{"id":3559,"date":"2011-12-08T12:00:59","date_gmt":"2011-12-08T12:00:59","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=3559"},"modified":"2026-01-16T14:06:45","modified_gmt":"2026-01-16T20:06:45","slug":"evaluating-resonance-forms-1-the-rule-of-least-charges","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2011\/12\/08\/evaluating-resonance-forms-1-the-rule-of-least-charges\/","title":{"rendered":"Evaluating Resonance Structures &#8211; The Fewer Charges, The Better"},"content":{"rendered":"<p><strong>Evaluating Resonance Forms &#8211; The Rule Of Least Charges<\/strong><\/p>\n<ul>\n<li>So far we&#8217;ve introduced\u00a0 <a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/11\/23\/introduction-to-resonance\/\" target=\"_blank\" rel=\"noopener noreferrer\">resonance<\/a> (<span style=\"color: #993366;\"><em>See article &#8211; <a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/11\/23\/introduction-to-resonance\/\">Introduction to Resonance<\/a><\/em><\/span>)\u00a0 and introduced the <a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/11\/30\/introduction-to-resonance-2-curved-arrows\/\" target=\"_blank\" rel=\"noopener noreferrer\">curved arrow formalism<\/a> (<span style=\"color: #993366;\"><em>See article &#8211; <a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/11\/30\/introduction-to-resonance-2-curved-arrows\/\">Curved Arrows For Resonance<\/a><\/em><\/span>) to show the movement of electrons.<\/li>\n<li>Importantly, we&#8217;ve talked about how the &#8220;true&#8221; picture of a molecule is a <strong>hybrid<\/strong> of its resonance forms (and <strong>not<\/strong> an equilibrium between forms).<\/li>\n<li>Here, we&#8217;re going to start evaluating the\u00a0<strong>relative importance\u00a0<\/strong>of various resonance forms and the contribution they make to the overall resonance\u00a0<strong>hybrid<\/strong>.<\/li>\n<li>The\u00a0<strong>first\u00a0<\/strong>rule, we&#8217;ll discuss here, is that\u00a0<strong>the resonance form with the least number of point charges\u00a0<\/strong>will be the\u00a0<strong>most important<\/strong> resonance form, and make the\u00a0<strong>greatest contribution<\/strong> to the resonance hybrid.<\/li>\n<\/ul>\n<p><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-38636\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/12\/0-summary-evaluating-resonance-forms-the-rule-of-least-charges-neutral-resoannce-forms-more-important-than-charged-resoannce-forms.gif\" alt=\"summary-evaluating resonance forms the rule of least charges neutral resonance forms more important than charged resoannce forms\" width=\"640\" height=\"435\" \/><\/a><\/p>\n<p><strong>Table of Contents<\/strong><\/p>\n<ol>\n<li><a href=\"#one\">Recap: The Three &#8220;Legal Moves&#8221; Of Resonance Curved Arrows<\/a><\/li>\n<li><a href=\"#two\">The First Rule Of Evaluating Resonance Structures: Minimize Charges<\/a><\/li>\n<li><a href=\"#three\">Evaluating The Resonance Structures Of Ethene (&#8220;Ethylene&#8221;)<\/a><\/li>\n<li><a href=\"#four\">Evaluating The Resonance Structures Of The Allyl Cation<\/a><\/li>\n<li><a href=\"#five\">Evaluating The Resonance Structures Of The Allyl Anion<\/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><a id=\"one\"><\/a>1. Recap: The Three &#8220;Legal Moves&#8221; Of Resonance Curved Arrows<\/h2>\n<p>Here&#8217;s a recap of the different &#8220;moves&#8221; we can perform on a molecule to get different resonance forms. <em>Every resonance form we can draw for a molecule can be made through a combination of these three moves.\u00a0<\/em><\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-42752\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/11\/1-three-legal-moves-of-resonance-revisited-pi-bond-to-lone-pair-lone-pair-to-pi-bond-pi-bond-to-pi-bond.gif\" alt=\"three legal moves of resonance revisited pi bond to lone pair lone pair to pi bond pi bond to pi bond\" width=\"640\" height=\"377\" \/><\/a><\/p>\n<p>We can apply these &#8220;moves&#8221; <strong>so long as we don&#8217;t break the octet rule. <\/strong>The thing is that if we start looking at even simple molecules, we can soon get lost in a maze of potential resonance forms if we apply the arrow-pushing rules willy-nilly. So let&#8217;s cut to the main problem: <strong>how do we evaluate the importance of different resonance forms?<\/strong><\/p>\n<p>In other words, which make a greater contribution to the overall resonance hybrid (<strong>more significant<\/strong>) and which make less of a contribution to the overall hybrid\u00a0 (<strong>less significant<\/strong>)?<\/p>\n<h2><a id=\"two\"><\/a>2. The First Rule Of Evaluating Resonance Forms: Minimize Charges<\/h2>\n<p>Here&#8217;s the punch line for today:<\/p>\n<p><img decoding=\"async\" class=\"alignnone wp-image-42753\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/11\/2-first-rule-of-evaluating-resonance-forms-the-resonance-form-with-least-number-of-charges-will-be-the-most-stable-and-contribute-the-most-to-the-resonance-hybrid.gif\" alt=\"first rule of evaluating resonance forms the resonance form with least number of charges will be the most stable and contribute the most to the resonance hybrid\" width=\"640\" height=\"118\" \/><\/a><\/p>\n<p>Let&#8217;s look at three examples. Really simple ones, but they&#8217;ll do.<\/p>\n<h2><a id=\"three\"><\/a>3. Evaluating The Resonance Forms Of Ethene (&#8220;Ethylene&#8221;)<\/h2>\n<p>The simplest molecule with a \u03c0 bond is ethene. If we draw a resonance structure for it, we can move the \u00a0\u03c0 bond to the lone pair of one of the end carbons (doesn&#8217;t matter which one) to give a <strong>carbocation<\/strong> and a<strong> lone pair<\/strong>. This is a &#8220;legal&#8221; resonance form, since we&#8217;re not breaking the octet rule.<\/p>\n<p>The question is, how significant is the resonance form with the two charges? The answer from experiment is: <strong>not very much\u00a0<\/strong>because ethene behaves essentially like how we&#8217;d expect a neutral, uncharged molecule to behave &#8211; it has a low boiling point and doesn&#8217;t mix with water. [<a href=\"#noteone\">Note 1<\/a>]<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-42754\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/11\/3-resonance-forms-of-ethene-aka-ethylene-significant-and-insignificant-ethene-is-non-polar.gif\" alt=\"resonance forms of ethene aka ethylene significant and insignificant ethene is non polar\" width=\"640\" height=\"459\" \/><\/a><\/p>\n<p>(One thing to note &#8211; the <b><i>net<\/i><\/b>\u00a0charge of each resonance structure is the same here (i.e. they both have a <strong>net charge of zero<\/strong>) since on the resonance form on the right, the opposite charges cancel).<\/p>\n<p>&nbsp;<\/p>\n<h2><a id=\"four\"><\/a>4. Evaluating The Resonance Forms Of The Allyl Cation<\/h2>\n<p>Let&#8217;s now look at the resonance structures of the <strong>allyl carbocation<\/strong>.\u00a0 The net charge on the allyl carbocation is +1.<\/p>\n<p>When we apply the relevant arrow-pushing &#8220;moves&#8221; to it, we get something like this, with 4 resonance structures &#8211; A, A&#8217; , B, and C. Resonance structures A and A&#8217; both have one charge. Resonance structures B and C have three charges. [<em><span style=\"color: #993366;\">Although again, note that B and C each have a<\/span> <strong>net<\/strong>\u00a0charge of +1 when you cancel the opposite charges<\/em>].<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-42755\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/11\/4-allyl-cation-resonance-form-evaluation-only-forms-with-one-charge-are-significant-interconvert-with-curved-arrows.gif\" alt=\"allyl cation resonance form evaluation only forms with one charge are significant interconvert with curved arrows\" width=\"639\" height=\"322\" \/><\/a><\/p>\n<p>So what does experiment tell us? Our best experimental evidence for the structure of the allyl carbocation tells us that the <strong>positive<\/strong> <strong>charge<\/strong> is distributed <strong>equally<\/strong> between the two end carbons, with a bond length <strong>intermediate<\/strong> between that of a C\u2013C single bond (1.54 \u00c5) and a C\u2013C \u03c0 bond (1.34 \u00c5).<\/p>\n<p>That is to say, the resonance hybrid of fhe allyl carbocation is a 1:1 mixture of resonance forms <strong>A<\/strong>\u00a0and <strong>A&#8217;<\/strong>.<\/p>\n<p>Resonance forms B and C don&#8217;t contribute to the resonance hybrid to any significant extent.<\/p>\n<h2><a id=\"five\"><\/a>5. Evaluating The Resonance Forms Of The Allyl Anion<\/h2>\n<p>Finally, let&#8217;s look at the <strong>allyl<\/strong> <strong>anion<\/strong>. We can use a similar application of the arrow pushing rules to get the different resonance forms, although here&#8217;s a twist.<\/p>\n<p>Note that we can&#8217;t move the end lone pair to form a new\u00a0\u03c0 bond without breaking the octet rule! So in this case we actually have to draw <strong>two<\/strong>\u00a0arrows to make it legal. This gets us to the equivalent resonance form, A&#8217;.<\/p>\n<p>On the other hand the other resonance structures (B and C) can be obtained through a fairly straightforward process (move a \u03c0 bond to form a lone pair on either carbon.)<\/p>\n<p>Again, however, these resonance forms don&#8217;t contribute much to the overall resonance hybrid. Experiment tells us that the electron density on the allyl anion is on the ends, and the bond lengths are again <strong>intermediate<\/strong> between a C\u2013C single bond and a C\u2013C \u03c0 bond.<\/p>\n<p>The best interpretation of this data is that the allyl carbanion is a 1:1 mixture of the resonance forms A and A&#8217;.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-42756\" src=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/11\/5-allyl-anion-resonance-forms-only-significant-resonance-forms-have-a-single-charge-lone-pair-to-pi-bond-pi-bond-to-lone-pair-two-moves-to-avoid-breaking-octet-rule.gif\" alt=\"allyl anion resonance forms only significant resonance forms have a single charge lone pair to pi bond pi bond to lone pair two moves to avoid breaking octet rule\" width=\"640\" height=\"599\" \/><\/a><\/p>\n<p>So again, the bottom line is that the <strong>most stable (and significant) resonance structures will be those with the fewest charges.\u00a0<\/strong><\/p>\n<p>You&#8217;ve probably noticed however that these examples are pretty simple &#8211; we&#8217;re just dealing with carbon atoms. What happens when we deal with <strong>unsymmetrical <\/strong>resonance structures? We&#8217;ll need to introduce a new principle for that!<\/p>\n<p><strong>Next Post: <\/strong><a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/12\/12\/evaluating-resonance-structures-2-applying-electronegativity\/\" target=\"_blank\" rel=\"noopener noreferrer\">Evaluating Resonance Forms (2): Applying Electronegativity<\/a><\/p>\n<hr \/>\n<h2 style=\"text-align: left;\"><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\/2012\/01\/17\/how-to-apply-electronegativity-and-resonance-to-understand-reactivity\/\" class=\"\"><span>How to apply electronegativity and resonance to understand reactivity<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/12\/12\/evaluating-resonance-structures-2-applying-electronegativity\/\" class=\"\"><span>How To Find The Best Resonance Structure By Applying Electronegativity<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2012\/01\/10\/common-mistakes-how-not-to-draw-resonance-curved-arrows\/\" class=\"\"><span>Drawing Resonance Structures: 3 Common Mistakes To Avoid<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/12\/13\/evaluating-resonance-forms-3-where-to-put-the-negative-charge\/\" class=\"\"><span>Evaluating Resonance Structures With Negative Charges<\/span><\/a><\/li><li><a href=\"https:\/\/www.masterorganicchemistry.com\/2011\/12\/14\/evaluating-resonance-forms-4-positive-charges\/\" class=\"\"><span>Evaluating Resonance Structures With Positive Charge<\/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\/organic-chemistry-practice-problems\/resonance-structures-practice\/\" class=\"\"><span>Resonance Structures Practice (MOC Membership)<\/span><\/a><\/li><\/ul><\/div>\n<p><strong><a id=\"noteone\"><\/a>Note 1.\u00a0<\/strong>If you measure the electronic and structural properties of ethene, it&#8217;s clear that the carbon-carbon bond length is shorter than that of a normal carbon-carbon single bond, implying a strong interaction between the two atoms.<\/p>\n<p>Furthermore, if the resonance structure on the right were significant, we&#8217;d expect ethene to have a relatively high boiling point (since it&#8217;s charged) and be quite soluble in polar solvents like water. In fact, ethene boils at a very <strong>low<\/strong> temperature (\u201388 \u00b0C) and is practically insoluble in polar solvents. \u00a0So it&#8217;s safe to conclude that the resonance form on the right is quite insignificant.<\/p>\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\/3661-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\/3665-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\/3667-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>Evaluating Resonance Forms &#8211; The Rule Of Least Charges So far we&#8217;ve introduced\u00a0 resonance (See article &#8211; Introduction to Resonance)\u00a0 and introduced the curved arrow <\/p>\n","protected":false},"author":1,"featured_media":38636,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1406],"tags":[406,335,191,298,353,213,267],"post_folder":[],"class_list":["post-3559","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-bonding-structure-resonance","tag-cations","tag-charge-stability","tag-charges","tag-curved-arrows","tag-electron-flow","tag-formal-charge","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>Evaluating Resonance Structures - The Fewer Charges, The Better<\/title>\n<meta name=\"description\" content=\"When evaluating resonance forms, the first rule to apply is this: generally speaking, the fewer the charges, the more important the resonance form is.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.masterorganicchemistry.com\/2011\/12\/08\/evaluating-resonance-forms-1-the-rule-of-least-charges\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Evaluating Resonance Structures - The Fewer Charges, The Better\" \/>\n<meta property=\"og:description\" content=\"When evaluating resonance forms, the first rule to apply is this: generally speaking, the fewer the charges, the more important the resonance form is.\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.masterorganicchemistry.com\/2011\/12\/08\/evaluating-resonance-forms-1-the-rule-of-least-charges\/\" \/>\n<meta property=\"og:site_name\" content=\"Master Organic Chemistry\" \/>\n<meta property=\"article:publisher\" content=\"https:\/\/www.facebook.com\/Master-Organic-Chemistry-242610599108055\/\" \/>\n<meta property=\"article:published_time\" content=\"2011-12-08T12:00:59+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2026-01-16T20:06:45+00:00\" \/>\n<meta property=\"og:image\" content=\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2024\/12\/0-summary-evaluating-resonance-forms-the-rule-of-least-charges-neutral-resoannce-forms-more-important-than-charged-resoannce-forms.gif\" \/>\n\t<meta property=\"og:image:width\" content=\"880\" \/>\n\t<meta property=\"og:image:height\" content=\"598\" \/>\n\t<meta property=\"og:image:type\" content=\"image\/gif\" \/>\n<meta name=\"author\" content=\"James Ashenhurst\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"James Ashenhurst\" \/>\n\t<meta name=\"twitter:label2\" content=\"Est. reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"6 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\\\/\\\/schema.org\",\"@graph\":[{\"@type\":\"Article\",\"@id\":\"https:\\\/\\\/www.masterorganicchemistry.com\\\/2011\\\/12\\\/08\\\/evaluating-resonance-forms-1-the-rule-of-least-charges\\\/#article\",\"isPartOf\":{\"@id\":\"https:\\\/\\\/www.masterorganicchemistry.com\\\/2011\\\/12\\\/08\\\/evaluating-resonance-forms-1-the-rule-of-least-charges\\\/\"},\"author\":{\"name\":\"James Ashenhurst\",\"@id\":\"https:\\\/\\\/www.masterorganicchemistry.com\\\/#\\\/schema\\\/person\\\/78d83ec7d02b4b7365bade2cedaef80c\"},\"headline\":\"Evaluating Resonance Structures &#8211; 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