Organic Chemistry and Stoichiometry
What the exam tests
Nomenclature, functional-group identification, isomerism, the main reaction types, and stoichiometry applied to organic reactions, especially combustion and esterification. Expect a structure-drawing or formula-finding task plus a mass or volume calculation.
Core rules
- Nomenclature: find the longest carbon chain containing the principal functional group, number to give that group the lowest locant, then list substituents alphabetically. Suffix priority: carboxylic acid > ester > amide > aldehyde > ketone > alcohol > amine > alkene > alkane.
- Isomerism: chain, position and functional-group isomers; cis/trans requires a C=C with two different groups on each carbon; optical isomerism requires a chiral centre with four different groups.
- Alkanes undergo free-radical substitution with halogens under UV light. Alkenes undergo electrophilic addition; Markovnikov's rule places H on the carbon with more hydrogens.
- Primary alcohols oxidise to aldehydes and then carboxylic acids; secondary alcohols to ketones; tertiary alcohols resist mild oxidation.
- Esterification: carboxylic acid + alcohol ⇌ ester + water, acid-catalysed and reversible.
- Combustion: C_xH_y + (x+y/4)O₂ → xCO₂ + (y/2)H₂O.
Worked example
Complete combustion of 0.10 mol of a hydrocarbon gives 0.30 mol CO₂ and 0.40 mol H₂O. Find its formula. Carbon: 0.30/0.10=3 C per molecule. Hydrogen: each H₂O contains 2 H, so 0.40×2=0.80 mol H, giving 0.80/0.10=8 H per molecule. Formula C₃H₈ (propane). Check oxygen: 3+8/4=5 mol O₂ per mol fuel, i.e. 0.50 mol O₂ for 0.10 mol.
Traps
- Forgetting that esterification produces water, so the equation is not a simple addition.
- Applying Markovnikov's rule backwards to an unsymmetrical alkene.
- Claiming cis/trans isomerism for a terminal alkene such as propene.
- Counting the hydrogen atoms in water without doubling to get H atoms per fuel molecule.
- Confusing the molecular formula with the empirical formula.