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A Primer On Organic Reactions

By James Ashenhurst

Curved Arrows (2): Initial Tails and Final Heads

Last updated: March 27th, 2021 |

Curved Arrows In Organic Chemistry: Always Change Two (And Only Two!) Charges

Here’s a handy little trick for accounting for charges when you draw curved arrows. You only ever change two formal charges: the charge at the “initial tail” becomes more positive, and the charge at the “final head” becomes more negative.

1. Curved Arrows Are The Chemical Accounting System For Showing How Electrons Move, From Electron Donor To Electron Acceptor

You know by now that curved arrows are our accounting system for showing how electrons move, from the tail (electron donor, source of electrons) to the head (electron accepting, destination of electrons).

There are only three moves you can do:

  • lone pair to bond
  • bond to lone pair
  • bond to bond.

With any given arrow, the source (tail) becomes more positive (by 1), and the destination (head) becomes more negative (by 1).

Like this.

in-curved-arrow-formalism-only-two-charges-change-the-initial-tail-and-the-final-head

However… sometimes (often!!) you will need to draw more than one arrow, in sequence. What do we do about the charges then?

It’s pretty simple, actually: you still only ever change two charges.

2. No Matter How Many Arrows You Draw In Sequence, You Only Ever Change Two Charges: The Initial “Tail” And The Final “Head”.

Here’s an example showing the deprotonation of H-Cl by hydroxide ion.

curved-arrow-example-with-two-arrows-initial-tail-and-final-head-reaction-of-hydroxide-ion-with-hcl-giving-h2o-and-cl-

Here, the HO(-) is “giving away” a pair of electrons, so it becomes more positive by 1 (from -1 to 0) . And the Cl is “accepting” a pair of electrons so its charge goes from neutral (0) to negative (-1). Meanwhile, the H in between is both accepting and donating electrons, so its charge remains the same.

Here’s two more examples – an example with three arrows, and an exaggerated (but plausible) example with six (!!!) arrows. Notice that no matter how many arrows are drawn, only two charges change.

example-of-curved-arrow-formalism-with-three-arrows-but-only-charges-change-on-the-initial-tail-and-final-head

curved-arrow-formalism-with-six-arrows-but-only-two-charges-change-initial-tail-and-final-head

Knowing that you’ll only ever change two formal charges will make electron accounting easier. By the way, I can’t claim credit for this trick: I got it from watching Steven (Freelance Teacher) – here’s his (useful) website and a link to his videos.

Next Post:Seven Factors that Stabilize Positive Charge in Organic Chemistry

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

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