What Are Functional Groups?
Functional group: A functional group is a specific arrangement of atoms within a molecule that determines the molecule's characteristic chemical reactions and properties.
Functional groups are the "active zones" of organic molecules , they dictate how a molecule behaves, reacts, and interacts with other substances. Two molecules with the same carbon skeleton but different functional groups can have dramatically different chemical and physical properties.
Think of a functional group as the "personality" of a molecule. Just as people with similar backgrounds can have very different personalities, organic molecules with the same carbon chain can behave completely differently depending on which functional group is attached.
For example, the hydroxyl group (–OH) in alcohols makes these compounds polar and capable of forming hydrogen bonds, influencing their solubility in water and their boiling points , properties that are critical in both natural processes and industrial applications.
In IB Chemistry, you are expected to recognise and name the key functional groups, identify them in structural and skeletal formulas, and predict how they influence the physical and chemical properties of organic compounds.
Representing Organic Compounds
A single organic compound can be represented in multiple ways. Each representation offers a different level of detail.
Empirical formula: The empirical formula represents the simplest whole-number ratio of atoms of each element in a compound. It gives relative proportions but not the actual number of atoms.
Molecular formula: The molecular formula specifies the exact number of atoms of each element in one molecule of a compound.
Structural formula: The structural formula shows how atoms are connected in a molecule. It can be displayed as a full, condensed, or skeletal formula.
The three types of structural formula are:
- Full structural formula: Every atom and bond is shown explicitly.
- Condensed structural formula: Atoms are grouped to simplify the representation (e.g., for ethanol).
- Skeletal formula: Carbon atoms are shown as vertices of a zigzag line; hydrogen atoms bonded to carbon are omitted. Functional groups (e.g., –OH, –COOH) are always shown explicitly.
For propan-2-ol ():
- Molecular formula:
- Condensed structural formula:
- Skeletal formula: A zigzag of two bonds with an –OH group on the middle carbon vertex.
For ethanol ():
- Full structural formula: shows every C–H and C–O bond explicitly
- Condensed:
- Skeletal: a single line (one bond) with –OH at the right-hand end
When drawing skeletal formulas, always include functional groups such as –OH, C=O, –COOH, –NH₂. Only C–H bonds on the carbon skeleton are omitted. Missing a functional group is a very common error in exams.