{"id":10335,"date":"2017-01-24T16:43:47","date_gmt":"2017-01-24T21:43:47","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=10335"},"modified":"2025-06-09T19:17:09","modified_gmt":"2025-06-10T00:17:09","slug":"conjugation-and-resonance","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2017\/01\/24\/conjugation-and-resonance\/","title":{"rendered":"Conjugation And Resonance In Organic Chemistry"},"content":{"rendered":"<p><strong>Conjugation In Organic Chemistry: Definition, Examples, Exploration, and Consequences<\/strong><\/p>\n<p>This is the first in a series of posts that\u00a0 cover conjugation, pi systems, molecular orbital theory, dienes, 1,2- and 1,4- additions, the Diels Alder reaction and other pericyclic reactions. We&#8217;re going to start by reviewing the basics!<\/p>\n<ul>\n<li>pi-bonds (\u03c0-bonds) are formed by the overlap of two adjacent p-orbitals<\/li>\n<li>if a p-orbital is present on an atom\u00a0<strong>adjacent<\/strong> to the pi bond, then it can also overlap with the p-orbitals from the pi-bond,\u00a0<strong>provided\u00a0<\/strong>that it is aligned in the\u00a0<strong>same plane<\/strong><\/li>\n<li>This overlap is called <strong>conjugation<\/strong> and allows for the de-localization of electrons we call\u00a0<strong>resonance<\/strong>.<\/li>\n<li>Atoms that can contain a p-orbital include <strong>carbocations<\/strong>, atoms bearing\u00a0<strong>lone pairs<\/strong>, atoms that participate in an adjacent\u00a0<strong>pi-bond<\/strong>, and atoms bearing a\u00a0<strong>free radical<\/strong><\/li>\n<li>Conjugation of an atom with an adjacent pi-bond will affect its bond lengths and electron distribution<\/li>\n<li>Conjugation is not possible on the <strong>bridgehead<\/strong> atom of many bicyclic molecules, since the p-orbital on the bridgehead cannot properly overlap with p-orbitals on adjacent atoms.<\/li>\n<\/ul>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-33855\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2023\/01\/0-summary-conjugation-and-resonance-necessity-of-orbital-overlap-1.gif\" alt=\"-summary conjugation and resonance - necessity of orbital overlap\" width=\"640\" height=\"761\" \/><\/a><\/p>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li><a href=\"#one\">Revisiting the Pi Bond (and Pi bonding): &#8220;Side-On&#8221; Orbital Overlap Between Adjacent p-Orbitals<\/a><\/li>\n<li><a href=\"#two\">The Importance Of Orbital Overlap For Pi Bonding<\/a><\/li>\n<li><a href=\"#three\">Beyond Pi Bonds: &#8220;Conjugation&#8221; Of 3 Or More p Orbitals<\/a><\/li>\n<li><a href=\"#four\">The Distinction Between Conjugation And Resonance<\/a><\/li>\n<li><a href=\"#five\">Consequences of Conjugation (1): \u00a0Bond Lengths<\/a><\/li>\n<li><a href=\"#six\">When Resonance Forms Are Not Identical, The Resonance Hybrid Will Be A &#8220;Weighted&#8221; Hybrid Of The Most Important Resonance Forms<\/a><\/li>\n<li><a href=\"#seven\">Consequences Of Conjugation (2): &#8220;Partial&#8221; Double Bonds<\/a><\/li>\n<li><a href=\"#eight\">Consequences of Conjugation (3): The Reactivity Of A Conjugated System Is Often Revealed By Its &#8220;Second-Best&#8221; Resonance Form<\/a><\/li>\n<li><a href=\"#nine\">Orbital Overlap (All p-Orbitals In The Same Plane) Is Required For Conjugation (And Resonance)<\/a><\/li>\n<li><a href=\"#ten\">Bridgehead Amides Are Not Conjugated, And Are Much More Easily Broken Than &#8220;Ordinary&#8221; Amides<\/a><\/li>\n<li><a href=\"#eleven\">More Consequences Of Conjugation: Color And Cycloadditions<\/a><\/li>\n<li><a href=\"#twelve\">Next Up: Molecular Orbital Theory<\/a><\/li>\n<li><a href=\"#notes\">Notes<\/a><\/li>\n<li><a href=\"#quizzes\">Quiz Yourself!<\/a><\/li>\n<\/ol>\n<hr \/>\n<h2><strong><a id=\"one\"><\/a>1. Revisiting the Pi Bond (and Pi bonding): &#8220;Side-On&#8221; Orbital Overlap Between Adjacent p-Orbitals<\/strong><\/h2>\n<p>One of the first things you learn about alkenes is that<strong> rotation about the C-C pi (\u03c0) bond does not occur<\/strong>. For instance, at normal temperatures and pressures.,\u00a0<em>trans<\/em>-2-butene (shown below left) is never observed to spontaneously convert to\u00a0<em>cis<\/em>-2-butene (right) . They&#8217;re separable compounds, with different melting and boiling points. You can buy each of them separately from <a href=\"http:\/\/www.sigmaaldrich.com\/catalog\/product\/aldrich\/400890?lang=en&amp;region=US\">Aldrich.<\/a> This wouldn&#8217;t be possible\u00a0if there was free rotation about the double bond.<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-15543\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-free-rotation-about-double-bonds-does-not-happen-trans-does-not-convert-to-cis-because-pi-bonding-is-side-on-overlap.gif\" alt=\"free rotation about double bonds does not happen trans does not convert to cis because pi bonding is side on overlap\" width=\"600\" height=\"263\" \/><\/p>\n<p>Digging deeper into this, we&#8217;ve seen that this is due to a phenomenon called &#8220;<a href=\"https:\/\/en.wikipedia.org\/wiki\/Pi_bond\">pi bonding<\/a>&#8221; &#8211; a <strong>side-on overlap of two adjacent p orbitals<\/strong>, each containing an electron, which results in a preferred orientation where the p-orbitals &#8220;line up&#8221; next to each other, like soldiers. Due to the dumbbell-like geometry of the p-orbital, overlap isn&#8217;t possible when the two p-orbitals are at 90\u00b0 to each other, which accounts for that &#8220;rotational barrier&#8221;.<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-15544\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-side-on-overlap-of-two-p-orbitals-in-pi-bonding-p-orbitals-are-in-same-plane-if-they-are-at-90-degrees-no-bonding-occurs.png\" alt=\"side on overlap of two p orbitals in pi bonding p orbitals are in same plane if they are at 90 degrees no bonding occurs\" width=\"450\" height=\"170\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-side-on-overlap-of-two-p-orbitals-in-pi-bonding-p-orbitals-are-in-same-plane-if-they-are-at-90-degrees-no-bonding-occurs.png 600w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-side-on-overlap-of-two-p-orbitals-in-pi-bonding-p-orbitals-are-in-same-plane-if-they-are-at-90-degrees-no-bonding-occurs-300x113.png 300w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-side-on-overlap-of-two-p-orbitals-in-pi-bonding-p-orbitals-are-in-same-plane-if-they-are-at-90-degrees-no-bonding-occurs-320x121.png 320w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-side-on-overlap-of-two-p-orbitals-in-pi-bonding-p-orbitals-are-in-same-plane-if-they-are-at-90-degrees-no-bonding-occurs-360x136.png 360w\" sizes=\"(max-width: 450px) 100vw, 450px\" \/><\/p>\n<p><span style=\"color: #993366;\"><em>[The shaded (blue) and unshaded (white) lobes of each p-orbital represents a property called &#8220;phase&#8221;, which is a property of\u00a0p-orbitals &#8211; I&#8217;d suggest revisiting this topic if this makes you queasy, because we&#8217;ll be using it a lot in the coming posts. As in waves, there is constructive interference between lobes\u00a0of &#8220;like&#8221; phase, and destructive interference between lobes\u00a0of &#8220;unlike&#8221; phase]\u00a0<\/em><\/span><\/p>\n<p>This has other physical consequences besides the rotation barrier: it influences <strong>molecular geometry<\/strong> as well. \u00a0Since \u00a0Pi-bonding is a phenomenon exclusive to p-orbitals, that means that each pi bond an atom participates in will leave one fewer p-orbital available for &#8220;<strong>hybridization<\/strong>&#8221; with the s orbital [and remaining p orbital(s)] on the atom (<em>See article: <a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/10\/10\/hybrid-orbitals\/\">Hybrid Orbitals<\/a><\/em>). This results in the familiar &#8220;trigonal planar&#8221; (sp<sup>2<\/sup> ) geometry for typical alkene carbons and &#8220;linear&#8221; geometry (sp) for alkyne carbons.<\/p>\n<p>Hence, alkenes are &#8220;flat&#8221;, as opposed to alkyl carbons, which adopt a tetrahedral geometry.<\/p>\n<h2><strong><a id=\"two\"><\/a>2. The Importance Of Orbital Overlap For Pi Bonding<\/strong><\/h2>\n<p>A vivid illustration of the importance of orbital overlap is presented by a case\u00a0where we might naively think a double bond &#8220;should&#8221; form &#8211; <em>but does not.<\/em> <a href=\"https:\/\/en.wikipedia.org\/wiki\/Julius_Bredt\">Bredt <\/a>observed in 1924 that alkenes tend not to form on &#8220;bridgehead&#8221; positions, such as in the molecule at bottom left, an observation that came to be called &#8220;<a href=\"https:\/\/www.masterorganicchemistry.com\/2014\/09\/02\/bredts-rule-and-summary-of-cycloalkanes\/\">Bredt&#8217;s rule<\/a>&#8220;.<\/p>\n<p>Why not?\u00a0If you make the model, you&#8217;ll see that the geometry of the bicyclic ring forces those p orbitals to be oriented at right angles. <strong>There&#8217;s no overlap between the p orbitals<\/strong>. Therefore, it resembles a carbon with two adjacent <strong>radicals<\/strong> more than it does a real pi bond!<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15545\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-pi-bonding-is-not-possible-in-bridgehead-alkenes-since-p-orbitals-are-at-right-angles-to-each-other-not-properly-aligned-bredts-rule.gif\" alt=\"pi bonding is not possible in bridgehead alkenes since p orbitals are at right angles to each other not properly aligned bredts rule\" width=\"600\" height=\"323\" \/><\/p>\n<p>Three dimensional drawings on a flat surface don&#8217;t really do justice to glory of the 3-dimensional structure. Here&#8217;s a model and accompanying video.\u00a0<span style=\"color: #993366;\">\u00a0<em>[RIP\u00a0Vine, which was awesome for short organic chemistry videos]<\/em><\/span><\/p>\n<p>Those pink things are supposed to be the p-orbitals. See how they&#8217;re at right angles to each other? That means they don&#8217;t overlap.<\/p>\n<p><iframe class=\"giphy-embed\" src=\"https:\/\/giphy.com\/embed\/l0ExbVmCs01p81jWg\" width=\"480\" height=\"480\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<p><a href=\"https:\/\/giphy.com\/gifs\/l0ExbVmCs01p81jWg\">via GIPHY<\/a><\/p>\n<p><strong>The bottom line is that you need overlap between p orbitals in order for a Pi bond to form.<\/strong> In acyclic systems that&#8217;s generally not a problem [see <a href=\"#noteone\">Note 1<\/a> for a prominent exception]<\/p>\n<h2><a id=\"three\"><\/a>3. Beyond Pi Bonds: &#8220;Conjugation&#8221; Of 3 Or More p Orbitals<\/h2>\n<p>In first semester organic chemistry we learn that this overlap of p\u00a0orbitals\u00a0is not necessarily confined to two adjacent\u00a0p orbitals. Overlap can extend beyond two p orbitals to include<strong> three, four, five, and even more consecutive p orbitals on consecutive\u00a0atoms<\/strong>, building\u00a0larger &#8220;pi-systems&#8221;\u00a0<span style=\"color: #993366;\"><em>(witness <strong><a style=\"color: #993366;\" href=\"https:\/\/en.wikipedia.org\/wiki\/Lycopene\">lycopene<\/a><\/strong>, for instance).<\/em><\/span><\/p>\n<p>We also see that definition of\u00a0\u00a0&#8220;p orbital&#8221; is somewhat flexible, and can\u00a0include examples such as<\/p>\n<ul>\n<li>an empty p orbital (such as that in a carbocation, or the empty p orbital on boron)<\/li>\n<li>an orbital containing a lone pair (e.g. on nitrogen, oxygen, fluorine, etc.)<\/li>\n<li>p-orbitals of a pi bond [such as another alkene, C=O (carbonyl), etc.]<\/li>\n<li>a half-filled orbital (e.g. a radical)<\/li>\n<\/ul>\n<p>We call this &#8220;building up&#8221;\u00a0of p orbitals into larger &#8220;pi systems&#8221;, \u00a0&#8220;<strong>conjugation&#8221;.\u00a0<\/strong>In each of the middle molecules below, the alkene (pi bond) is conjugated with an adjacent p orbital.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15546\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-examples-of-conjugation-pi-bond-with-carbocation-lone-pair-conjugation-with-other-pi-bond-conjugation-with-radical-example-of-no-conjugation.gif\" alt=\"examples of conjugation pi bond with carbocation lone pair conjugation with other pi bond conjugation with radical example of no conjugation\" width=\"630\" height=\"542\" \/><\/p>\n<p>The &#8220;conjugation killer&#8221; to watch out for is an\u00a0atom lacking lone pairs connected to only single bonds, such as CH<sub>2<\/sub> in the example below-right (1,4-pentadiene). These two pi bonds are <strong>not<\/strong> conjugated.<\/p>\n<p><span style=\"color: #993366;\"><em>[A note on the second example. We usually think of the\u00a0geometry of a nitrogen with three single bonds\u00a0as<strong> trigonal pyramidal<\/strong>\u00a0(e.g. as in NH<sub>3<\/sub>).\u00a0But\u00a0in the presence of an adjacent pi bond, there is a slight &#8220;re-hybridization&#8221; of the nitrogen from sp<sup>3<\/sup> to sp<sup>2<\/sup> (<strong>trigonal planar<\/strong>) such that the lone pair is in a p orbital, not an sp<sup>3<\/sup> orbital. \u00a0This is a tradeoff: \u00a0the slightly increased strain of the eclipsed N-H bonds is made up for by a decrease in overall energy due to better overlap of a p orbital with the pi bond. We usually think of this as <strong>&#8220;resonance energy&#8221;<\/strong>]<\/em><\/span><\/p>\n<p>Here&#8217;s a fun\u00a0 trick question. Are the the double bonds in the molecule below (<strong>allene<\/strong>) conjugated? Why or why not?<\/p>\n<div class=\"wq-quiz-wrapper\" data-id=\"40393\"><style type=\"text\/css\" id=\"wq-flip-custom-css\">.wq-quiz-wrapper[data-id=\"40393\"] {\n--wq-question-width: 100%;\n--wq-question-color: #009cff;\n--wq-question-height: auto;\n--wq-font-color: #444;\n}\n\n\t\t\t.wq-quiz-wrapper[data-id=\"40393\"] {\n\t\t\t\t--wq-question-width: 600px;\n\t\t\t}\n\n\t\t\t@media screen and (max-width: 600px) {\n\t\t\t\t.wq-quiz-wrapper[data-id=\"40393\"] .wq_singleQuestionWrapper { width:100% !important; height:auto !important; }\n\t\t\t}\n\t\t<\/style><!-- wp quiz -->\n<div id=\"wp-quiz-40393\" class=\"wq_quizCtr single flip_quiz wq-quiz wq-quiz-40393 wq-quiz-flip wq-layout-single wq-skin-traditional wq-should-show-correct-answer\" data-quiz-id=\"40393\">\n<div class=\"wq-questions wq_questionsCtr\">\n\t<div class=\"wq-question wq_singleQuestionWrapper wq-question-iiff1\" data-id=\"iiff1\">\n\n\t\n\t<div class=\"item_top\">\n\t\t<div class=\"title_container\">\n\t\t\t<div class=\"wq_questionTextCtr\">\n\t\t\t\t<h4 class=\"wq-question-title\"><\/h4>\n\t\t\t<\/div>\n\t\t<\/div>\n\t<\/div>\n\n\t<div class=\"card \">\n\t\t<div class=\"front\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2414-Front.gif\" \/>\n\t\t\n\t\t\n\t\n\t\n\t\t\t<span class=\"top-desc\">Click to Flip<\/span>\n\t<\/div>\n\t\t<div class=\"back\" >\n\t\n\t\t\t\t\t<img decoding=\"async\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/quiz-images\/2414-Reverse.gif\" \/>\n\t\t\n\t\t\n\t\n\t<\/div>\n\t<\/div>\n\n\t\n<\/div>\n<\/div>\n<\/div>\n<!-- \/\/ wp quiz-->\n<\/div><!-- End .wq-quiz-wrapper -->\n<h2><a id=\"four\"><\/a>4. The Distinction Between Conjugation And Resonance<\/h2>\n<p>You might well ask: this just sounds like <a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/12\/22\/in-summary-resonance\/\">resonance<\/a>. What&#8217;s the difference?<\/p>\n<p>Let&#8217;s take a second to distinguish\u00a0<em>conjugation<\/em> and\u00a0<em>resonance<\/em>.<\/p>\n<ul>\n<li><strong>Conjugation<\/strong> is what we call it when 3 or more p orbitals<strong>\u00a0join together<\/strong> into a larger &#8220;pi system&#8221;.<\/li>\n<li>These conjugated pi systems contain electrons, which we often call &#8220;pi electrons&#8221; to distinguish them from the electrons that comprise single bonds in the molecule.<\/li>\n<li>The different arrangements of electrons within that &#8220;pi system&#8221; are called <strong>resonance forms<\/strong>.<\/li>\n<\/ul>\n<p>A rough analogy could go\u00a0like this:<\/p>\n<ul>\n<li>Think of p orbitals as being\u00a0a bit like &#8220;rooms&#8221; for electrons (maximum occupancy: 2)<\/li>\n<li>Joining several rooms together into a larger\u00a0building\u00a0is\u00a0conjugation<\/li>\n<li>The different allowable <em>arrangements of people (electrons) within that building\u00a0<\/em>are <strong>resonance forms.<\/strong><\/li>\n<\/ul>\n<p>The key requirement for conjugation is\u00a0<em>orbital overlap<\/em>, which we&#8217;ll\u00a0expand on in a bit.<\/p>\n<p>For now, let&#8217;s review some consequences of conjugation.<\/p>\n<h2><strong><a id=\"five\"><\/a>5. Consequences of Conjugation (1): \u00a0Bond Lengths\u00a0<\/strong><\/h2>\n<p>As I just said, we&#8217;re more\u00a0used to conjugation in the context of &#8220;<a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/12\/22\/in-summary-resonance\/\">resonance<\/a>&#8220;, a concept\u00a0we&#8217;ve covered before (and since this series is going into second-semester territory, is worth re-familiarizing yourself with)<\/p>\n<p>For instance, with the acetate\u00a0ion (CH<sub>3<\/sub>CO<sub>2<\/sub>)<sup>&#8211;<\/sup>\u00a0 and the allyl cation (both shown below),\u00a0we saw that there&#8217;s two different\u00a0ways of arranging the pi electrons, which we call &#8220;resonance forms&#8221;.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15548\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/7-consequences-of-conjugation-resonance-bond-lengths-orbital-hybrid-acetate-ion-has-c-o-bond-length-of-126-angstrom-allyl-cation-hybrid-carbon-carbon-bond-length-140-angstrom.gif\" alt=\"consequences of conjugation resonance bond lengths orbital hybrid acetate ion has c o bond length of 126 angstrom allyl cation hybrid carbon carbon bond length 140 angstrom\" width=\"630\" height=\"539\" \/><\/p>\n<p>The important point to note is that the pi-electrons in these\u00a0are not constantly switching back-and-forth between atoms; rather,<strong> the &#8220;true&#8221; structure of the molecule is a hybrid of these resonance forms.<\/strong><\/p>\n<p>One important consequence of resonance is <strong>bond lengths that are intermediate between two forms.\u00a0<\/strong><\/p>\n<p>For example, the C-O bond length in the acetate ion (1.26 \u00c5) \u00a0is\u00a0<strong>between what we&#8217;d expect for a C-O pi bond (1.20 \u00c5) and a C-O single bond (1.4 \u00c5).\u00a0<\/strong><\/p>\n<h2><strong><a id=\"six\"><\/a>6. When Resonance Forms Are Not Identical, The Resonance Hybrid Will Be A &#8220;Weighted&#8221; Hybrid Of The Most Important Resonance Forms<\/strong><\/h2>\n<p>In the acetate ion and the allyl cation the two important resonance forms are <strong>equivalent<\/strong>, so both end up contributing equally to the hybrid.<\/p>\n<p>A more common situation is found molecules like the ones below there&#8217;s a blending of\u00a0<span style=\"text-decoration: underline;\">unequal<\/span> resonance forms. Some resonance forms are more important than others.\u00a0<span style=\"color: #993366;\"><em>[to go back to determining <a style=\"color: #993366;\" href=\"https:\/\/www.masterorganicchemistry.com\/2011\/12\/22\/in-summary-resonance\/\" target=\"_blank\" rel=\"noopener noreferrer\">how to evaluate resonance forms, go to this series of posts<\/a>. ].<\/em><\/span><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15549\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/8-nonequivalent-resoanance-forms-major-and-minor-contributors-example-butadiene-and-acetamide-butadiene-central-c-c-bond-length-shorter-than-normal.gif\" alt=\"nonequivalent resoanance forms major and minor contributors example butadiene and acetamide butadiene central c c bond length shorter than normal\" width=\"630\" height=\"340\" \/><\/p>\n<p>Our visual language of chemistry with its sharp distinction between single and double bonds does not accurately depict the electron density in the molecules, which are a <strong>weighted<\/strong>\u00a0<em>hybrid<\/em> of resonance forms. Minor resonance contributors influence the bond lengths in the molecule, making them shorter or longer than normal.<\/p>\n<p>In the\u00a0top\u00a0molecule (butadiene), that central C-C bond is a little bit\u00a0<strong>shorter\u00a0<\/strong>than a &#8220;normal&#8221; C-C single bond (i.e. it has a bit of double bond character) due to the influence of the minor resonance contributor to the hybrid.<span style=\"color: #993366;\"> <em>[Note that the bond length is not halfway between single and double C-C bond, as it was in the allyl cation: that&#8217;s because the two resonance forms are not equally important (i.e. do not make equal contributions to the resonance hybrid)]\u00a0<\/em><\/span><\/p>\n<p>In the bottom\u00a0molecule (&#8220;acetamide&#8221;) the C-O bond is a little bit<strong> longer<\/strong> than a &#8220;normal&#8221; C=O bond (i.e. has more single-bond character) and the C-N bond is a little bit\u00a0<strong>shorter<\/strong> than a &#8220;normal&#8221; C-N bond (i.e. has more double-bond character). This reflects the influence of the &#8220;minor contributor&#8221; (or &#8220;second best&#8221; contributor, if you like) in which there is a C-N pi bond and a C-O single bond.<\/p>\n<p>Let&#8217;s make our first conceptual leap.<\/p>\n<h2><a id=\"seven\"><\/a>7. Consequences Of Conjugation (2): &#8220;Partial&#8221; Double Bonds<\/h2>\n<p>There&#8217;s an interesting consequence of that &#8220;partial double bond character&#8221; in the C-N bond. <strong>It has a &#8220;barrier to rotation&#8221; just like we&#8217;d expect from a &#8220;double bond&#8221;!<\/strong> \u00a0The barrier to rotation in the C-N bond of amides is about\u00a015-20 kcal\/mol in peptide bonds (compare to about 2-3 kcal\/mol for most C-C bonds).<\/p>\n<p>What this means is that the two conformations<i>\u00a0<\/i>can<em>\u00a0<\/em>still\u00a0interconvert,\u00a0but they do so relatively slowly at room temperature. In the molecule below (<em>N<\/em>-methyl acetamide) \u00a0it&#8217;s possible to observe the\u00a0<em>s<\/em>-cis<em>\u00a0<\/em>conformer (both green methyl groups on the same side of the C-N bond) and the\u00a0<em>s-trans<\/em> conformer (green methyl groups on opposite sides of the C-N bond) separately. [<a href=\"#notefour\">Note 4<\/a>] This usually isn&#8217;t possible for conformers unless you take the temperature down to 100 Kelvin or so!<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15550\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/9-one-consequence-of-partial-double-bond-character-in-amides-is-that-they-have-restricted-rotation-s-cis-s-trans-barrier-to-rotation-about-15-to-20-kcal-per-mol.gif\" alt=\"one consequence of partial double bond character in amides is that they have restricted rotation s cis s trans barrier to rotation about 15 to 20 kcal per mol\" width=\"600\" height=\"371\" \/><\/p>\n<p>For the image above, the crucial concept here is just the partial double bond character. If\u00a0<em>s<\/em>-cis and\u00a0<em>s<\/em>-trans doesn&#8217;t make sense to you after thinking about it for a bit, it&#8217;s OK. Not crucial for the rest of the discussion.<\/p>\n<h2><strong><a id=\"eight\"><\/a>8. Consequences of Conjugation (3): The Reactivity Of A Conjugated System Is Often Revealed By Its &#8220;Second-Best&#8221; Resonance Form<\/strong><\/h2>\n<p>We&#8217;ve shown the effect that conjugation (and by extension, resonance) has on bond lengths.<\/p>\n<p>Let&#8217;s take a closer look at its effect on\u00a0<em>electron density<\/em>, which ultimately influences\u00a0<em>reactivity<\/em>.<\/p>\n<p>For illustrative purposes, let&#8217;s continue to look at alkenes.<\/p>\n<p>The reactivity of an alkene can be modified dramatically through the attachment of various groups.<\/p>\n<p>Look at the &#8220;second best&#8221; resonance form when we attach a \u00a0<a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/12\/15\/exploring-resonance-pi-donation\/\"><strong>pi-donor<\/strong><\/a> such as N(CH<sub>3<\/sub>)<sub>2<\/sub> to an alkene. This results in a build-up of negative charge (\u03b4<sup>&#8211;<\/sup>) on terminal carbon of the alkene, with the result that this alkene (which we call an <strong>enamine<\/strong>) is an excellent nucleophile. To take just one prominent example, enamines react with alkyl halides (such as CH<sub>3<\/sub>I) and other electrophiles in a class of reactions sometimes referred to as<a href=\"https:\/\/www.masterorganicchemistry.com\/2010\/05\/24\/imines-and-enamines\/\"><strong> Stork Enamine<\/strong><\/a> reactions after their discoverer, <a href=\"https:\/\/en.wikipedia.org\/wiki\/Gilbert_Stork\" target=\"_blank\" rel=\"noopener noreferrer\">Gilbert Stork<\/a>. \u00a0Ordinary alkenes such as 2-butene (below) don&#8217;t work in this reaction.<\/p>\n<p>Attachment of a<a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/12\/19\/exploring-resonance-pi-acceptors\/\"> <strong>pi-acceptor<\/strong><\/a> such as C=O results in a \u00a0build-up of positive charge (\u03b4<sup>+<\/sup>)\u00a0on the terminal carbon of the alkene, with the result that this species (which we call an \u03b1, \u03b2\u00a0unsaturated aldehyde, Michael acceptor, or enone) is an excellent electrophile. \u03b1,\u00a0\u03b2\u00a0 unsaturated carbonyls react with nucleophiles (such as CH<sub>3<\/sub>S<sup>&#8211;<\/sup>)<sup>\u00a0<\/sup>and many other classes of nucleophiles\u00a0in a general type of reaction we call <strong>conjugate additions<\/strong> or sometimes Michael reactions.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15551\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/10-consequence-of-conjugation-is-major-and-minor-contributors-to-hybrid-second-best-resonance-form-is-very-helpful-.gif\" alt=\"consequence of conjugation is major and minor contributors to hybrid second best resonance form is very helpful\" width=\"630\" height=\"459\" \/><\/p>\n<p>This is just the tip of the iceberg as to how conjugation can affect reactivity, but it&#8217;s enough for today.<\/p>\n<h2><a id=\"nine\"><\/a>9. Orbital Overlap (All p-Orbitals In The Same Plane) Is Required For Conjugation (And Resonance)<\/h2>\n<p>So far we&#8217;ve seen that:<\/p>\n<ul>\n<li>overlap between p orbitals is necessary to form pi bonds<\/li>\n<li>some &#8220;single bonds&#8221; can have &#8220;pi bond character&#8221; due to contribution from a minor resonance form (such as amides, for example)<\/li>\n<\/ul>\n<p>Here is the logical consequence of these two statements:<\/p>\n<ul>\n<li><strong>In order for conjugation to exist, and therefore in order for resonance to occur, all the p orbitals must overlap. They must therefore all be aligned in the same plane.<\/strong><\/li>\n<\/ul>\n<p>Remember the\u00a0&#8220;allyl cation&#8221; that is &#8220;stabilized by resonance&#8221;? In order\u00a0for the carbocation to gain this\u00a0&#8220;resonance stabilization&#8221;, the empty p orbital on the carbocation\u00a0<strong>must<\/strong>\u00a0be lined up with the adjacent pi bond.<\/p>\n<p>If the p orbital is at an angle of 90 degrees from the p orbitals in the pi bond, there is no conjugation and thus no resonance stabilization.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15557\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/11-allyl-cation-is-more-stable-than-normal-carbocation-but-stabilization-only-occurs-if-p-orbitals-overlap.gif\" alt=\"allyl cation is more stable than normal carbocation but stabilization only occurs if p orbitals overlap\" width=\"630\" height=\"572\" \/><\/p>\n<p>As we might predict, there is a barrier to rotation in the allyl cation, just as there is a barrier to rotation in an alkene.\u00a0The barrier to rotation in the allyl cation is about <strong>37 kcal\/mol<\/strong> &#8211; a little more than half the strength of a C-C pi bond.<\/p>\n<p>We also saw that\u00a0the C-N bond in amides has partial double bond character, with a barrier to rotation of about 15-20 kcal\/mol.<\/p>\n<p>Likewise, this &#8220;partial double bond&#8221; character is only\u00a0possible if the p orbital containing the lone pair is able to overlap with the p orbitals comprising the C=O pi bond.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15552\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/12-amide-c-n-bonds-have-partial-double-bond-character-restricted-rotation-need-to-have-p-orbital-aligned-with-carbonl-pi-system.gif\" alt=\"amide c n bonds have partial double bond character restricted rotation need to have p orbital aligned with carbonl pi system\" width=\"600\" height=\"569\" \/><\/p>\n<p>The partial double bond character of the C-N bond in amides has long been thought to be the main reason why they are much more resistant to breakage than, say, esters <span style=\"color: #993366;\"><em>[for an advanced treatment, see<a style=\"color: #993366;\" href=\"#notetwo\"><strong> Note 2<\/strong><\/a>]<\/em>.<\/span> Since the proteins in our body are joined by peptide (amide) linkages, this is a matter of no small importance! Life-forms based on ester rather than amide linkages would be much more fragile!<\/p>\n<h2><strong><a id=\"ten\"><\/a>10. Bridgehead Amides Are Not Conjugated, And Are Much More Easily Broken Than &#8220;Ordinary&#8221; Amides<\/strong><\/h2>\n<p><strong>Bridgehead amides\u00a0<\/strong>give an illustration of what happens to amides when overlap is impossible.<\/p>\n<p>Just as we saw in bridgehead alkenes, in bridgehead\u00a0<strong>amides<\/strong>, orbital overlap between the nitrogen lone pair and carbonyl carbon is impossible due to twisting. The result is that the C-N bond does NOT have partial double bond character and it is much easier to break than a &#8220;normal&#8221; amide.<\/p>\n<p>The bridgehead amide below is &#8220;quinuclidone&#8221;, a twisted amide that eluded synthesis for decades. It was only in 2006 that it was finally made (as its conjugate acid) through a clever route by\u00a0the lab of <a href=\"http:\/\/www.nature.com\/nature\/journal\/v441\/n7094\/pdf\/nature04842.pdf\">Brian Stoltz at Caltech<\/a>.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15553\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/13-bridgehead-amide-not-in-conjugation-with-carbonyl-nitrogen-twisted-90-from-co-orbitals-note-x-ray-structure.png\" alt=\"bridgehead amide not in conjugation with carbonyl nitrogen twisted 90 from co orbitals note x ray structure\" width=\"450\" height=\"162\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/13-bridgehead-amide-not-in-conjugation-with-carbonyl-nitrogen-twisted-90-from-co-orbitals-note-x-ray-structure.png 600w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/13-bridgehead-amide-not-in-conjugation-with-carbonyl-nitrogen-twisted-90-from-co-orbitals-note-x-ray-structure-300x108.png 300w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/13-bridgehead-amide-not-in-conjugation-with-carbonyl-nitrogen-twisted-90-from-co-orbitals-note-x-ray-structure-320x115.png 320w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/13-bridgehead-amide-not-in-conjugation-with-carbonyl-nitrogen-twisted-90-from-co-orbitals-note-x-ray-structure-360x130.png 360w\" sizes=\"(max-width: 450px) 100vw, 450px\" \/><\/p>\n<p>The X-ray crystal structure bears witness to the lack of conjugation in this amide. The C-N bond length is 1.52\u00a0\u00c5 (typical of a single C-N bond) and the C=O bond length is 1.19 \u00c5, which is typical of a bond length in an aldehyde or ketone (1.20 \u00c5). We would therefore expect that it is quite a bit more unstable towards nucleophilic attack than a normal amide, which was borne out in the Stoltz lab&#8217;s study.<\/p>\n<p>Here&#8217;s a 3-D model of quinuclidine (courtesy of <a href=\"https:\/\/www.rowansci.com\/\">Rowan<\/a>). The lone pair on nitrogen is not visible here, but just like the bridgehead C-H bond, it points straight out from the bridgehead and cannot overlap with the C-O pi bond.<\/p>\n<p><iframe title=\"\u201cQuinuclidinone\u201d\" src=\"https:\/\/labs.rowansci.com\/iframe2\/calculations\/b393f3e7-8468-495a-9f06-ca162d596335\" width=\"640\" height=\"640\"><\/iframe><\/p>\n<p>&nbsp;<\/p>\n<h2><strong><a id=\"eleven\"><\/a>11. More Consequences Of Conjugation: Color And Cycloadditions<\/strong><\/h2>\n<p>So far, this post has pretty much been a review of 1st semester concepts. \u00a0There&#8217;s nothing in the discussion above that couldn&#8217;t be reasonably explained by what we&#8217;ve already learned about conjugation and resonance.<\/p>\n<p>However, this simplistic approach can only take us so far.<\/p>\n<p>Two quick examples, because this post has gone on long enough.<\/p>\n<p><strong>First: Conjugation And Color<\/strong><\/p>\n<p>If you saw this post on <a href=\"https:\/\/www.masterorganicchemistry.com\/2016\/09\/08\/how_bleach_works\/\">how bleach works<\/a>, you learned that as we\u00a0lengthen the conjugation length, we change the wavelength at\u00a0which molecules absorb light. Some very brightly coloured molecules such as carotene, chlorophyll and lycopene all have very long conjugated double bonds.<\/p>\n<p>For instance, lycopene is responsible for the red colour of tomatoes. If we remove the double bonds, we remove the colour. Why?<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15554\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/14-consequences-of-conjugation-is-color-eg-lycopene-is-deep-red-and-lycopane-is-colorless.gif\" alt=\"consequences of conjugation is color eg lycopene is deep red and lycopane is colorless\" width=\"630\" height=\"372\" \/><\/p>\n<p>What&#8217;s the relationship between color and conjugation? Why would the number of double bonds have an influence on this? Resonance fails to illuminate this subject.<\/p>\n<p><strong>Second: Reactions of Dienes That Form Rings<\/strong><\/p>\n<p>Here&#8217;s some weird reactions for you.<\/p>\n<p>If you treat butadiene with the molecule to its right (methyl maleate) you obtain a new product containing a six membered ring. Nothing we&#8217;ve seen so far prepares us for this type of reaction, which is called a &#8220;cycloaddition&#8221;.<\/p>\n<p>Interestingly, if you treat ethene with the same molecule, nothing happens<span style=\"color: #993366;\">\u00a0<\/span><em><span style=\"color: #993366;\">(except if you treat it with UV light. Then you get a 4-membered ring, but I digress).<\/span>\u00a0<\/em><\/p>\n<p><em>WHY?<\/em><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15555\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/15-diels-alder-reaction-of-butadiene-with-maleic-acid-gives-new-six-membered-ring-but-resonance-doesnt-really-help-to-understand-why.gif\" alt=\"diels alder reaction of butadiene with maleic acid gives new six membered ring but resonance doesnt really help to understand why\" width=\"600\" height=\"396\" \/><\/p>\n<p>Resonance doesn&#8217;t help us here either.<\/p>\n<h2><strong><a id=\"twelve\"><\/a>12. Next Up: Molecular Orbital Theory<\/strong><\/h2>\n<p>What <strong>will<\/strong> help us answer these questions, as well as many others going forward is a concept called\u00a0<strong>molecular orbital theory.\u00a0<\/strong><\/p>\n<p>In the following posts in this series, we&#8217;re going to dig more deeply into how p orbitals overlap to form\u00a0<strong>molecular orbitals<\/strong>, and we&#8217;ll examine the energy levels of these orbitals. We&#8217;ll also see how this influences the reactivity of molecules and allows us to make predictions about their chemical behaviour.<\/p>\n<p>As we&#8217;ll see, molecular orbital theory provides us with a very powerful set of concepts that will help us understand chemical reactivity at a much deeper level.<\/p>\n<p><strong>Thanks to Tom\u00a0Struble\u00a0for all his help with this post.\u00a0<\/strong><\/p>\n<hr \/>\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\/2017\/02\/14\/molecular-orbital-pi-bond\/\" class=\"\"><span>Bonding And Antibonding Pi Orbitals<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/12\/22\/in-summary-resonance\/\" class=\"\"><span>In Summary: Evaluating Resonance Structures<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2016\/09\/08\/conjugation_and_color\/\" class=\"\"><span>Conjugation And Color (+ How Bleach Works)<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2017\/02\/28\/pi-molecular-orbitals-of-butadiene\/\" class=\"\"><span>Pi Molecular Orbitals of Butadiene<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2018\/02\/28\/amides-properties-synthesis-and-nomenclature\/\" class=\"\"><span>The Amide Functional Group: Properties, Synthesis, and Nomenclature<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/organic-chemistry-practice-problems\/molecular-orbital-theory-practice\/\" class=\"\"><span>Molecular Orbital Theory Practice<\/span><\/a><\/li><\/ul><\/div>\n<p><a id=\"noteone\"><\/a>\u00a0<strong>Note 1. <\/strong>A-1,2 strain is the reason why<a href=\"https:\/\/en.wikipedia.org\/wiki\/Tetra-tert-butylethylene\"> tetra-tert-butylethylene <\/a>has not yet been synthesized.<\/p>\n<p><strong><a id=\"notetwo\"><\/a>Note 2. <\/strong>The pi bonds are\u00a0<em>not<\/em> conjugated. Look at the orbitals comprising the two pi bonds. They are at right angles to each other and cannot overlap.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-15556\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-orbitals-in-allene-are-at-right-angles-to-each-other-and-therefore-are-not-conjugated.png\" alt=\"orbitals in allene are at right angles to each other and therefore are not conjugated\" width=\"450\" height=\"217\" srcset=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-orbitals-in-allene-are-at-right-angles-to-each-other-and-therefore-are-not-conjugated.png 400w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-orbitals-in-allene-are-at-right-angles-to-each-other-and-therefore-are-not-conjugated-300x145.png 300w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-orbitals-in-allene-are-at-right-angles-to-each-other-and-therefore-are-not-conjugated-320x154.png 320w, https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/F1-orbitals-in-allene-are-at-right-angles-to-each-other-and-therefore-are-not-conjugated-360x174.png 360w\" sizes=\"(max-width: 450px) 100vw, 450px\" \/><\/p>\n<p><strong><a id=\"notethree\"><\/a>Note 3. <\/strong>recent work suggests that resonance is not as important in amide bond restriction as was thought previously. See <a href=\"https:\/\/books.google.com\/books?id=gY-Sxijk_tMC&amp;pg=PA23&amp;lpg=PA23&amp;dq=peptide+bond+rotation+barrier&amp;source=bl&amp;ots=euzHhfMjXe&amp;sig=5FO1CqA02PF1WlgJERE6HWxCUqc&amp;hl=en&amp;sa=X&amp;ved=0ahUKEwi52va45cnRAhVm9IMKHQySCvUQ6AEIfDAS#v=onepage&amp;q=peptide%20bond%20rotation%20barrier&amp;f=false\">here<\/a> (Modern Physical Organic Chemistry by Anslyn and Dougherty, p. 23)]<\/p>\n<p><strong><a id=\"notefour\"><\/a>Note 4. <\/strong>Using NMR, nuclear magnetic resonance spectroscopy. These conformational isomers are sometimes called &#8220;<a href=\"https:\/\/en.wikipedia.org\/wiki\/Conformational_isomerism\">rotational isomers<\/a>&#8221; or &#8220;rotamers&#8221; and they can complicate NMR spectra considerably. The peaks can usually be made to coalesce by heating the sample probe.<\/p>\n<p>This stands in contrast to, say, different chair forms of cyclohexane, which generally can still interconvert at room temperature and coalesce to a single peak. However if you cool a substituted cyclohexane enough you can &#8220;freeze out&#8221; the different conformers and observe them separately.<\/p>\n<hr \/>\n<h2><a id=\"quizzes\"><\/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\/3205-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\/3209-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\/3210-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\/3211-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\/2514-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\/3220-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>Conjugation In Organic Chemistry: Definition, Examples, Exploration, and Consequences This is the first in a series of posts that\u00a0 cover conjugation, pi systems, molecular orbital <\/p>\n","protected":false},"author":1,"featured_media":33855,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1163],"tags":[195,1164,1165,1126,267],"post_folder":[],"class_list":["post-10335","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-dienes-and-mo-theory","tag-conjugation","tag-mo-theory","tag-orbital-overlap","tag-pi-systems","tag-resonance"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.7 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Conjugation And Resonance In Organic Chemistry<\/title>\n<meta name=\"description\" content=\"What&#039;s &quot;conjugation&quot; in organic chemistry? 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