AQA A Level Chemistry 3.3.1 Introduction to Organic Chemistry Exam Questions

About AQA A Level Chemistry 3.3.1 Introduction to Organic Chemistry

This topic sets up the language of AQA A Level organic chemistry. You need to name compounds using IUPAC rules, draw and interpret molecular, empirical, general, structural, displayed and skeletal formulae, recognise functional groups and homologous series, and understand how curly arrows show the movement of electron pairs in mechanisms. You also study structural isomerism (chain, position and functional group) and E/Z stereoisomerism using the Cahn-Ingold-Prelog priority rules. Everything else in organic chemistry relies on these skills. Examiners regularly ask you to name or draw isomers, identify E or Z forms and explain why stereoisomers exist.

What you need to know

• Formulae Molecular formula gives the actual number of atoms; empirical formula the simplest ratio; general formula a whole homologous series (e.g. CnH2n+2 for alkanes). Structural, displayed and skeletal formulae show how the atoms are arranged.
• Nomenclature Find the longest carbon chain, number from the end giving the lowest locants, list substituents alphabetically with prefixes such as di- and tri-, and use the functional group suffix (e.g. -ol, -al, -one, -oic acid).
• Functional groups and homologous series A homologous series has the same functional group and general formula, with each member differing by CH2. Members show a gradual change in physical properties and similar chemical reactions.
• Mechanisms Curly arrows show the movement of a pair of electrons, starting from a lone pair or a bond. Free radicals have an unpaired electron and are shown with a dot. Heterolytic fission gives ions; homolytic fission gives radicals.
• Structural isomers Same molecular formula, different structural formula. Chain isomers have different carbon skeletons, position isomers have the functional group in different places, and functional group isomers have different functional groups (e.g. an aldehyde and a ketone).
• E/Z isomerism Occurs because of restricted rotation about a C=C bond when each carbon of the double bond has two different groups. Use the CIP rules: the higher atomic number atom has priority. Z has the higher priority groups on the same side; E has them on opposite sides.

Common mistakes in 3.3.1 Introduction to Organic Chemistry

• Not numbering the chain to give the lowest locants, or listing substituents by number instead of alphabetically.
• Starting curly arrows from atoms or charges rather than from a lone pair or a bond.
• Using cis and trans when the question asks for E or Z. Apply the CIP priority rules, which can give a different label from cis/trans.
• Claiming E/Z isomerism exists when one carbon of the C=C carries two identical groups, as in but-1-ene.
• Drawing skeletal formulae with hydrogen atoms on carbon or with a carbon missing at the end of a chain.

Try three 3.3.1 Introduction to Organic Chemistry exam-style questions