Organic Chemistry and Reaction Mechanisms
What the exam tests
Organic Chemistry and Reaction Mechanisms covers nomenclature, homologous series, functional groups, isomerism, characteristic reactions, mechanisms and qualitative tests for organic compounds.
Core rules
- Name by the longest carbon chain, then number to give the principal functional group the lowest locant.
- Homologous series differ by CH₂ and share a general formula: alkanes CnH2n+2, alkenes CnH2n, alkynes CnH2n−2.
- Isomers have the same molecular formula but different structures; structural isomers differ in connectivity.
- Addition occurs at double bonds; substitution occurs in alkanes and benzene; elimination forms a double bond.
- Free-radical substitution has initiation, propagation and termination steps.
- Tests: bromine water decolourises with alkenes; Tollens' reagent gives a silver mirror with aldehydes; Fehling's gives a red precipitate; the iodoform test detects a CH₃CO– group.
Worked example
How many structural isomers have the formula C₄H₁₀? Only single bonds are possible, so the isomers are unbranched butane (CH₃CH₂CH₂CH₃) and branched 2-methylpropane (CH₃CH(CH₃)CH₃). That is two isomers. A branch drawn at carbon 1 of a four-carbon chain is the same molecule, which is the usual error.
Traps
- Numbering the chain from the wrong end so the functional group gets a higher number.
- Drawing the same structure twice and counting it as two isomers.
- Saying bromine water tests for alkanes rather than alkenes.
- Confusing an aldehyde with a ketone during Tollens' test.
- Omitting the conditions (catalyst, heat, UV light) that distinguish reaction types.