Bonding Trends in Organic Chemistry

knowledge of the entire periodic table is not necessary to easily recall those things seen commonly with organic molecules. The harder part of organic chemistry is determining how to write out these molecules.

BONDING TRENDS IN ORGANIC CHEMISTRY

Using the typical methods used for identifying organic molecules does not work very well, especially when the molecules are large and when there are basically strings of carbon atoms in varying arrangements. For this reason, there are things you need to know in order to write the different organic molecules. All organic molecules are carbon-based and, as you know, it has the ability to form four separate bonds. There are different ways to write this using the chemical abbreviation for carbon. Figure 9 shows what it looks like to write carbon in smaller molecules:

Figure 9.

Hydrogen is also relatively easy. It has one electron and can only have one bond and will have no formal charge. The exception is the single proton H+, and the hydride ion, H. The hydride ion is a proton plus two electrons. These are highly reactive molecules that do not exist in nature but are important in discussing the chemistry of organic molecules.

Next comes oxygen. This exists in one of three ways. It can exist in a neutral way with two bonds and two lone pairs of electrons. It can also become single-bonded with a negative-one negative charge. Lastly, it can come triple-bonded with a formal positive-

one charge. A formal charge of negative-one is called a hydroxide ion; a formal charge of positive-one is called a hydronium ion.

Nitrogen has two major patterns of bonding. It can exist as a neutral nitrogen atom with three separate bonds, a double bond and a single bond, or a triple bond. Nitrogen can also be positively-charged with four bonds to other atoms. These can be four single bonds, a double bond and two single bonds, or a triple bond and a single bond. There is no double “double-bonding situation” in nitrogen and no formal negative charge on nitrogen, except in highly unstable situations.

Two other atoms should be noted as they apply to organic molecules. The first is sulfur and the second is phosphorus. Sulfur generally follows the same pattern as oxygen, while phosphorus is almost always seen in the form of the phosphate ion or PO4 with a three-negative charge. Phosphorus has five bonds, no lone pairs and a zero formal charge. Because it is a larger molecule, it has an expanded valence number with dorbitals in which the octet rule does not apply. Figure 10 shows the phosphate molecule and its charges:

Figure 10.

The halogens, like chlorine, fluorine, iodine, and bromine, are important in organic chemistry but aren’t seen in organic molecules as much as the molecules already discussed. Halogens will have a formal charge of zero and will have a single bond. Rarely, such as in the case of bromine, there will be a three-membered ring with carbon and a formal charge of plus one.