<\/em> and <em>Z<\/em> Notation For Alkenes<\/a><\/li>\n<li><a href=\"#seven\">Breaking Ties: The Method of Dots<\/a><\/li>\n<li><a href=\"#eight\">Conclusion: <em>E<\/em> and <em>Z<\/em> Notation For Alkenes<\/a><\/li>\n<li><a href=\"#notes\">Notes<\/a><\/li>\n<li><a href=\"#quiz\">Quiz Yourself!<\/a><\/li>\n<\/ol>\n<hr \/>\n<p><em>This post was co-authored with Matt Pierce of\u00a0<a href=\"http:\/\/organicchemistrysolutions.com\">Organic Chemistry Solutions<\/a>.\u00a0 Ask Matt about scheduling an online tutoring session\u00a0<a href=\"https:\/\/masterorganic.wufoo.com\/forms\/q1yg3qx8076h7gx\/\">here<\/a>.<\/em><\/p>\n<h2>Quick Review: <em>cis<\/em>&#8211; And\u00a0<em>trans-<\/em> Isomerism (&#8220;Geometrical Isomerism&#8221;) In Rings<\/h2>\n<p>Earlier on our MOC series on cycloalkanes, we saw that a\u00a0key feature of small rings is that they can\u2019t be turned &#8220;inside out&#8221; without breaking bonds.(<em>See post: <a href=\"https:\/\/www.masterorganicchemistry.com\/2014\/03\/20\/cycloalkanes-dashes-and-wedges\/\">Cycloalkanes &#8211; Dashes and Wedges<\/a><\/em>)<\/p>\n<p>One of the most important consequence of this is that it can lead to the existence of <em>stereoi<\/em><em>somers &#8211;\u00a0<\/em>molecules which share the same molecular formula and the same connectivity but have a different arrangement of atoms in space.<\/p>\n<p>These two versions of\u00a0<em>\u00a0<\/em>1,2 dichlorocyclopentane (below) are an example.\u00a0They have the same connectivity &#8211; both are 1,2-dichlorocyclopentane &#8211; \u00a0but have different\u00a0arrangements of their atoms in space. The chlorines are on the same side of the ring in the left-hand isomer (both &#8220;wedges&#8221;, coming out of the page) \u00a0and on the opposite sides \u00a0(one wedged, one dashed) on the right-hand isomer.<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-14936\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-geometrical-isomers-cis-and-trans-in-rings-1-2-dichlorocyclopentane.gif\" alt=\"geometrical isomers cis and trans in rings 1 2 dichlorocyclopentane\" width=\"640\" height=\"362\" \/><\/p>\n<p>These two molecules <strong>cannot be interconverted through rotation\u00a0 of the C-C bond without rupturing the ring<\/strong> (use a model kit and try, if you like). They are therefore\u00a0<em>isomers.<\/em><\/p>\n<p>Molecules which have the <strong>same connectivity but different arrangement in space<\/strong> are known as\u00a0<em><strong>stereoisomers<\/strong>.\u00a0<\/em><\/p>\n<p>Specifically, the relationship between the two molecules above is that of\u00a0<em>diastereomers: <\/em>stereoisomers which are not mirror images of each other. (<em>See post: <a href=\"https:\/\/www.masterorganicchemistry.com\/2018\/09\/10\/types-of-isomers\/\">Types of Isomers<\/a><\/em>)<\/p>\n<p>These two molecules have different physical properties &#8211; different boiling points, melting points, reactivities, spectral characteristics and so on.<\/p>\n<p><em>\u00a0<\/em>[<span style=\"color: #993366;\"><em>Just to note, the other subclass of stereoisomer is &#8220;enantiomers&#8221;. We apply this to two stereoisomers which are (non-superimposable) mirror images of each other.\u00a0Also:\u00a0keep in mind that the terms &#8220;diastereomer&#8221; and &#8220;enantiomer&#8221; denote comparative relationships, like the terms &#8220;brother&#8221; or &#8220;cousin&#8221;.<\/em> <\/span>]<\/p>\n<h2><a id=\"one\"><\/a>1. When Do We Use\u00a0<em>cis-\u00a0<\/em>And\u00a0<em>trans-\u00a0<\/em>Notation In Rings?<\/h2>\n<p>We use the terms cis- and\u00a0<em>trans<\/em>&#8211; \u00a0to denote the<em>\u00a0<\/em><em>relative\u00a0<\/em>configuration of two groups to each other in situations where there is restricted rotation.<\/p>\n<p><span style=\"color: #993366;\"><em>[Side note: the &#8220;restricted rotation&#8221; is how cis- and\u00a0trans-\u00a0subtly differs from \u00a0syn and\u00a0anti,\u00a0which we use\u00a0in cases where there is free rotation, such as the orientation of methyl groups in &#8220;eclipsed&#8221; and &#8220;staggered&#8221; butane. Bottom line:\u00a0<strong>syn and\u00a0anti\u00a0forms\u00a0can generally be interconverted through bond rotation: cis and trans\u00a0forms cannot<\/strong>.\u00a0]<\/em><\/span><\/p>\n<p>In nomenclature, \u00a0&#8220;<em>cis<\/em>&#8221; is used to distinguish the isomer where<strong> two identical\u00a0groups<\/strong> (e.g. the two chlorines in 1,2-dichlorocyclopentane) are pointing in the<strong> same<\/strong> direction from the plane of the ring, and <em>trans<\/em> to distinguish the isomer where they point in <strong>opposite<\/strong> directions. [<span style=\"color: #993366;\"><em>You might also hear organic chemists say, &#8220;the chlorines are\u00a0cis to each other&#8221; or &#8220;the hydrogens are\u00a0trans to one another&#8221;.<\/em><\/span>]<\/p>\n<p>A common name for these so-called &#8220;<em>cis-trans<\/em>&#8221; isomers is &#8220;geometric isomers&#8221;. Those scolds at IUPAC actually discourage the term &#8220;geometric isomers&#8221;, and for once, I agree: \u00a0the term is somewhat redundant and can cause confusion. In the rest of this post I&#8217;ll just use the term &#8220;<em>cis-trans<\/em>&#8221; isomers.<\/p>\n<p>In order for\u00a0<em>cis- trans-\u00a0<\/em>isomerism to exist in rings, we need two conditions:<\/p>\n<ul>\n<li>two (and only two) carbons<strong> each<\/strong>\u00a0bearing non-identical substituents above and below the ring<\/li>\n<li>the two carbons have <em>at least<\/em> one of those substituents in common<\/li>\n<\/ul>\n<p>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: \u00a0H and Cl ).<\/p>\n<p>Here&#8217;s another example: <em>cis-\u00a0<\/em>and\u00a0<em>trans<\/em>&#8211; 1-ethyl-2-methylcyclobutane. Note that they <strong>each<\/strong> have two carbons which <strong>each<\/strong> bear non-identical substituents above and below the ring (H and CH<sub>3<\/sub>; H and CH<sub>2<\/sub>CH<sub>3<\/sub>). They also have at least one substituent in common (<span style=\"color: #ff0000;\"><strong>H<\/strong><\/span>). So we can refer to\u00a0<em>cis<\/em>-1-ethyl-2-methylcyclohexane as the isomer where the two hydrogens are pointing in the same direction, and\u00a0<em>trans<\/em> where they point in opposite directions.<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-14937\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-cis-and-trans-in-cyclobutane-at-least-one-substituent-identical.gif\" alt=\"cis and trans in cyclobutane at least one substituent identical\" width=\"600\" height=\"312\" \/><\/p>\n<p>If you&#8217;ve covered chirality, you might also note an interesting fact: there are two ways to draw each of the\u00a0<em>cis-\u00a0<\/em>and\u00a0<em>trans<\/em>&#8211; isomers, and they can&#8217;t be superimposed on each other. These are\u00a0<em>enantiomers<\/em>, by the way. (<em>See post:<a href=\"https:\/\/www.masterorganicchemistry.com\/2019\/03\/08\/enantiomers-diastereomers-or-the-same-1-using-models\/\"> Enantiomers, Diastereomers or the Same<\/a><\/em>)<\/p>\n<p>So\u00a0<em>cis-\u00a0<\/em>and\u00a0<em>trans-<\/em> doesn&#8217;t specify which enantiomer (it can be applied to either). It&#8217;s just describing the\u00a0<em>relative<\/em> configuration of the two groups (<strong><span style=\"color: #ff0000;\">H<\/span>\u00a0<\/strong>in this case). If we want to specify a particular enantiomer, we need to use the\u00a0 Cahn-Ingold-Prelog (CIP) system of assigning <em>R <\/em>and <em>S<\/em> configurations,\u00a0which provides us with the &#8220;absolute&#8221; configuration. In that case, <em>cis<\/em>&#8211; and\u00a0<em>trans-\u00a0<\/em>is redundant. (<em>See post: <a href=\"https:\/\/www.masterorganicchemistry.com\/2016\/10\/20\/introduction-to-assigning-r-and-s-the-cahn-ingold-prelog-rules\/\">Cahn-Ingold-Prelog System<\/a><\/em>)<\/p>\n<p>Because\u00a0<em>cis<\/em>&#8211; and\u00a0<em>trans<\/em>&#8211; is relative, it doesn&#8217;t work\u00a0if the two carbons don&#8217;t share a common substituent. In that case you also have to use\u00a0<em>(R<\/em>)\/<em>(S)\u00a0<\/em>.<\/p>\n<p>We&#8217;re taking too long to go through rings here, so let&#8217;s just illustrate 2 examples where &#8220;cis&#8221; and trans&#8221; doesn&#8217;t work in rings and leave it there.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-14938\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-no-geometric-isomers-present-in-rings-where-only-1-carbon-has-different-groups-also-3-or-more-carbons-above-below.gif\" alt=\"no geometric isomers present in rings where only 1 carbon has different groups also 3 or more carbons above below\" width=\"600\" height=\"339\" \/><\/p>\n<h2><a id=\"two\"><\/a>2. <em>c<\/em><em>is<\/em>&#8211; and <em>trans-<\/em> Isomerism (Geometric Isomerism) In Alkenes<\/h2>\n<p><em>cis-trans<\/em> isomerism\u00a0\u00a0is also possible for alkenes. \u00a0As in small rings, rotation about pi bonds is also constrained: due to the &#8220;side-on&#8221; overlap of pi bonds, <strong>one can&#8217;t rotate a pi bond without breaking it<\/strong>. This stands in contrast to conventional sigma bonds (single bonds) in acyclic molecules, where free rotation is possible: witness 1,2-dichloroethane (below left).<\/p>\n<p>Hence we can have molecules such as <i>cis-<\/i>1,2-dichloroethene [boiling point 60\u00b0C] and\u00a0<em>trans<\/em>-1,2-dichloroethene [boiling point: 48\u00b0C] which can be separated from each other due to their differing physical properties.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-14939\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/4-geometric-isomers-in-alkenes-possible-because-no-rotation-about-double-bond-cis-and-trans.gif\" alt=\"geometric isomers in alkenes possible because no rotation about double bond cis and trans\" width=\"600\" height=\"480\" \/><\/p>\n<p>We can also use the\u00a0<em>cis<\/em>&#8211;<em>trans\u00a0<\/em>nomenclature to distinguish isomers such as 2-methyl-3-hexene (above right). In the\u00a0<em>cis<\/em> isomer, the two hydrogens are on the same side of the pi bond, and in the\u00a0<em>trans<\/em> isomer, the two hydrogens are on the opposite side of the bond.<span style=\"color: #993366;\"><em> [Note: this risks a &#8220;tsk-tsk&#8221; with accompanying finger-wag from IUPAC , but it nevertheless gets the right structure: see the <span style=\"text-decoration: underline;\"><a style=\"color: #993366; text-decoration: underline;\" href=\"#noteone\">Note 1<\/a> <\/span>below for a digression as to why]<\/em><\/span><\/p>\n<p>As with rings, the minimum requirement for\u00a0<em>cis-trans\u00a0<\/em>isomerism in alkenes is that\u00a0<strong>each carbon is bonded to two different groups,\u00a0<\/strong>and that\u00a0<strong>the two carbons have at least one substituent in common.\u00a0<\/strong><\/p>\n<p>As with rings, <em>cis-trans<\/em>\u00a0isomerism isn&#8217;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&#8217;re not convinced.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-14940\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-cis-and-trans-not-possible-in-alkenes-if-two-groups-on-one-carbon-are-identical.gif\" alt=\"cis and trans not possible in alkenes if two groups on one carbon are identical\" width=\"600\" height=\"364\" \/><\/p>\n<h2><a id=\"three\"><\/a>3. Watch Out For Ambiguous Names Where Cis\/Trans Isomerism Is Possible<\/h2>\n<p>A quick digression: one\u00a0consequence of our newfound appreciation of geometrical isomerism is that many\u00a0simple-sounding molecule names \u00a0are actually ambiguous.<\/p>\n<p>For instance, the descriptor &#8220;3-hexene&#8221; does not unambiguously\u00a0describe a specific molecule. <em>\u00a0<span style=\"color: #993366;\">[The same is true for 2-butene: try it! ]<\/span><\/em>. To nail down the specific molecule, \u00a0we need to specify\u00a0<em>cis<\/em>&#8211; or\u00a0<em>trans<\/em>&#8211; 3-hexene.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-14941\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-sometimes-condensed-formula-can-lead-to-ambiguous-name-e-g-3-hexene-could-be-cis-or-trans-hex-3-ene.gif\" alt=\"sometimes condensed formula can lead to ambiguous name e g 3 hexene could be cis or trans hex 3 ene\" width=\"600\" height=\"233\" \/><\/p>\n<p>Note that 1-hexene is still OK, since the 1-position of 1-hexene is attached to two identical groups (hydrogens) and thus no\u00a0<em>cis<\/em>&#8211;<em>trans<\/em> isomers are possible.<\/p>\n<h2><a id=\"four\"><\/a>4. <em>Cis<\/em>&#8211;<em> Trans- <\/em>Isomerism For Cyclic Alkenes<\/h2>\n<p><em>cis-<\/em> and\u00a0<em>trans <\/em>can also be\u00a0applied to alkenes in rings. For example,\u00a0<em>on paper<\/em> it&#8217;s possible to draw\u00a0<em>cis<\/em>&#8211; and\u00a0<em>trans<\/em>&#8211; cyclohexene, since the pi bond fulfills the requirements for\u00a0<em>cis- trans-\u00a0<\/em>isomerism.\u00a0In reality,\u00a0<em>trans<\/em>-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<em> trans<\/em>&#8211; double bond in\u00a0\u00a0a six membered ring .\u00a0\u00a0[<a href=\"#notetwo\">Note 2<\/a>]<\/p>\n<p>For this reason, for ring sizes 7 and below, it&#8217;s safe to ignore writing &#8220;<em>cis<\/em>&#8221; : the configuration is assumed.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-14942\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/7-in-small-rings-cis-and-trans-is-omitted-since-e-alkenes-in-small-rings-are-too-unstable-eg-trans-cyclohexene.gif\" alt=\"in small rings cis and trans is omitted since e alkenes in small rings are too unstable eg trans cyclohexene\" width=\"600\" height=\"282\" \/><\/p>\n<p>At ring sizes of 8 and above, we\u00a0<em>do<\/em> need to put a\u00a0<em>cis<\/em>&#8211; or\u00a0<em>trans-\u00a0<\/em>in the name, because the\u00a0<em>trans<\/em>&#8211; isomer becomes feasible. (Imagine\u00a0trying to kiss yourself on the tailbone if you had the neck of a giraffe: suddenly not impossible!)<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-14943\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/8-large-rings-cis-and-trans-are-both-possible-such-as-cis-and-trans-cyclodecene.gif\" alt=\"large rings cis and trans are both possible such as cis and trans cyclodecene\" width=\"600\" height=\"203\" \/><\/p>\n<h2><strong><a id=\"five\"><\/a>5. A Solution For When &#8220;Cis&#8221; and &#8220;Trans&#8221; Fails: The E\/Z System<\/strong><\/h2>\n<p>We saw that <em>cis<\/em> and <em>trans<\/em> fails in rings when the two carbons lacked a common substituent. It also fails for alkenes under these circumstances.<\/p>\n<p>Case in point: try to apply\u00a0<em>cis<\/em> and\u00a0<em>trans<\/em> to the alkene below:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-14944\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/9-e-and-z-must-be-used-when-cis-trans-fails-e-g-double-bonds-with-no-carbons-bearing-identical-substituents.gif\" alt=\"e and z must be used when cis trans fails e g double bonds with no carbons bearing identical substituents\" width=\"600\" height=\"370\" \/><\/p>\n<p>See the problem?<\/p>\n<p>In the absence of two identical groups, <strong>we have no reference point!<\/strong><\/p>\n<p>On the left, the chlorine is\u00a0<em>cis<\/em> to Br and\u00a0<em>trans\u00a0<\/em>to F. But does that really justify calling the isomer &#8220;<em>cis<\/em>&#8221; ? How do we decide?<\/p>\n<p>What we need is some way to determine\u00a0<strong>priorities<\/strong> in these situations.<\/p>\n<p><span style=\"color: #993366;\"><em>[note: some textbooks may still refer to this alkene as exhibiting &#8220;cis-trans isomerism&#8221; even though we must use E and Z]<\/em><\/span><\/p>\n<h2><strong><a id=\"six\"><\/a>6. The <em>E<\/em> and <em>Z\u00a0<\/em>Notation For Alkenes<\/strong><\/h2>\n<p>Thankfully, we can apply the ranking system developed by Cahn, Ingold, and Prelog for chiral\u00a0centers (<a href=\"https:\/\/www.masterorganicchemistry.com\/2016\/10\/20\/introduction-to-assigning-r-and-s-the-cahn-ingold-prelog-rules\/\">as touched on in this earlier post on (R)\/(S) nomenclature<\/a>) for this purpose.<\/p>\n<p>The protocol is as follows:<\/p>\n<ul>\n<li>Each carbon in the pi bond is attached to two substituents. For <strong>each<\/strong> <strong>carbon<\/strong>, these two\u00a0substituents are\u00a0<strong>ranked<\/strong> (1 or 2) according to the atomic numbers of the atom directly attached to the carbon. (e.g. Cl &gt; F )<\/li>\n<li>If both substituents ranked 1 are on the <strong>same side<\/strong> of the pi bond, the bond is given the descriptor\u00a0<em>Z<\/em>\u00a0(short for German\u00a0<em>Zusammen<\/em>, which means &#8220;together&#8221;).<\/li>\n<li>If both substituents ranked 1 are on the\u00a0<strong>opposite side<\/strong><em>\u00a0<\/em>of the pi bond, the bond is given the descriptor\u00a0<em>E\u00a0<\/em>(short for\u00a0German <em>Entgegen<\/em>, which means &#8220;opposite&#8221;).<\/li>\n<\/ul>\n<p>So\u00a0<em>Z<\/em> resembles\u00a0<em>&#8220;cis&#8221;<\/em> and\u00a0<em>E<\/em> resembles\u00a0<em>&#8220;trans&#8221;<\/em>\u00a0 .\u00a0<span style=\"color: #993366;\"><em>\u00a0(Note: \u00a0they are\u00a0not necessarily the same and do not always correlate: see <span style=\"text-decoration: underline;\"><a style=\"color: #993366; text-decoration: underline;\" href=\"#notetwo\">Note 2<\/a><\/span> for an example of a cis alkene which is\u00a0E .\u00a0The\u00a0E\/Z system is\u00a0comprehensive\u00a0for all alkenes capable of geometric isomerism, including the\u00a0cis\/trans alkene examples above. We often\u00a0use\u00a0cis\/trans for convenience, but\u00a0E\/Z is the &#8220;official&#8221;, IUPAC approved way to name alkene stereoisomers].<\/em><\/span><\/p>\n<p><em>\u00a0<\/em>One easy way to remember\u00a0<em>Z<\/em> is to say &#8220;Zee Zame Zide&#8221; in a German accent.\u00a0My way of doing it was\u00a0pretending that the Z stands for\u00a0<em>&#8220;z<\/em><i>is&#8221;.\u00a0<\/i>Whatever works for you.<\/p>\n<p>Here&#8217;s a practical example:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-14945\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/10-e-and-z-notation-for-alkenes-rank-two-atoms-directly-attached-to-pi-bond-by-atomic-number-alkene-where-highest-priority-groups-on-same-side-is-z.gif\" alt=\"e and z notation for alkenes rank two atoms directly attached to pi bond by atomic number alkene where highest priority groups on same side is z\" width=\"600\" height=\"668\" \/><\/p>\n<p>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.<\/p>\n<h2><a id=\"seven\"><\/a>7. Breaking Ties: The Method of Dots<\/h2>\n<p>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?<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-14946\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/11-breaking-ties-method-of-dots-for-complex-alkenes-e-and-z.gif\" alt=\"breaking ties method of dots for complex alkenes e and z\" width=\"600\" height=\"204\" \/><\/p>\n<p>In the case of ties, we must apply the\u00a0<strong>method of dots. \u00a0<\/strong>Dots are handy placeholders which is why I like to use this method.<\/p>\n<ul>\n<li>Place a dot on each of the two atoms you are comparing.<\/li>\n<li>List the 3 atoms\u00a0each atom is\u00a0attached to, in order of atomic number.<\/li>\n<li>Compare the lists, <strong>much like you\u00a0would compare a set of three playing cards<\/strong>. Just as a hand of (8, 8, 7) would beat (8, 7, 7), so would (C, C, H) beat (C, H, H).<\/li>\n<li>If the lists are identical, move the dots outward to the highest priority atom on the list.<\/li>\n<li>At the <strong>first point of difference,\u00a0<\/strong>assign (<em>E<\/em> or\u00a0<em>Z<\/em>).<\/li>\n<li>If there is no difference&#8230; then the groups are identical, and\u00a0<em>E \/ Z\u00a0<\/em>does not apply.<\/li>\n<\/ul>\n<p>Here&#8217;s a practical example of the &#8220;method of dots&#8221;.\u00a0<a href=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2016\/10\/11-methodofdots2.png\"><br \/>\n<\/a><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-14947\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/12-method-of-dots-determining-e-and-z-breaking-ties-example.gif\" alt=\"method of dots determining e and z breaking ties example\" width=\"630\" height=\"670\" \/><\/p>\n<p>Here&#8217;s a more complex example with multiple alkenes. In this case each pi bond is designated by a number with its own separate\u00a0<em>E<\/em> or\u00a0<em>Z<\/em> configuration.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-14948\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/13-complex-example-of-molecule-with-multiple-alkenes-with-designated-e-and-z-configuration.gif\" alt=\"complex example of molecule with multiple alkenes with designated e and z configuration\" width=\"600\" height=\"160\" \/><\/p>\n<p>OK, this was long. But hopefully useful.<\/p>\n<p>Watch out for a future post in which we go into more detail on the &#8220;method of dots&#8221;.<\/p>\n<h2><a id=\"eight\"><\/a>8. Conclusion:\u00a0 <em>E<\/em> and <em>Z<\/em> Notation For Alkenes<\/h2>\n<p><em>cis-trans-\u00a0<\/em> is OK for describing simple alkene stereoisomers, but only works in certain cases. Furthermore, \u00a0it\u00a0only gives <em>relative<\/em> configurations.\u00a0\u00a0The\u00a0<em>E\/Z\u00a0<\/em>system is comprehensive and describes the\u00a0<em>absolute\u00a0<\/em>configuration of the molecule.<\/p>\n<p>See below for an example of an\u00a0<em>E\u00a0<\/em>alkene which is &#8220;cis&#8221; and a\u00a0<em>Z<\/em> alkene which is\u00a0<em>&#8220;trans&#8221;.\u00a0<\/em><\/p>\n<p><strong>Just a reminder: this post\u00a0was co-authored by Matt Pierce of\u00a0<a href=\"http:\/\/organicchemistrysolutions.com\">Organic Chemistry Solutions<\/a>.\u00a0 Ask Matt about scheduling an online tutoring session\u00a0<a href=\"https:\/\/masterorganic.wufoo.com\/forms\/q1yg3qx8076h7gx\/\">here<\/a>.<\/strong><\/p>\n<hr \/>\n<h2><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-14949\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-Eazy-E-teaches-E-and-Z-2E-4Z-6E-Octatri-246-ene-2.png\" alt=\"Eazy-E-teaches-E-and-Z-2E-4Z-6E-Octatri-2,4,6-ene-2\" width=\"400\" height=\"266\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-Eazy-E-teaches-E-and-Z-2E-4Z-6E-Octatri-246-ene-2.png 600w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-Eazy-E-teaches-E-and-Z-2E-4Z-6E-Octatri-246-ene-2-300x200.png 300w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-Eazy-E-teaches-E-and-Z-2E-4Z-6E-Octatri-246-ene-2-320x213.png 320w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-Eazy-E-teaches-E-and-Z-2E-4Z-6E-Octatri-246-ene-2-360x239.png 360w\" sizes=\"(max-width: 400px) 100vw, 400px\" \/><\/h2>\n<h2><a id=\"notes\"><\/a>Notes<\/h2>\n<div class=\"related-articles\"><p><strong>Related Articles<\/strong><\/p><ul><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2016\/10\/20\/introduction-to-assigning-r-and-s-the-cahn-ingold-prelog-rules\/\" class=\"\"><span>Introduction to Assigning (R) and (S): The Cahn-Ingold-Prelog Rules<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/01\/17\/determining-rs-2-the-method-of-dots\/\" class=\"\"><span>Assigning Cahn-Ingold-Prelog (CIP) Priorities (2) \u2013 The Method of Dots<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2020\/04\/30\/alkene-stability\/\" class=\"\"><span>Alkene Stability<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2018\/09\/10\/types-of-isomers\/\" class=\"\"><span>Types of Isomers: Constitutional Isomers, Stereoisomers, Enantiomers, and Diastereomers<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2019\/03\/08\/enantiomers-diastereomers-or-the-same-1-using-models\/\" class=\"\"><span>Enantiomers vs Diastereomers vs The Same? Two Methods For Solving Problems<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2014\/03\/20\/cycloalkanes-cis-and-trans\/\" class=\"\"><span>Geometric Isomers In Small Rings: Cis And Trans Cycloalkanes<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2010\/07\/02\/stereoselective-stereospecific\/\" class=\"\"><span>Stereoselective and Stereospecific Reactions<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2013\/01\/22\/alkene-addition-regioselectivity-syn-anti\/\" class=\"\"><span>Alkene Addition Reactions: \u201cRegioselectivity\u201d and \u201cStereoselectivity\u201d (Syn\/Anti)<\/span><\/a><\/li><\/ul><\/div>\n<p><strong><a id=\"noteone\"><\/a>Note 1<\/strong>: It&#8217;s possible to have an alkene we&#8217;d describe as &#8216;<em>cis&#8217;\u00a0<\/em>be\u00a0<em>E<\/em> and vice versa.<\/p>\n<p><em>E\/Z\u00a0<\/em>is the preferred, more comprehensive nomenclature since it describes\u00a0<strong>absolute<\/strong> configuration, whereas\u00a0<em>cis- trans-\u00a0<\/em>merely describes\u00a0<em>relative\u00a0<\/em>configuration.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-14950\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F2-cis-alkenes-are-not-always-z-and-trans-alkenes-are-not-always-e.gif\" alt=\"cis alkenes are not always z and trans alkenes are not always e\" width=\"630\" height=\"417\" \/><\/p>\n<p><strong><a id=\"notetwo\"><\/a>Note 2<\/strong>:\u00a0<em>trans-<\/em>cyclopropene,\u00a0<em>trans<\/em>-cyclobutene,\u00a0and\u00a0<em>trans<\/em>-cyclopentene have never been synthesized or observed.\u00a0<em>trans<\/em>-cyclohexene is a laboratory curiosity, stable at a few degrees above absolute zero.\u00a0<a href=\"http:\/\/pubs.acs.org\/doi\/abs\/10.1021\/jo00389a067\"><em>trans<\/em>-cycloheptene has an extremely short half-life at room temperature.<\/a>\u00a0<em>trans<\/em>-cyclooctene is a stable molecule [it also exhibits <a href=\"https:\/\/en.wikipedia.org\/wiki\/Cyclooctene\">axial\u00a0chirality<\/a>, which is interesting! ].<\/p>\n<hr \/>\n<h2><a id=\"quiz\"><\/a>Quiz Yourself!<\/h2>\n\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/1591-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <\/p>\n\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/2941-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. &nbsp;<\/p>\n\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/1592-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <br \/>\n<\/p>\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/1593-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <br \/>\n<\/p>\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/1594-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <br \/>\n<\/p>\n<p class=\"p1\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-26714\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-previews\/3110-Front-Image-Only.png\" alt=\"\" width=\"640\" height=\"616\" \/><\/p>\n<p><a href=\"https:\/\/www.masterorganicchemistry.com\/moc-membership\/\"><strong>Become a\u00a0 MOC member<\/strong><\/a> to see the clickable quiz with answers on the back. <\/p>\n","protected":false},"excerpt":{"rendered":"<p>E and Z Notation For Alkenes Unlike C\u2013C single bonds, C\u2013C double bonds can&#8217;t undergo rotation without breaking the pi bond One consequence of this <\/p>\n","protected":false},"author":1,"featured_media":37380,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1418],"tags":[879,527,1135,878,242,273,476,1136],"post_folder":[],"class_list":["post-10224","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-alkene-reactions","tag-cahn-ingold-prelog","tag-cis","tag-e-and-z","tag-entgegen","tag-nomenclature","tag-stereochemistry","tag-trans","tag-zusammen"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>E and Z Notation For Alkenes (+ Cis\/Trans) - Master Organic Chemistry<\/title>\n<meta name=\"description\" content=\"When do we use cis- and trans- and when do we use E and Z? 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