When organic chemists look at a molecular structure, they mentally disassemble the structure into its constituent parts. Figure 1 takes you inside the mind of a chemist who is looking at the structure of sulfanilamide, one of the earliest antibiotic drugs.
Thinking Like an Organic Chemist
The constituent parts of a molecule are called its functional groups. More specifically, a functional group is a molecular sub-unit that displays characteristic chemical reactions. Since the chemical reactivity of a given functional group is generally independent of the rest of the structure of which it is a part, functional groups simplify the task of understanding chemical reactivity. Consider the reactions presented in Equations 1 and 2. As Equation 1 indicates, when sulfanilamide is treated with hydrochloric acid, it is converted into its hydrochloride salt.
By the same token, methylamine reacts with hydrochloric acid to produce methylamine hydrochloride:
The fact that the amino groups in sulfanilamide and methylamine react in the same way means that it is possible to generalize the reactivity pattern common to Equations 1 and 2:
Here R-NH2 represents any compound that contains an amino group. R stands for that portion of the compound that is not involved in the reaction with HCl.
Examples of Common Functional Groups
Figure 1 introduces most of the functional groups of interest to organic chemists. As before, R stands for that portion of the structure that is not part of the functional group. The functional group is highlighted in blue. The name of each functional group is listed beneath each structure. A specific example of each functional group follows the general structure.
Figure 2 collects all of the functional groups shown in Figure 1 into a single page. You should be able to identify all of the functional groups within a molecule.
Alkanes are saturated hydrocarbons. Technically speaking alkanes contain two functional groups, the C-H bond and the C-C bond. While saturated hydrocarbons undergo several interesting and important reactions, C-H and C-C bonds in these molecules are relatively inert in comparison to the functional groups in other families of organic compounds. For that reason, C-H and C-C bonds in alkanes are not included in the list of functional groups shown in Figure 2. However, we do need to be aware of a classification scheme that organic chemists use to describe different types of carbon atoms in alkanes. This scheme is summarized in Figure 4.
The Five Classes of Saturated Carbon Atoms
This classification scheme is also used to differentiate between hydrogen atoms: a 1o hydrogen atom is one that is bonded to a 1o carbon atom; a 2o hydrogen atom is one that is connected to a 2o carbon atom; etc.
The Carbonyl Group
You may have noticed that several functional groups contain a C=O unit. This is called a carbonyl group. The substituents attached to the carbon atom of the carbonyl group determine whether a functional group is an aldehyde, ketone, carboxylic acid, ester, or amide.
In the interest of economy, chemists often condense Lewis structures. For example, they frequently draw a carboxylic acid as RCO2H. Beginning students, especially those who don't do their homework!!, often mistakenly interpret this formula as R-C-O-O-H, forgetting that carbon is tetravalent. Likewise, a common representation for aldehydes is RCHO, which tends to disguise the fact that the sequence of atoms is R-C-H (with the O branching off of the C) and not R-C-H-O or R-C-O-H. Bottom line- draw lots of structures.