Electrons carry a negative charge, so they are drawn toward positive charge. Curly arrows run from places with electrons to spare to places that are short of them.
An electron-rich place has electrons to give: a lone pair, a negative ion, or a C=C double bond. A double bond holds four electrons between two carbons. Its second pair sits above and below the line between the atoms, where other molecules can reach it.
An electron-poor place is short of electrons. An H⁺ has none at all. A δ+ atom, as in lesson 11, has had part of its share pulled away by a stronger puller next to it: the hydrogen of an O–H bond, or the carbon of a C=O.
Givers and takers
Chemists have a name for each partner. A nucleophile gives a pair of electrons to make a new bond. The name means "nucleus-loving": it heads for positive charge, like the positive nucleus of an atom. An electrophile takes a pair of electrons, and its name means "electron-loving".
When ammonia picked up an H⁺, ammonia was the nucleophile and the H⁺ the electrophile. A curly arrow that makes a new bond starts on a nucleophile and points to an electrophile.
| Nucleophile | Electrophile |
|---|---|
| electron-rich | electron-poor |
| gives a pair of electrons | takes a pair of electrons |
| ammonia, OH⁻, ethene | H⁺, the δ+ carbon of a C=O |