CHEM1011 · Fundamentals Of Chemistry 1a
Organic Chemistry: Groups, Naming and Isomers
This chapter is the organic-chemistry block of CHEM1011 (USyd Fundamentals of Chemistry 1A): how to read a carbon structure, name it by IUPAC rules, classify its isomers, and assign R/S to a chiral centre. It is the single largest organic load on the closed-book final, and the skills are procedural — once you drill the recipes (skeletal reading, the six naming steps, the isomer tree, the CIP rotation), the marks are reliable and repeatable.
What this chapter covers
- 011. Why carbon: tetravalency, catenation, and the three representations (molecular, condensed, skeletal/line)
- 022. Hydrocarbon families and general formulas: alkane CₙH₂ₙ₊₂, alkene CₙH₂ₙ, alkyne CₙH₂ₙ₋₂
- 033. The chain stems meth- to dec- (1 to 10 carbons)
- 044. The ~13 functional groups: recognise, predict reactivity, name (suffix/prefix)
- 055. IUPAC naming: the six-step recipe and the lowest-locant rule
- 066. The isomer tree: constitutional vs stereo; conformational vs configurational; enantiomers vs diastereomers
- 077. E/Z geometry and cis/trans on a C=C
- 088. Chirality, stereocentres, and R/S assignment by CIP priority
- 099. Optical activity: equal-and-opposite rotation, and the zero rotation of a racemate
Assign R or S to a halogen-bearing stereocentre
- +1Confirm the stereocentre: the carbon carries four different groups (Br, Cl, F, H), so it is a genuine chiral (stereogenic) centre and a single R/S label applies.
- +1Rank by CIP priority using atomic number (highest Z = priority 1): Br (35) > Cl (17) > F (9) > H (1), giving Br = 1, Cl = 2, F = 3, H = 4.
- +1Orient the molecule so the lowest priority group (H, #4) points directly away from you, along the bond going into the page.
- +1Trace the path 1 → 2 → 3, i.e. Br → Cl → F: the rotation is clockwise, so the centre is R. (Anticlockwise would be S.)
Key terms
- Functional group
- A characteristic grouping of atoms (e.g. –OH, –COOH, C=C) that gives a molecule its typical reactivity and determines its IUPAC name suffix or prefix; R denotes the rest of the carbon skeleton.
- Skeletal (line) structure
- A shorthand drawing where each line-end and vertex is a carbon atom, C–H bonds are implied (added mentally until each carbon has four bonds), and only heteroatoms such as the O of –OH or the N of an amine are drawn explicitly.
- Constitutional isomers
- Molecules with the same molecular formula but different connectivity (a different sequence of which atom is bonded to which); includes chain, position, and functional-group isomers.
- Stereocentre (chiral centre)
- A carbon atom bonded to four different groups; its molecule is non-superimposable on its mirror image, giving a pair of enantiomers.
- Enantiomers
- A pair of stereoisomers that are non-superimposable mirror images of each other; they share most physical properties but rotate plane-polarised light by equal and opposite amounts and behave differently toward other chiral objects such as enzymes.
- R/S (CIP) configuration
- The absolute label for a stereocentre: rank the four groups by Cahn–Ingold–Prelog priority (highest atomic number first), point the lowest priority away, and trace 1→2→3 — clockwise = R (rectus), anticlockwise = S (sinister).
Organic Chemistry: Groups, Naming and Isomers FAQ
How do I know which way to number the carbon chain in IUPAC naming?
Number from the end that gives the principal functional group (or the multiple bond) the lowest locant first. Only if there is a tie do you then minimise the substituent locants. So a four-carbon chain with a terminal double bond is but-1-ene, not but-3-ene — always check both directions before committing.
What is the difference between an aldehyde, a ketone and a carboxylic acid?
All three contain a C=O (carbonyl), but the position and neighbours differ. An aldehyde has the C=O at a chain end with an H on it (–CHO, suffix -al); a ketone has the C=O between two carbons (suffix -one); a carboxylic acid has an –OH on the carbonyl carbon (–COOH, suffix -oic acid) and is acidic. Mis-classifying the carbonyl is a classic short-answer slip.
How do I assign R or S to a chiral centre?
Rank the four groups by CIP priority (highest atomic number = priority 1, read Z off the provided periodic table). Point the lowest-priority group away from you, then trace from priority 1 to 2 to 3: clockwise = R, anticlockwise = S. If the lowest priority points toward you, read the rotation and then flip the answer.
Are conformers (eclipsed vs staggered) counted as separate isomers?
No. Conformations interconvert just by rotating about single bonds, so they are not separable, distinct compounds. Only configurational changes — which require a bond to break — give genuinely different, isolable isomers such as enantiomers and diastereomers. Do not list a conformer as a different compound in the exam.
What is on the exam for organic chemistry, and what must I memorise?
The closed-book final provides a datasheet and periodic table but does NOT give the chain stems, the general formulas, the functional-group list, or the CIP/R-S rules. You must memorise meth- to dec-, the three general formulas (CₙH₂ₙ₊₂ / CₙH₂ₙ / CₙH₂ₙ₋₂), the ~13 functional groups with their suffixes, and the R/S procedure. You only get atomic numbers (for CIP priority) from the provided table.
Exam move
Treat this chapter as four memorised recipes, then drill each on a real structure. (1) Lock the rote facts the datasheet will NOT give you: stems meth-...dec-, the three general formulas, and the ~13 functional groups with their suffix/prefix. (2) Practice reading skeletal structures cold — every vertex and line-end is a carbon, fill H to four bonds, and only the heteroatoms are drawn. (3) Run the six-step IUPAC recipe on five different chains until the lowest-locant rule is automatic. (4) For isomers, walk the tree top-down by asking the fork question each time: different connectivity? bond rotation enough? mirror images? (5) For R/S, always write rank, point-lowest-away, trace 1→2→3, and check the toward-you flip. The whole topic is procedural, so the marks come from executing the steps in order, not from insight.
