Difficulty: Intermediate | Prerequisites: General chemistry (Lewis structures, bonding, electronegativity), familiarity with basic carbon skeletal structures.
Naming organic molecules is the foundation everything else in organic chemistry builds on. If you cannot read a name and draw the structure (or vice versa), reaction mechanisms and synthesis problems become guesswork. This section covers IUPAC nomenclature for alkenes and cycloalkanes, including how to assign E/Z stereochemistry using the Cahn-Ingold-Prelog priority rules. You should already be comfortable naming simple alkanes and drawing line-angle structures before tackling this material.
IUPAC names describe exactly one structure: pick the longest carbon chain containing the functional group, number from the end that gives the lowest locants, and list substituents alphabetically. For alkenes with two different groups on each carbon of the double bond, use Cahn-Ingold-Prelog priority rules to assign E (higher-priority groups on opposite sides) or Z (same side).
IUPAC nomenclature
The standardised system for naming organic compounds published by the International Union of Pure and Applied Chemistry. In simple terms, it is the universal "address" for a molecule so that any chemist in the world draws the same structure from the name.
Parent chain
The longest continuous carbon chain that includes the principal functional group (e.g. the double bond in an alkene). Think of it as the backbone the rest of the name hangs from.
Locant
The number assigned to a carbon in the parent chain to specify where a substituent or functional group is attached.
Substituent
An atom or group replacing a hydrogen on the parent chain (e.g. methyl, chloro). Listed alphabetically in the IUPAC name, ignoring multiplying prefixes like di- or tri-.
Alkene
A hydrocarbon containing at least one carbon-carbon double bond. The suffix changes from -ane to -ene.
E/Z designation
A stereochemical label for alkenes. E (from German "entgegen," meaning opposite) places the two higher-priority substituents on opposite sides of the double bond. Z (from "zusammen," meaning together) places them on the same side.
Cahn-Ingold-Prelog (CIP) priority rules
The ranking system used to assign priority to substituents on a stereogenic element. Higher atomic number at the first point of difference wins. In simple terms, heavier atoms beat lighter ones.
Constitutional isomers
Molecules with the same molecular formula but different connectivity of atoms. They are entirely different compounds with different physical properties.
Degree of substitution (alkene stability)
The number of carbon substituents attached to the double-bond carbons. More substituted alkenes are generally more stable due to hyperconjugation.
Identify the longest carbon chain containing the C=C double bond. This is your parent chain, and the suffix is -ene.
Number from whichever end gives the double bond the lowest locant. In names like "pent-2-ene," the 2 tells you the double bond starts at carbon 2.
List substituents alphabetically with their locants. Multiplying prefixes (di-, tri-) do not affect alphabetical order.
Example from exam: (E)-hex-3-en-1-ol. The parent chain is hexene (six carbons, one double bond), the -ol suffix indicates a hydroxyl group at carbon 1, and the double bond is at carbon 3 with E stereochemistry.
Example from exam: 3,3-dimethylcyclopropene. A three-membered ring with a double bond and two methyl groups both on carbon 3.
Start by drawing the parent chain with the correct number of carbons.
Place the double bond at the specified position.
Add substituents at their numbered positions.
Example from exam: 5-chloro-2-methylpent-2-ene. Draw a five-carbon chain, place a double bond starting at C2, attach a methyl group at C2, and a chlorine at C5.
Applies only when each carbon of the double bond bears two different groups.
Use CIP priority rules: at each double-bond carbon, rank the two attached groups by atomic number at the first point of difference.
Higher atomic number wins. Cl (17) beats C (6). F (9) beats H (1).
If the atoms at the first point of difference are the same, move outward along the chain until you find a difference.
Z (zusammen): the two higher-priority groups are on the same side of the double bond.
E (entgegen): the two higher-priority groups are on opposite sides.
Exam example 1: an alkene with F and Cl on one side, CH3 and H on the other. Cl > CH3 on the left carbon, F > H on the right. The higher-priority groups (Cl, F) are on opposite sides, so it is E.
Exam example 2: an alkene with Cl and H3C on one side, Cl and H on the other. The higher-priority groups end up on the same side, making it Z.
More substituted alkenes are more stable (Zaitsev's rule connects here: the more substituted alkene is the thermodynamic product).
Trisubstituted > disubstituted > monosubstituted > unsubstituted.
Trans (E) alkenes are generally more stable than cis (Z) due to reduced steric strain, unless forced into a ring.
Exam example: among four constitutional isomers, the most substituted, internal alkene (option c) was the most stable.
Students often confuse E/Z with cis/trans. Cis/trans works only for simple cases (two identical groups). E/Z uses CIP priorities and works for every alkene, so it is the more general system.
Students sometimes number the parent chain from the wrong end, giving substituents higher locants than necessary. Always number to give the principal functional group (the double bond in alkenes, the -OH in alcohols) the lowest locant first.
Forgetting that multiplying prefixes (di-, tri-, tetra-) are ignored when alphabetising substituents. "Dimethyl" is still filed under M, not D.
Assuming E always means trans and Z always means cis. This is often true for simple cases, but not always. E/Z is determined by CIP priority, not by spatial arrangement of "like" groups.
Naming and drawing structures from IUPAC names is virtually guaranteed on every organic chemistry exam. Get fast at it.
E/Z assignments are a favourite exam question because they test both your understanding of CIP priorities and your ability to read a structure carefully.
Alkene stability ranking appears in both direct comparison questions and as the basis for predicting major products in elimination reactions (Zaitsev's rule).
The exam tested all three skills together: name a molecule with stereochemistry, draw from a name, and assign E or Z to drawn structures.
True or false: The prefix "di-" counts when alphabetising substituents in an IUPAC name. (False)
Fill in the blank: In E/Z nomenclature, Z stands for ______, meaning the higher-priority groups are on the ______ side of the double bond. (zusammen; same)
True or false: A trisubstituted alkene is more stable than a disubstituted alkene. (True)
Fill in the blank: When numbering the parent chain of an alkene, the double bond should receive the ______ possible locant. (lowest)
True or false: E/Z and cis/trans always give the same designation for a given alkene. (False)
Q: Provide the IUPAC name, including stereochemistry, for a six-carbon chain with a double bond at C3, a hydroxyl group at C1, and E configuration.
A: (E)-hex-3-en-1-ol.
Q: Draw the structure of 5-chloro-2-methylpent-2-ene.
A: A five-carbon chain with a double bond between C2 and C3, a methyl group on C2, and a chlorine on C5.
Q: An alkene has F and Cl on one double-bond carbon, and CH3 and H on the other. The higher-priority groups (Cl, F) are on opposite sides. Is this E or Z?
A: E. The higher-priority group on each carbon (Cl on the left, F on the right) are on opposite sides.
Q: Rank the following alkenes from most to least stable: monosubstituted, trisubstituted, disubstituted.
A: Trisubstituted > disubstituted > monosubstituted.
Q: What is the key difference between using E/Z and cis/trans nomenclature?
A: E/Z uses CIP priority rules and works for all alkenes. Cis/trans relies on comparing "like" groups and fails when all four substituents are different.
Nomenclature is the language you will use in every subsequent topic. Elimination reactions (E1, E2) produce alkenes, and you need to name and predict the stereochemistry of those products. Alkene stability directly predicts which product dominates in elimination reactions via Zaitsev's rule. CIP priority rules also return when assigning R/S configuration at chirality centres in substitution reactions.
IUPAC naming, organic nomenclature, E/Z alkene, cis-trans isomerism, Cahn-Ingold-Prelog rules, CIP priority, alkene stability, degree of substitution, Zaitsev rule, stereochemistry of alkenes, constitutional isomers, line-angle structures, parent chain, locant, substituent naming, organic chemistry I, OChem exam review