{"id":3325,"date":"2011-11-30T12:00:40","date_gmt":"2011-11-30T12:00:40","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=3325"},"modified":"2026-01-16T13:58:44","modified_gmt":"2026-01-16T19:58:44","slug":"curved-arrows-to-interconvert-resonance-structures","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2011\/11\/30\/curved-arrows-to-interconvert-resonance-structures\/","title":{"rendered":"How To Use Curved Arrows To Interconvert Resonance Structures"},"content":{"rendered":"

Curved Arrows And Resonance Structures<\/strong><\/p>\n

Previously in this series on resonance, we saw that resonance forms represent two (or more) different ways to draw the same molecule, which differ only<\/strong> in their distribution of electrons (see article: <\/span>Introduction to Resonance<\/span><\/a><\/em>)<\/p>\n

In this post, we’ll explore how to use the important “bookkeeping” tool of\u00a0curved electron-pushing arrows<\/strong> to show the movement of electrons.<\/p>\n

At the bottom of the article, you can test your application of these concepts with some additional quizzes.<\/p>\n

\"summary<\/a><\/p>\n

Table of Contents<\/strong><\/p>\n

    \n
  1. What Bonds Formed, What Bonds Broke?<\/a><\/li>\n
  2. Introducing Curved Arrows, A Tool For Showing The Movement Of Electrons Between Resonance Structures<\/a><\/li>\n
  3. Every Resonance Form For A Molecule Can Be “Found” Through The Application Of Three Electron-Pushing Arrow “Moves”<\/a><\/li>\n
  4. Some Common “Dumb” Questions About Curved Arrows (That Are Not Dumb)<\/a><\/li>\n
  5. Three animated examples (GIFS) that illustrate the “3 legal moves”\u00a0<\/a><\/li>\n
  6. One Last Piece Of Advice About Curved Arrows And Resonance Forms<\/a><\/li>\n
  7. Notes<\/a><\/li>\n
  8. Quiz Yourself!<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. What Bonds Formed, What Bonds Broke?<\/h2>\n

    What’s different in the molecules below? Specifically, what bonds formed and broke? Where did the electrons actually go?<\/p>\n

    \"compare<\/a><\/p>\n

    In both cases the resonance form on the right contains all the same atoms of the molecule on the right, but the electrons have been moved around<\/strong> (or to be more specific, pairs<\/strong> of electrons).<\/p>\n

    There are two places we will find electron pairs: they will either be found in bonds<\/strong> or as lone pairs<\/strong> on atoms. That’s it. For the purposes of discussing resonance, we’ll confine our discussion of “bonds” to \u03c0\u00a0bonds exclusively.<\/p>\n

    <\/a>2. Introducing Curved Arrows, A Tool For Showing The Movement Of Electrons Between Resonance Structures<\/h2>\n

    Here’s the punch line: we can convert one resonance form into another by showing the movement of electrons between bonds and lone pairs (or vice versa).\u00a0<\/strong><\/p>\n

    We just need a graphical tool to do it. Thankfully, Robert Robinson <\/a>devised such a tool for us to use. It’s called the “curved arrow”.<\/p>\n

    \"introduction<\/a><\/p>\n

    The curved arrow shows “movement” of a pair of electrons. It’s an extremely useful accounting system that lets us keep track of changes in bonding<\/strong> and also in charge. <\/strong>Since electron pairs are present either in bonds or in lone pairs, there are really only four combinations of “moves”. Only three of them are actually legal.<\/p>\n

    <\/a>3. Every Resonance Form For A Molecule Can Be “Found” Through The Application Of Three Electron-Pushing Arrow “Moves”<\/h2>\n

    Let’s look at them in detail.<\/p>\n

    First, we have pi bond to lone pair:<\/p>\n

    \"three<\/a><\/p>\n

    Secondly, we have its reverse, pi bond to pi bond:<\/p>\n

    \"three<\/a><\/p>\n

    If you look closely, with each arrow we are changing the formal charge by 1. <\/strong>The charges change at the tail<\/strong>, which becomes more positive (since it’s giving away electrons), and the head<\/strong>, which becomes more negative (since it’s gaining electrons).<\/p>\n

    Third, we have pi bond to pi bond.<\/p>\n

    \"three<\/a><\/p>\n

    Note that last example, lone pair to lone pair, is not legal.\u00a0 It’s illegal because we are changing the formal charge at each carbon by 2 units<\/strong> (from \u20131 to +1 and from +1 to \u20131). This is not allowed for a single arrow.<\/p>\n

    \"-three<\/a><\/p>\n

    <\/a>4. Some Common “Dumb” Questions About Curved Arrows (That Are Not Dumb)<\/h2>\n

    \u00a0Here’s some common “dumb” questions about curved arrows.\u00a0<\/strong><\/p>\n