“Initial Tails” and “Final Heads”

by Kiley Lynch

Here’s a handy little trick for understanding curved arrows.

You know that curved arrows are our trick for showing how electrons move, from the tail (electron rich, nucleophile) to the head (electron poor, electrophile).

There are only three moves you can do: lone pair to bond, bond to lone pair, and bond to bond.

But here’s another important  trick: no matter how many arrows you draw, you only ever change two charges. The initial tail and the final head. This picture should hopefully explain what I mean.

The initial tail is the source of the electrons. Since it will be “giving away” a pair of electrons, it is going to “lose” an electron in doing so (for formal charge purposes), which means that the charge is going to be more positive (by 1).

So it will go from -1 to 0 (neutral) , or from 0 (neutral) to + 1

The final head is the destination of the electrons. Since it is “accepting” a pair of electrons, it is going to “gain” an electron in doing so (for formal charge purposes), which means that the charge is going to be more negative (by 1).

So it will go from +1 to 0 (neutral) or from 0 (neutral) to -1

Here’s the cool thing: no matter how many arrows you draw in sequence, you only ever change two charges: the initial “tail” and the final “head”.

Here’s two more examples – an example with two arrows, and an exaggerated (but plausible) example with six (!!!) arrows.


Tomorrow: let’s talk about carbocation stability!

Thanks for reading! James

P.S. Relevant post: Curved Arrows (for Reactions)

P.P.S. I picked up this trick from Freelance Teacher (Steven) – to visit his useful site click here.