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Bonding, Structure, and Resonance

By James Ashenhurst

How To Use Curved Arrows To Interchange Resonance Forms

Last updated: December 13th, 2024 |

Curved Arrows And Resonance Structures

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 in their distribution of electrons (see article: Introduction to Resonance)

In this post, we’ll explore how to use the important “bookkeeping” tool of curved electron-pushing arrows to show the movement of electrons.

summary how to draw curved arrows to interconvert resonance structures 3 legal moves pi bond lone pair

Table of Contents

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

1. What Bonds Formed, What Bonds Broke?

What’s different in the molecules below? Specifically, what bonds formed and broke? Where did the electrons actually go?

compare-bonds-formed-and-bonds-broken-between-resonance-forms-and-we-use-curved-arrows-to-illustrate

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 (or to be more specific, electron pairs).

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

2. Introducing Curved Arrows, A Tool For Showing The Movement Of Electrons Between Resonance Structures

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). 

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

introduction-to-curved-arrows-to-show-moevement-of-electrons-from-tail-to-head-electron-pair-only-three-moves-bond-to-lone-pair-lone-pair-to-bond-bond-to-bond

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 and also in charge. 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.

3. Every Resonance Form For A Molecule Can Be “Found” Through The Application Of Three Electron-Pushing Arrow “Moves”

Let’s look at them in detail.

three-legal-moves-of-resonance-pi-bond-to-lone-pair-lone-pair-to-pi-bond-pi-bond-to-pi-bond

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

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

4. Some Common “Dumb” Questions About Curved Arrows (That Are Not Dumb)

 Here’s some common “dumb” questions about curved arrows. 

  • Does it matter which side of the bond the arrows are on? No
  • If an atom has multiple lone pairs, does it matter which one you use? No
  • Are we ever allowed to give an atom more than 8 electrons? absolutely not (at least not with C, N, O, F).
  • I’m lazy. Can’t I just draw the “tail” as coming from the negative charge and skip putting in the lone pairs? YES

This brings up an excellent point. When it comes to drawing, chemists are ingenious at finding ways to be lazy.  We can also draw the tail of curved arrows as coming from negative charges (as long as there are electrons on that atom, remember how formal charge can be misleading).

negative-charges-can-stand-in-for-lone-pairs-when-using-curved-arrows

This makes our lives a little easier because who really wants to draw lone pairs if they don’t have to? From now on in this series I’m only going to draw in the lone pairs if absolutely necessary. Otherwise I’ll just draw the curved arrow as coming from the negative charge.

5. Animated Examples

Here are three animations of arrow pushing resonance forms. See if you can spot the three different legal “moves”!

Allyl Anion:

Allyl Cation:

Acetate Anion:

6. One Last Piece Of Advice About Curved Arrows And Resonance Forms

Just be careful, however – if the atom is neutral, you MUST draw in a lone pair of electrons. Never draw the tail of a curved arrow from an atom with no lone pairs. 

This covers the basics of the curved arrow formalism. Now that we can start to use curved arrows to draw resonance structures, we can also think about how to evaluate the relative importance of some simple resonance structures. That’s the subject of the next post.

Next Post: Evaluating Resonance Forms (1): The Rule of Least Charges

Notes

Related Articles

Note 1. Here’s a detailed breakdown of arrow pushing in the carboxylate ion we discussed last time. Yes, it’s ridiculous in detail but sometimes that helps.

curved-arrows-help-keep-track-of-bonds-and-charges-charge-always-becomes-more-positive-at-the-initial-head-and-more-negative-at-the-final-tail

 

00 General Chemistry Review
01 Bonding, Structure, and Resonance
02 Acid Base Reactions
03 Alkanes and Nomenclature
04 Conformations and Cycloalkanes
05 A Primer On Organic Reactions
06 Free Radical Reactions
07 Stereochemistry and Chirality
08 Substitution Reactions
09 Elimination Reactions
10 Rearrangements
11 SN1/SN2/E1/E2 Decision
12 Alkene Reactions
13 Alkyne Reactions
14 Alcohols, Epoxides and Ethers
15 Organometallics
16 Spectroscopy
17 Dienes and MO Theory
18 Aromaticity
19 Reactions of Aromatic Molecules
20 Aldehydes and Ketones
21 Carboxylic Acid Derivatives
22 Enols and Enolates
23 Amines
24 Carbohydrates
25 Fun and Miscellaneous
26 Organic Chemistry Tips and Tricks
27 Case Studies of Successful O-Chem Students

Comments

Comment section

15 thoughts on “How To Use Curved Arrows To Interchange Resonance Forms”

  2. Pingback: Can A Curved Arrow Begin At A Pi Bond?

  3. Hello!

    I really don’t understand how C3 went from CH3 to CH2 with a double bond, and the + charge remains localized on C1. Please explain and thanks so much in advance.

    Reply

  4. This is fantastic. Clears up a lot of dumb mistakes I could have avoided in Orgo 1. This semester will be better! Thanks so much James!

    Reply

  5. I agree that hyperconjugation is a “possible” example of resonance ( a special case). However, I don’t like the example you cite because it is really an E2 reaction. I see the area you are dealing with (the gauche effect) and why you might like to call it resonance. Anyway, I’d make the case that it does involve pi systems because the one form contains a new pi bond.
    But why not just call it hyperconjugation which can it’s own unique characteristics that can be used to keep it distinct ? I think that can only help students of organic chemistry in the long run.
    The FCH2CH2F certainly is an interesting example. I just looked at a few references to check C-F bonds lengths…. for 1,2-difluoroethane Xray data gave 1.39A. A general list of typical sp3C-F data cited 1.4A. At a first assessment this rationale for the geometric preference might be flawed because it doesn’t seem to rationalise the C-F bond length trend correctly.

    Reply

  6. Interestingly, you never mention the commonly spouted but incorrect idea that conversion of resonance structures always involves the movement of pi electrons and/or the formation of pi bonds. Poppycock!

    http://imgur.com/fXRDB

    I like the way you put it: “two different ways to draw the same molecule, which differ only in their distribution of electrons”—that says nothing about the nature of said electrons as being part of lone pairs and pi bonds. I’d love a post with your thoughts about how to evaluate the “goodness” of resonance structures… :-)

    Reply

    3. That drawing has an error, it should have a pi bond in it. And it seems like either an example of an E2 or of hyperconjugation….

      Reply

      1. Gak; My fault. Let’s try that again, with a more useful example. Don’t mean to start a flame war.

        http://imgur.com/q9yfi

        Hyperconjugation (or “double-bond no-bond resonance”) is the idea; the point is that hyperconjugation is really just a fancy form of resonance. When the applications are structural (conformation, bond lengths/angles, etc.) it can be called resonance because the electron flow doesn’t really reflect chemical change—it helps us explain structural reality.

        Reply

  7. a small mistake:
    “The charges change at the head, which becomes more positive (since it’s giving away electrons), and the tail, which becomes more negative (since it’s gaining electrons).”

    the opposite is true:
    electrons move from the tail to the head, so it is the head that becomes more negative,
    and electrons move away from the tail, so the tail becomes more positive.

    Reply

  9. #FirstWorldChemDrawProblems I find if I use the built in 90o or 120o arrows in ChemDraw, my arrow heads sometimes look like single-barbed arrows like in the last drawing on this page.

    I’ve found using the Edit Curve function (directly above the Arrow Tools function), I can change the arc angle to my liking and make the double-barbed arrows clear. It also helps with carbocation rearrangements and such. And I don’t have to switch between clockwise/counterclockwise :)

    For example: http://i.imgur.com/EdMBk.png

    Reply

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