Functional groups: classification of organic compounds

Chemistry · Structure 3: Classification of matter. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All Chemistry topics

The idea

Organic compounds are sorted by their functional group, the atom or group of atoms that gives the compound its typical reactions. Compounds with the same functional group form a homologous series: they share a general formula, each member differs from the next by CH₂, and they have similar chemical properties. Boiling points rise along a series because longer molecules have stronger London forces.

One compound can be shown by several formulas: empirical, molecular, full structural, condensed structural and skeletal. In a skeletal formula every line end and every corner is a carbon atom, and hydrogen atoms on carbon are not drawn.

Structural isomers have the same molecular formula but different structural formulas.

Rules to remember

Groups: halogeno –X, hydroxyl –OH, carbonyl C=O, carboxyl –COOH, alkoxy –OR, amino –NH₂, amido –CONH₂, ester –COO–, phenyl –C₆H₅
Chain names: meth 1, eth 2, prop 3, but 4, pent 5, hex 6 carbon atoms
Naming: find the longest carbon chain, then number it so that the functional group and branches get the lowest numbers
Alcohols and halogenoalkanes: primary, secondary or tertiary = 1, 2 or 3 carbon atoms bonded to the carbon that carries the –OH or halogen
Amines: primary, secondary or tertiary = 1, 2 or 3 carbon atoms bonded to the nitrogen atom
Isomers: chain (different carbon skeleton), position (same group in a different place), functional group (different group, such as alcohol and ether, or aldehyde and ketone)
Compounds that can form hydrogen bonds (alcohols, carboxylic acids) boil at higher temperatures than alkanes of similar molar mass

Common mistake

Numbering the chain from the wrong end, which gives names such as butan-3-ol. Number from the end that gives the functional group the lowest number: butan-2-ol.

Worked example

What is the IUPAC name of (CH₃)₂CHCH₂OH, and is it a primary, secondary or tertiary alcohol?

The longest chain that includes the carbon carrying –OH has 3 carbon atoms: propan.
Number from the –OH end: –OH is on carbon 1 and a methyl branch is on carbon 2.
The carbon carrying –OH is bonded to only one other carbon atom.

Answer: 2-methylpropan-1-ol, a primary alcohol

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Read the full notes

Structure 3.2 Functional Groups: Classification of Organic Compounds

Keep going

← The periodic table: classification of elements Measuring enthalpy changes →Take a Chemistry test