M7 Intro and Resources

Wait a minute, before we learn to name organic compounds, we need a summary of the different ways to write and draw formulas for the compounds:

The molecular formula of a compound gives the exact number of each different type of atom (i.e. type of element) present in one molecule of the compound. This is useful because it allows you to work out the molecular weight of the compound, but doesn’t give you any information about the bonds in the molecule, and might not even tell you about which functional groups are present (functional groups are the parts of the organic molecule that dictate its particular reactivity with other chemicals.)

The empirical formula is the simplest ratio of the elements present in a molecule. They are of some use for organic compounds – they can sometimes help us work out the molecular formula if we don’t know it. (They are much more frequently used for ionic compounds, such as table salt, sodium chloride (NaCl). Ionic compounds have giant lattice structures – a large number of oppositely charged ions electrostatically attracted to each other. It wouldn’t be practical to try and write down the exact number of each type of atom present in a crystal of salt, because the number would be huge – so we just use the simple empirical formula instead.)

The condensed formula is often used in text. Each carbon in the molecule is written out in turn, with the atoms attached to it written directly after. Brackets are used to indicate branching in the molecule, or multiple identical groups. This is more often seen for organic acids when writing acid-base chemical equations since it highlights the acid functional group.

The displayed formula represents the molecule by showing all of the atoms, and all of the bonds between those atoms. Bonds are represented by lines, with the number of lines representing the strength of the bond. Single bonds and double bonds (2 lines) are relatively common, but triple bonds (3 lines) are also possible in some molecules. These are obviously useful, in that they show the molecule in as much detail as possible, but they also take up a lot of space, and can be time-consuming to draw. The structural formula attempts to tackle this by omitting some bonds, and grouping some atoms together, but still isn’t the quickest way to represent an organic molecule.

The skeletal formula is the simplest way of representing organic molecules, and so is commonly seen in both textbooks and research publications. It represents the main carbon chain as a zig-zag line, where the end of lines and the vertices (points) represent carbon atoms. All hydrogens are omitted, unless they are part of a functional group (eg -OH, COOH), when they are always shown; atoms other than carbon or hydrogen are also always shown.

The general formula doesn’t apply to just one compound. It applies to families of compounds; it gives a way to predict the molecular formula of a compound based on the number of carbons it contains. For an alkane the general formula is CnH2n+2. This means that if you know the number of carbons, you just need to multiply that number by two, then add two, to find the number of hydrogens. Every simple organic molecule has a general formula that can be used to work out the molecular formula of particular members of that family of compounds.