You should certainly use the methods you have learned to check that these formal charges are correct for the examples given above. You may encounter carbenes in more advanced chemistry courses, but they will not be discussed any further in this book). (One last possibility is a highly reactive species called a ‘carbene’, in which a carbon has two bonds and one lone pair of electrons, giving it a formal charge of zero. Another possibility is a carbon with three bonds and a single, unpaired (free radical) electron: in this case, the carbon has a formal charge of zero.
FORMAL CHARGE OF CARBON PLUS
If, on the other hand, it has three bonds plus a lone pair of electrons, it will have a formal charge of -1. If a carbon has only three bonds and an unfilled valence shell (in other words, if it does not fulfill the octet rule), it will have a positive formal charge. Later on in this chapter and throughout this book we will see examples of organic ions called ‘carbocations’ and carbanions’, in which a carbon atom bears a positive or negative formal charge, respectively.
In carbon dioxide, the carbon atom has double bonds to oxygen on both sides (O=C=O). This is a pattern that holds throughout most of the organic molecules we will see, but there are also exceptions. If you look at the simple structures of methane, methanol, ethane, ethene, and ethyne in the figures from the previous section, you should quickly recognize that in each molecule, the carbon atom has four bonds, and a formal charge of zero. Carbon is said to be tetravalent, meaning that it tends to form four bonds. Let’s start with carbon, the most important element for organic chemists. Fortunately, this ability is not terribly hard to come by - all it takes is a few shortcuts and some practice at recognizing common bonding patterns. Clearly, you need to develop the ability to quickly and efficiently draw large structures and determine formal charges. It would be unrealistic, for example, to ask you to draw the Lewis structure below (of one of the four nucleoside building blocks that make up DNA) and determine all formal charges by adding up, on an atom-by-atom basis, the valence electrons.Īnd yet, as organic chemists, and especially as organic chemists dealing with biological molecules, you will be expected soon to draw the structure of large molecules such as this on a regular basis. But as you can imagine, these methods become unreasonably tedious and time-consuming when you start dealing with larger structures. The methods reviewed above for drawing Lewis structures and determining formal charges on atoms are an essential starting point for a novice organic chemist, and work quite will when dealing with small, simple structures. To illustrate this method, let’s calculate the formal charge on the atoms in ammonia (NH 3) whose Lewis electron structure is as follows:Ĭommon bonding patterns in organic structures Bonding electrons are divided equally between the bonded atoms.įor each atom, we then compute a formal charge:.Nonbonding electrons are assigned to the atom on which they are located.To calculate formal charges, we assign electrons in the molecule to individual atoms according to these rules:
A formal charge does not represent a true charge on an atom in a covalent bond but is simply used to predict the most likely structure when a compound has more than one valid Lewis structure. The formal charge is a way of computing the charge distribution within a Lewis structure the sum of the formal charges on the atoms within a molecule or an ion must equal the overall charge on the molecule or ion.
FORMAL CHARGE OF CARBON FREE
In these situations, we can choose the most stable Lewis structure by considering the formal charge on the atoms, which is the difference between the number of valence electrons in the free atom and the number assigned to it in the Lewis electron structure. It is sometimes possible to write more than one Lewis structure for a substance that does not violate the octet rule, as we saw for CH 2O, but not every Lewis structure may be equally reasonable.
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