Cambridge O Level Chemistry · Syllabus 5070 · Organic Chemistry
Functional Group
What is Functional Group?
An atom or group of atoms within a molecule that determines that molecule's characteristic chemical reactions, such as the C=C of an alkene, the -OH of an alcohol or the -COOH of a carboxylic acid.
This definition is part of the Organic Chemistry chapter in Cambridge O Level Chemistry.
Functional Group in context
Organic chemistry is the chemistry of carbon compounds. Every question in Topic 11 is the same question in a different costume: read the carbon backbone, find the functional group, name or draw the molecule, choose the reaction and its conditions, then redraw only the part that changed and check that every atom still has the right number of bonds.
A homologous series is a family of organic compounds that share the same functional group and the same general formula, whose successive members differ by a -CH2- unit and show a gradual trend in physical properties with similar chemical properties. Alkanes (CnH2n+2) are saturated hydrocarbons with only single bonds and limited reactivity; alkenes (CnH2n) are unsaturated, containing a reactive C=C double bond; alcohols (CnH2n+1OH) contain -OH; and carboxylic acids (CnH2n+1COOH) contain -COOH and behave as acids. Substitution replaces one atom and displaces another; addition opens a double bond to form a single product; condensation polymerisation releases a small molecule at each linkage, while addition polymerisation releases nothing.
Common mistakes with Functional Group
- “All polymerisation reactions release water.” why it fails This over-generalises from condensation polymers to every polymer. correct Addition polymerisation releases nothing at all — the polymer is the only product. Condensation polymerisation releases a small molecule, and you can only say which one by looking at the functional groups actually being joined. Taught in 11.8B.
- 17. “Nylon is made from ethanoic acid and an amine.” defect Condensation monomers given only one functional group each. why With one reactive end apiece, the two would join once and stop. You would get a small molecule, not a polymer. repair Use a dicarboxylic acid and a diamine: HOOC–□–COOH and H2N–▢–NH2. transfer Explain why an amino acid can build a protein on its own, despite there being only one kind of monomer.
Questions students ask about Functional Group
What is the difference between addition polymerisation and condensation polymerisation?
Addition polymerisation joins many unsaturated monomers by opening each C=C double bond, so the polymer is the only product and nothing is released. Condensation polymerisation joins monomers that each carry two functional groups, releasing a small molecule — usually water — at every linkage formed. A polyamide forms a -CO-NH- linkage from a dicarboxylic acid and a diamine; a polyester forms a -CO-O- linkage from a dicarboxylic acid and a diol.