Chemistry · Ch 9 — Organic Chemistry – Some Basic Principles and Techniques
Functional Group
Functional Group
The Functional Group: The Source of Chemical Identity
An organic compound is far more than just a chain of carbon atoms. The carbon skeleton provides the structure, but the functional group is the atom or group of atoms attached to that chain that dictates nearly all of the compound's characteristic chemical behaviour. Think of the carbon skeleton as the chassis of a car and the functional group as the engine — the chassis gives the car its shape, but the engine determines what it can do.
A single functional group can transform a simple hydrocarbon into an alcohol, an aldehyde, or an acid, each with a completely different set of reactions. For instance, the hydroxyl group () makes a compound an alcohol, which can be oxidised to an aldehyde and then to a carboxylic acid. The aldehyde group () is easily reduced to an alcohol, while the carboxylic acid group () is acidic and can form salts. These are not just labels; they are the keys to predicting and understanding organic reactions.
The functional group is the reactive centre of the molecule. The rest of the carbon chain, while it can influence the reactivity (through inductive, resonance, or steric effects), is generally less reactive than the functional group itself.
Examples of Common Functional Groups
The textbook introduces three fundamental examples, but the principle extends to many others you will encounter throughout organic chemistry.
| Functional Group Name | Structure (Condensed) | Structure (Displayed) | General Formula for a Saturated Compound |
|---|---|---|---|
| Hydroxyl (Alcohol) | or | ||
| Aldehyde | or | ||
| Carboxylic Acid | or |
In the table, represents the rest of the carbon chain (an alkyl group). The general formulas show how the presence of the functional group changes the overall molecular formula compared to an alkane (). For example, an alcohol has two fewer hydrogen atoms than the corresponding alkane because the group replaces a hydrogen atom.
Do not confuse the functional group with the rest of the molecule. The in ethanol () is the functional group; the part is the ethyl group, which is the hydrocarbon skeleton. The chemical properties of ethanol are overwhelmingly those of the group, not the ethyl group.
The Core Concept: Why Functional Groups Matter
The entire system of organic chemistry classification rests on functional groups. Compounds with the same functional group belong to the same homologous series and exhibit similar chemical reactions. This is a powerful simplification — instead of memorising the reactions of thousands of individual compounds, you learn the reactions of a few dozen functional groups. Once you know how an aldehyde reacts, you know how almost all aldehydes will react, with only minor variations due to the size and shape of the group. …