Electron Pairs and Partial Charges
("Electronics")

A curly arrow always starts at a region of relatively high electron density and ends at a region of lower electron density.  Thus, the first step in drawing a curly arrow is to identify the areas of high and low electron density in a given molecule. For this, you will need to recall what you have learned about Lewis structures for atoms and molecules, and about electronegativity and bond polarity.

The Electronics section of Curly Arrows is designed to help you learn to do this, by asking you to annotate various structures with lone pairs of electrons and partial negative and positive charges d+ and d–.

Start the Java Applet by clicking on the Electronics link.  The screen will have a structure in the middle of the screen, and some buttons on the right, like the one shown here.

Start by clicking in the radio button beside the first symbol, which represents a lone pair of electrons, then click on the structure to add electron pairs to appropriate atoms (click on the atom itself).  Click in turn on the symbols for partial negative and partial positive charges, then on appropriate atoms. When you believe your answer is correct and complete, click on the Done button. If you have made any errors or omissions, feedback will appear, suggesting how you may modify your answer.  Otherwise, you will get a message saying Correct, and the next example will be displayed after a few seconds.

Note that in this section, the program requires you to add all the lone pairs of electrons and partial charges to each structure, although in practice, these would not all be drawn in, in a reaction scheme, particularly by experienced chemists.  For example, a chlorine atom should normally have all three lone pairs of electrons drawn.

Three specimen answers are displayed below:

Note that no partial negative charge is necessary on the enolate oxygen as it already has a formal, full negative charge.

The curly arrow formalism for ionic reaction mechanisms represents movements of 2 electrons at a time. Organic chemists sometimes draw curly arrows from negative charges when, strictly, they should be drawn fom lone pairs. This can often cause students confusion.

It is therefore a good idea to spend some time actually putting on all the lone pairs and partial positive charges so you know exactly where they are.

If you are not very confident about lone pairs and partial charges, attempt this section first and the mechanism questions should be easier to negotiate. If you are quite confident, by all means go straight in to the mechanism sections and use this section later on if necessary.