Curved Arrows (2): Initial Tails and Final Heads

by James

in Drawing Reaction Mechanisms, Organic Chemistry 1

Here’s a handy little trick for accounting for charges when you draw curved arrows.

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, and 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.

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 only ever change two charges.

No matter how many arrows you draw in sequence, you only ever change two charges: the initial “tail” and the final “head”.

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.

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

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