CHEM10007 · Fundamentals Of Chemistry
Organic Chemistry I: Alkanes, Isomerism & Stereochemistry
The first organic week builds the structural toolkit. You learn alkane bonding and IUPAC nomenclature, distinguish structural (constitutional) isomers, and read Newman projections (staggered vs eclipsed conformations). Stereochemistry adds chirality, enantiomers and R/S assignment, and the chapter introduces the functional-group map you will recognise and name for the rest of the subject.
What this chapter covers
- 01Alkanes CnH2n+2; tetrahedral sp3 bonding in CH4 and C2H6; condensed and skeletal drawing
- 02Structural (constitutional) isomers: same formula, different connectivity
- 03IUPAC nomenclature: longest chain as parent, lowest locants, alphabetical substituents
- 04Newman projections: staggered vs eclipsed conformations (interconvert by rotation)
- 05Stereoisomerism vs conformation: configurational isomers need bond breaking
- 06Chirality, stereocentre, enantiomers and racemates
- 07R/S assignment using CIP priority rules
- 08Functional-group map: alcohol, amine, ether, thiol, aldehyde, ketone, carboxylic acid, ester, amide, halide, alkene, alkyne, arene
IUPAC naming and functional-group identification
- 1 mark — parent chainThe longest chain has 6 carbons, so the parent is hexane.
- 1 mark — locants and orderingNumber to give the lowest locants and list substituents alphabetically (bromo/chloro before methyl): chloro and methyl both on C-3.
- 1 mark — correct IUPAC nameFull name: 3-chloro-3-methylhexane.
- 1 mark — all four groups identifiedFunctional groups: −OH alcohol, −NH2 (primary) amine, −COOH carboxylic acid, C≡C alkyne.
Key terms
- Structural (constitutional) isomer
- Molecules with the same molecular formula but different atom-to-atom connectivity, e.g. butane and 2-methylpropane.
- Conformation
- A spatial arrangement (e.g. staggered or eclipsed) interconverted by rotation about a single bond — not a distinct isomer, since no bonds are broken.
- Chirality / stereocentre
- A carbon bonded to four different groups is a stereocentre; a molecule lacking an internal mirror plane is chiral and exists as non-superimposable enantiomers.
- Enantiomers
- A pair of non-superimposable mirror-image stereoisomers; a 50:50 mixture is a racemate.
- R/S configuration
- An absolute descriptor of a stereocentre assigned by ranking the four groups by CIP priority and reading 1→2→3 with the lowest priority pointing away (clockwise = R, anticlockwise = S).
Organic Chemistry I: Alkanes, Isomerism & Stereochemistry FAQ
What is the difference between a conformation and a configurational isomer?
Conformations (staggered, eclipsed) interconvert freely by rotation about a single bond, so they are the same molecule. Configurational isomers (such as enantiomers or cis/trans) can only interconvert by breaking and remaking bonds, so they are genuinely different compounds.
How do I assign R or S to a stereocentre?
Rank the four attached groups by CIP priority (higher atomic number first). Orient the lowest-priority group away from you and trace 1→2→3: clockwise is R, anticlockwise is S.
Which Newman conformation is lowest in energy?
Staggered conformations are lower in energy than eclipsed ones because the bonds are 60° apart, minimising torsional strain. For butane, the anti-staggered conformation is the global minimum.
Exam move
Make IUPAC naming bidirectional — practise both naming a drawn structure and drawing a structure from its name, since the exam tests both. Build a one-page functional-group sheet and quiz yourself until recognition is instant. For stereochemistry, drill CIP priority and the R/S reading rule on simple stereocentres first, and be clear that conformations are not isomers — a classic multiple-choice distractor.
