Organic Molecular Charges

Figure 12.

ORGANIC MOLECULAR CHARGES

You should also have an understanding of what it means to be a charged molecule. This is relatively easy to understand when it comes to ionized molecules like sodium, which becomes Na+, and chlorine, which becomes Cl- when ionized. What you need to know now is that organic molecules can also be charged. An example of this is methanol, which is CH3OH, which can be neutral. It can also lose an H+ ion to make CH3O- (an anion) or can gain a hydrogen ion to become CH3OH2+. In these cases, the positive charge or negative charge is on the oxygen atom. This can be further categorized as the “formal charge”, which is the charge specifically on the oxygen atom rather than on the molecule as a whole or on any other part of the molecule.

To find the formal charge on different atoms of an organic molecule, it is necessary to add up the valence electrons. In the case of oxygen, an unbound oxygen atom has six valence electrons. When it is bound in the methanol molecule, there are two electrons used to make the bonds to carbon and hydrogen and four left over. These four electrons left over are all “owned” by the oxygen molecule. The bound electrons with hydrogen are “half-owned”. Taking the total valence electrons and subtracting the unbound electrons around the atom and half of the bound electrons, gives a net charge of 0 on a methanol molecule. Figure 13 demonstrates the formal charge of -1 on the oxygen atom in the methanol anion:

Figure 13.

In the case of methane cation, the extra electron given by hydrogen makes six bonding electrons and two nonbonding electrons. Half of six bonding electrons is three and the two electrons not involved in bonding makes 5 in total. This leads to a +1 charge. The formula for determining the formal charge on oxygen is to take the total valence electrons minus the unpaired electrons and minus half of the bound electrons. Taking six minus five, gives a plus-one charge on the oxygen.

Added to the issue is the fact that there can be both negative and positive formal charges on different atoms of a larger molecule. This is the issue with amino acids, in particular. There are zwitterions that have both positive and negative charges on the molecule. The total charge on the molecule is going to be zero. Even though there is a net zero charge on the molecule, it is necessary to show the location of the positive and negative charges on each atom.

Formal charges can help to determine which molecules are more stable than others. Let’s look at two possible ways to write CO2 to see which is most stable:

In CO2, the C is less electronegative, so that it is the central atom. It has four valence electrons and oxygen has six valence electrons, for a total of 16 valence electrons. The binding of carbon and oxygen gives an O-C-O molecule with double bonds between the two. This would not work with a single bond because it leaves too many unpaired electrons. It can also not be written with a single bond on one side and a triple bond on the other side. It must be symmetric. Figure 14 shows the Lewis dot structure on CO2.