Difficulty: Intermediate | Prerequisites: Lewis structures, basic bonding, functional group recognition
IUPAC nomenclature is the universal system for naming organic molecules so that any chemist can draw the same structure from the name alone. Stereoisomerism adds another layer: molecules with the same connectivity but different spatial arrangements (enantiomers, diastereomers) behave differently in biological systems and reactions. If you can name a compound and classify its relationship to another, you have the foundation for everything else in this course.
IUPAC nomenclature
The International Union of Pure and Applied Chemistry naming system that assigns a unique, unambiguous name to every organic compound based on its structure.
In simple terms, it is the "official" naming method so that chemists worldwide draw the same molecule from the same name.
Parent chain
The longest continuous carbon chain in a molecule that includes the highest-priority functional group; it determines the root name (methane, ethane, propane, butane, pentane, hexane, etc.).
Think of it as the backbone of the name; everything else is a substituent hanging off it.
Stereoisomers
Compounds with the same molecular formula and the same connectivity (same bonds), but a different arrangement of atoms in three-dimensional space.
In simple terms, they are molecules that are "built the same" but shaped differently.
Enantiomers
Stereoisomers that are non-superimposable mirror images of each other. They have identical physical properties except for the direction they rotate plane-polarised light and how they interact with other chiral molecules.
Think of your left and right hands: same parts, mirror-image arrangement, and they do not stack perfectly on top of each other.
Diastereomers
Stereoisomers that are not mirror images of each other. They differ at some (but not all) stereocentres and have different physical properties (melting point, boiling point, solubility).
In simple terms, when you have two or more chiral centres and only some of them flip, the resulting pair are diastereomers.
Constitutional isomers (structural isomers)
Compounds with the same molecular formula but different connectivity, meaning the atoms are bonded in a fundamentally different order.
Think of it as rearranging which atoms connect to which, not just rotating in space.
Meso compound
A molecule that contains chiral centres but is achiral overall because it has an internal plane of symmetry. The stereocentres cancel each other out.
In simple terms, it looks like it should be chiral, but the mirror-image halves within the molecule make it optically inactive.
R/S configuration (Cahn-Ingold-Prelog rules)
A system for assigning absolute configuration at a stereocentre. Substituents are ranked by atomic number; if the priority sequence 1 to 3 runs clockwise (with priority 4 pointing away), the centre is R; anticlockwise is S.
E/Z configuration
A system for describing the geometry of alkenes (or other double bonds) with two different substituents on each carbon. Z (zusammen) means higher-priority groups are on the same side; E (entgegen) means they are on opposite sides.
Think of Z as "zee same side" as a memory aid.
Identify the longest continuous chain (or the ring) as the parent.
Number the chain so that substituents get the lowest possible locants.
List substituents in alphabetical order (ignore multiplying prefixes like di-, tri-).
For cycloalkanes with substituents, the ring is the parent if it has more carbons than any chain, or if a functional group is directly on the ring.
Example: 1,4,4-trimethylcyclohexanol names a cyclohexane ring bearing an -OH group at C1 and three methyl groups (one at C1 implied by the -ol suffix, two at C4).
Change the parent suffix: -ane becomes -ene (double bond) or -yne (triple bond).
Number the chain so the multiple bond gets the lowest locant. The locant goes directly before the suffix: pent-2-ene, not 2-pentene (current IUPAC style, though older textbooks use the latter).
When both a double bond and a triple bond are present, -en- precedes -yn- in the name. Number to give the lowest set of locants overall; if there is a tie, give the double bond the lower number.
Example: (Z)-2-chloro-3-bromopent-2-en-4-yne has a five-carbon chain with a double bond at C2 (Z geometry), a triple bond at C4, chlorine at C2 and bromine at C3.
The -ol suffix takes naming priority over alkenes (-en-) and alkynes (-yn-).
The chain is numbered so the -OH group gets the lowest possible locant.
Example: (R)-4-methylpent-3-en-2-ol has a five-carbon chain, a double bond at C3, a methyl branch at C4 and an -OH at C2 with R configuration.
Rank substituents on each doubly bonded carbon by Cahn-Ingold-Prelog priority (higher atomic number wins).
Z: the two higher-priority groups are on the same side of the double bond.
E: the two higher-priority groups are on opposite sides.
Example: (Z)-3-bromo-5,5-dimethylhex-2-ene places the higher-priority groups (the bromo-bearing side and the larger alkyl side) on the same face.
Assign priorities 1 (highest) through 4 (lowest) to the four substituents using atomic number at the first point of difference.
Orient the molecule so that priority 4 points away from you.
Trace a path from 1 to 2 to 3: clockwise is R, anticlockwise is S.
Common pitfall: if priority 4 is not already pointing away in the drawing, you either physically reorient or invert the assignment after tracing (an odd number of swaps inverts R/S).
First ask: do the two structures have the same molecular formula? If not, they are not isomers at all.
Same formula but different connectivity (different bonds)? Constitutional isomers.
Same connectivity? They are stereoisomers. Now ask: are they mirror images?
Mirror images that cannot be superimposed: enantiomers.
Not mirror images (differ at some but not all stereocentres): diastereomers.
Fully superimposable: identical compounds.
Enantiomers (mirror-image pairs): Two structures drawn as Fischer projections or wedge-dash that, when you assign R/S at every centre, show the configuration is inverted at every stereocentre. All physical properties are the same except optical rotation.
Diastereomers (non-mirror-image stereoisomers): Two structures where at least one stereocentre has the same configuration and at least one differs. These have different physical properties. A classic exam pattern: show two Fischer projections of a molecule with two chiral centres, one R,S and the other R,R. One centre matches, one does not, so they are diastereomers.
Constitutional isomers: Same formula but different atom connectivity. Example: two diols on a cyclohexane ring where the -OH groups sit on different carbons (1,2 vs. 1,3 positions).
Meso compounds: A molecule with two or more stereocentres and an internal mirror plane. The key test: assign R/S at every centre. If one centre is R and its mirror-image partner within the same molecule is S, and the molecule has a plane of symmetry, it is meso. Meso compounds are optically inactive despite having stereocentres.
Look for an internal plane of symmetry (a line you can draw through the molecule so that one half mirrors the other).
Check that the molecule has stereocentres, yet the overall molecule is superimposable on its mirror image.
Cyclohexane with two identical substituents in a trans-1,2 arrangement where one centre is R and the other is S is a common meso example.
Students often confuse E/Z with cis/trans. E/Z uses Cahn-Ingold-Prelog priority rules; cis/trans only works reliably when each doubly bonded carbon has one hydrogen. For tri- or tetra-substituted alkenes, E/Z is the only correct descriptor.
Students often think that any molecule with stereocentres must be chiral. It does not follow. Meso compounds have stereocentres but are achiral because of an internal mirror plane.
Students sometimes assign R/S without first orienting priority-4 away from the viewer. If priority 4 faces towards you in the drawing, the apparent clockwise/anticlockwise assignment is inverted. Either reorient mentally or switch the answer.
Students often assume that diastereomers must differ at exactly one stereocentre. They can differ at any number of centres, as long as they are not mirror images. The defining feature is "stereoisomers that are not enantiomers."
⚠️ Nomenclature questions appear on nearly every organic chemistry exam. You will be asked both to draw a structure from a name and to write a name from a structure, including correct stereodescriptors (R/S, E/Z).
⚠️ Isomer classification is a high-value question type. Expect pairs of structures in wedge-dash or Fischer projection form; you must identify the relationship (constitutional isomers, enantiomers, diastereomers, identical, meso).
⚠️ Meso identification is a favourite test item because it catches students who rely on "stereocentre = chiral" without checking for a symmetry plane.
⚠️ When explaining why two compounds are diastereomers rather than enantiomers, the standard answer is: "One chiral centre has the same configuration in both structures; the other differs." That phrasing appears directly in answer keys.
True or false: E/Z and cis/trans always give the same designation for a given alkene. (False: they can disagree on tri- and tetra-substituted alkenes.)
Fill in the blank: Two stereoisomers that are non-superimposable mirror images are called ________. (Enantiomers.)
True or false: A meso compound has no stereocentres. (False: it has stereocentres but is still achiral due to an internal plane of symmetry.)
Fill in the blank: In IUPAC nomenclature the suffix -ol indicates the presence of a(n) ________ group. (Hydroxyl / alcohol / -OH.)
True or false: Diastereomers have identical melting points. (False: diastereomers have different physical properties.)
Q: Write the IUPAC name, including stereochemistry, for a six-carbon chain with a bromine at C3, two methyl groups at C5 and a double bond at C2 with Z geometry.
A: (Z)-3-bromo-5,5-dimethylhex-2-ene.
Q: A compound has two stereocentres. Structure A is (R,S) and Structure B is (R,R). What is their relationship?
A: They are diastereomers, because one stereocentre (the R centre) has the same configuration in both, while the other differs (S vs. R). They are not mirror images.
Q: You are given a cyclic compound with two chlorine substituents in a 1,2-trans arrangement on a cyclohexane. Both carbons bearing Cl are stereocentres. Could this compound be meso?
A: Yes. If the two stereocentres have opposite configurations (one R, one S) and the molecule has an internal plane of symmetry, it is meso and optically inactive.
Q: Draw and name the alcohol with the molecular formula C₅H₁₀O that has a double bond at C3, a methyl branch at C4 and the hydroxyl at C2 with R configuration.
A: (R)-4-methylpent-3-en-2-ol.
Q: Two structures have the same molecular formula, the same functional groups, but the -OH groups are attached at different positions on a cyclohexane ring. What is their relationship?
A: Constitutional isomers. Same formula, different connectivity.
Nomenclature feeds directly into reaction chemistry: you cannot predict a product if you cannot name the starting material or read a reagent scheme. Stereoisomer classification connects to substitution and elimination reactions (SN1 vs. SN2, E1 vs. E2), where the mechanism determines whether stereochemistry is retained, inverted or lost. Meso compounds reappear in discussions of addition reactions across symmetric alkenes (e.g., Br₂/H₂O addition to cyclopentene) and in NMR equivalence problems.
IUPAC naming, organic nomenclature, naming alkenes, naming alkynes, naming alcohols, E/Z configuration, cis/trans isomers, R/S assignment, Cahn-Ingold-Prelog priority, stereocentre, chiral centre, asymmetric carbon, enantiomers, diastereomers, constitutional isomers, structural isomers, meso compound, mirror image, superimposable, optical activity, plane of symmetry, Fischer projection, wedge-dash notation, stereoisomer classification, Organic Chemistry I, CHEM 2301, UMN