{"id":3647,"date":"2011-12-19T12:00:21","date_gmt":"2011-12-19T12:00:21","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=3647"},"modified":"2026-05-03T06:58:42","modified_gmt":"2026-05-03T11:58:42","slug":"exploring-resonance-pi-acceptors","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2011\/12\/19\/exploring-resonance-pi-acceptors\/","title":{"rendered":"Exploring Resonance: Pi-acceptors"},"content":{"rendered":"

Resonance In Organic Chemistry: What Are “Pi Acceptors”<\/strong><\/p>\n

Last time we saw that\u00a0 \u00a0\u03c0 donors<\/a> are atoms capable of forming a new \u03c0 bond with an adjoining C-C \u03c0 bond.\u00a0 The upshot of \u03c0 donation is that these molecules will have an important resonance form where the carbon at the far end of the \u03c0 bond (away from the \u03c0 donor) has a negative charge.<\/p>\n

Todays topic is \u03c0 acceptors, which is, as you might imagine, is exactly the opposite phenomenon as \u03c0 donation.<\/p>\n

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

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

    \n
  1. In Pi Bonds That Are Polarized Toward A More Electronegative Atom There Is An Important Resonance Form Where The Less Electronegative Atom Has A Positive Formal Charge<\/a><\/li>\n
  2. These Groups Are Called “Pi Acceptors” Since They Can “Accept” A Pi Bond In Conjugation With It<\/a><\/li>\n
  3. The Presence Of A “Pi-Acceptor” Attached To A C-C Pi Bond Results In An Important Resonance Form Where A Carbon On The Pi Bond Bears A Positive Charge<\/a><\/li>\n
  4. Notes<\/a><\/li>\n
  5. Quiz Yourself!<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. Pi Bonds That Are Polarized Toward A More Electronegative Atom Are Have An Important Resonance Form Where The Less Electronegative Atom Bears A Positive Charge<\/strong><\/h2>\n

    Recall that in \u03c0 bonds that are polarized toward a more electronegative atom, we have an important<\/strong> resonance form where the less electronegative atom (usually carbon) bears a positive<\/strong> charge. That would be the one on the left, below. (See article: Applying Electronegativity On Resonance Forms<\/a><\/em><\/span>)<\/p>\n

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

    However, when we have an additional double bond attached to an electron withdrawing group, we can also draw an additional<\/strong> resonance form where there is a positive charge on the far<\/strong> carbon. That would be the one on the right.<\/p>\n

    These are both important resonance forms. <\/em><\/p>\n

    Note that they both have an equal<\/strong> number of charges<\/strong> (two) and they are polarized so as to put the negative<\/strong> charge<\/strong> on the most<\/strong> electronegative<\/strong> atom (oxygen).<\/p>\n

    They will each make an important contribution to the resonance hybrid of this molecule.<\/p>\n

    (The question of which resonance form is more important for the purposes of reactivity<\/strong> is an important question I’m deferring to a later date. The short answer is that it depends greatly on the reaction conditions chosen).<\/em><\/span><\/p>\n

    <\/a>2. These Groups Are Called “Pi Acceptors” Since They Can “Accept” A Pi Bond In Conjugation With It<\/h2>\n

    Now: see how the double bond has “moved” toward the electron-withdrawing group? For this reason, we call these types of substituents “pi acceptors<\/strong>“, because they can “accept” the pair of electrons from a pi bond.<\/p>\n

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

    This is a general phenomenon not just for functional groups containing C=O and C=N groups, but also for functional groups such as nitriles, nitro groups, sulfonyls\u2026 essentially any polarized<\/em> group containing a \u03c0 bond.<\/p>\n

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

    <\/a>3. The Presence Of A “Pi-Acceptor” Attached To A C-C Pi Bond Results In An Important Resonance Form Where The Terminal Carbon On The Pi Bond Bears A Positive Charge<\/h2>\n

    Here’s the quick summary of this phenomenon:<\/p>\n

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

    Any functional group with a pi bond that is polarized can potentially be a pi-acceptor:<\/p>\n

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

    You can think of this as an extension of a phenomenon we observe for resonance forms that have an empty p orbital, such as carbocations and other groups that contain an empty p orbital (such as this boronic acid, pictured).<\/p>\n

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

    This is the last big post planned on introducing resonance concepts for the time being. <\/span><\/p>\n

    After a summary, and then some examples of what NOT to do,\u00a0<\/span>\u00a0I want to show how you can apply<\/strong>\u00a0this skill (that of evaluating resonance forms in determining\u00a0electron density<\/strong>\u00a0on a molecule) toward figuring out the reactivity of a given molecule. \u00a0<\/strong><\/p>\n

    Next Post: In Summary – Resonance\u00a0<\/a><\/strong><\/p>\n

    \n

    <\/a>Notes<\/h2>\n

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