Difficulty: Intermediate | Prerequisites: IUPAC naming of alkanes (Ch. 2–3), cis-trans and E/Z designation
Big picture: Once you can identify geometric isomers, you need to name them properly and understand how substitution patterns affect alkene stability. This set of notes covers the IUPAC rules for naming alkenes (including specifying E/Z and cis/trans in the name), how to draw structures from names and vice versa, how substituent count affects double-bond stability, and the specific conventions for naming cycloalkenes. These skills appear on virtually every CHM 255 exam from this point forward.
Alkenes are named using IUPAC rules by finding the longest chain containing the double bond, numbering to give the double bond the lowest locant, and adding stereochemical prefixes (E/Z or cis/trans) where applicable. More highly substituted double bonds are more stable (tetra > tri > di > mono). Cycloalkenes follow the same logic but the double bond always gets carbons 1 and 2, and cis/trans describes substituent relationships relative to the ring plane.
Alkene
A hydrocarbon containing at least one carbon-carbon double bond. The suffix in IUPAC naming is "-ene." Think of it as: the molecule has a C=C somewhere in it.
Degree of substitution
The number of non-hydrogen substituents (alkyl groups or other) attached to the two carbons of the double bond. A monosubstituted alkene has one such group, disubstituted has two, trisubstituted has three, and tetrasubstituted has four. In simple terms, how "surrounded" the double bond is by carbon groups rather than hydrogens.
Hyperconjugation
The stabilising interaction between the filled C-H (or C-C) sigma bonding orbitals on substituents adjacent to the double bond and the empty pi* antibonding orbital of the double bond. This is the main reason more substituted alkenes are more stable. Think of it as: nearby C-H bonds donate a small amount of electron density into the double bond, strengthening it.
Cycloalkene
A cyclic hydrocarbon containing a carbon-carbon double bond within the ring. Named by adding "-ene" to the cycloalkane root (e.g. cyclohexene). The double-bond carbons are always numbered 1 and 2.
Locant
The number assigned to a carbon in the IUPAC name indicating the position of a substituent or functional group. For alkenes, the locant specifies which carbon the double bond starts at.
Parent chain
The longest continuous carbon chain that includes the double bond. This chain determines the root name. In simple terms, even if a longer chain exists elsewhere in the molecule, if it does not contain the C=C, it is not the parent chain.
Find the parent chain. Identify the longest continuous carbon chain that contains the C=C double bond. This chain sets the root name (ethene, propene, butene, pentene, hexene, etc.).
Number the chain. Number from the end that gives the double bond the lowest possible locant. The double bond position is indicated by the lower-numbered carbon of the C=C.
Name and number substituents. Identify all branches (methyl, ethyl, isopropyl, etc.) and give each a locant based on the numbered parent chain.
Assign stereochemistry. If the double bond can have geometric isomers, add (E)- or (Z)- as a prefix, or cis-/trans- where appropriate. If the molecule also has a chiral centre, include (R)- or (S)- as well.
Assemble the name. Substituents go in alphabetical order before the parent name. The double bond locant goes before the "-ene" suffix.
2-methyl-1-propene: Parent chain is propene (3 carbons). Double bond at C1. Methyl group at C2. No geometric isomers (C1 has two H's).
2-ethyl-4-methyl-1-pentene: Parent chain is pentene (5 carbons). Double bond at C1. Ethyl at C2, methyl at C4.
trans-2,5-dimethyl-4-octene: Parent chain is octene (8 carbons). Double bond at C4. Methyl groups at C2 and C5. Trans configuration specified.
4-isopropyl-2-methyl-1-heptene: Parent chain is heptene (7 carbons). Double bond at C1. Isopropyl at C4, methyl at C2.
(4S,E)-4-chloro-2-hexene: Parent chain is hexene (6 carbons). Double bond at C2 with E configuration. Chloro at C4 with S configuration at that chiral centre.
(E)-2,3-dimethyl-2,4-hexadiene: Parent chain is hexadiene (6 carbons, two double bonds). Double bonds at C2 and C4. Methyl groups at C2 and C3. E configuration specified for the relevant double bond.
When given a name and asked to draw: start with the parent chain length, place the double bond at the indicated locant, attach substituents at their locants, then add the correct stereochemistry last. Drawing the carbon skeleton first and adding hydrogens to fill valences is faster than trying to draw the whole structure at once.
The central principle: more substituted alkenes are more stable. This is measured experimentally by heats of hydrogenation (the less heat released, the more stable the alkene was to begin with).
Tetrasubstituted > trisubstituted > disubstituted > monosubstituted > unsubstituted
Within disubstituted alkenes: trans is generally more stable than cis (less steric strain between groups on opposite sides)
Hyperconjugation is the primary explanation. Each C-H bond on a carbon adjacent to the double bond can overlap with the pi* orbital of the C=C, donating electron density and lowering the overall energy of the molecule. More alkyl substituents means more C-H bonds available for hyperconjugation.
A secondary factor is that alkyl groups are weakly electron-donating (through induction), which stabilises the electron-rich pi system.
The recitation asks you to name compounds and then rank them by increasing double-bond stability. The method is:
Draw each structure.
Count the number of non-hydrogen substituents on the two carbons of the double bond.
Rank by substitution count. If two alkenes have the same degree of substitution, the trans isomer is more stable than cis.
Alkene stability determines which product forms preferentially in elimination reactions. Zaitsev's rule (the more substituted alkene is the major product) is a direct consequence of this stability trend. It also matters in petroleum cracking and polymer chemistry, where the thermodynamic stability of intermediates governs product distribution.
The double bond carbons are always C1 and C2 (you do not need to specify "1-" in the name for monocyclic alkenes with one double bond, though it is acceptable).
Number around the ring in the direction that gives substituents the lowest locants.
Use cis/trans (not E/Z) for cycloalkenes to describe whether substituents on the ring are on the same face or opposite faces of the ring plane. The recitation explicitly notes: you do not need to assign E/Z to cycloalkenes.
For cyclic dienes (two double bonds in the ring), include both locants in the name (e.g. 1,5-cyclooctadiene).
cis-3-ethyl-4-isopropyl-1-cyclopentene: A five-membered ring with a double bond (C1=C2). Ethyl at C3, isopropyl at C4. Cis means both substituents point to the same face of the ring.
trans-1-chloro-3-isopropyl-1-cyclohexene: A six-membered ring with a double bond at C1. Chloro at C1, isopropyl at C3. Trans means these groups are on opposite faces.
4-vinyl-1-cyclohexene: A six-membered ring with a double bond at C1. A vinyl group (CH=CH₂) at C4.
3,3-dimethyl-1-cycloheptene: Draw a seven-membered ring, place a double bond between C1 and C2, attach two methyl groups to C3.
1,2-dimethyl-1-cyclobutene: Draw a four-membered ring, place a double bond between C1 and C2, attach a methyl to C1 and a methyl to C2.
(S)-3-isobutyl-1,5-cyclooctadiene: Draw an eight-membered ring with double bonds at C1 and C5. Attach an isobutyl group at C3 with S configuration at that chiral centre.
The recitation ends with a question about the maximum number of stereoisomers for a given molecule. The method is straightforward.
For a molecule with n stereocentres (chiral centres and/or double bonds capable of E/Z isomerism), the maximum number of stereoisomers is 2ⁿ.
Each chiral centre (sp3 carbon with four different groups) contributes a factor of 2 (R or S).
Each double bond with E/Z isomerism contributes a factor of 2 (E or Z).
Multiply: n total stereocentres gives a maximum of 2ⁿ stereoisomers.
Meso compounds reduce the count. If a molecule has an internal plane of symmetry despite having chiral centres, some "expected" stereoisomers turn out to be identical (meso forms). For the purposes of the Week 7 recitation, the question asks for the maximum, so 2ⁿ is typically the expected answer.
The recitation molecule (a structure with a cyclohexane ring, a double bond in a side chain, and what appears to be a branching point) requires you to count: how many E/Z-capable double bonds and how many chiral centres? Multiply the factors of 2. In the recitation answer, the result was 6 stereoisomers, consistent with the structure having multiple stereocentres and geometric isomerism sources (though the exact count depends on reading the structure carefully from the original worksheet).
Students often choose the longest carbon chain in the molecule as the parent chain, even when it does not contain the double bond. The parent chain must include the C=C.
Students sometimes number the chain to give substituents the lowest numbers rather than the double bond. The double bond takes numbering priority over substituents.
Students frequently confuse "isopropyl" and "isobutyl" when reading or writing names. Isopropyl is (CH₃)₂CH-, a three-carbon branched group. Isobutyl is (CH₃)₂CHCH₂-, a four-carbon branched group. Drawing them out each time avoids mix-ups.
Students often assume that a more substituted alkene is less stable because it is "more crowded." The opposite is true: more substitution means more hyperconjugation and greater stability.
When naming cycloalkenes, students sometimes assign arbitrary numbering rather than ensuring the double bond sits at C1-C2 and then minimising substituent locants.
⚠️ "Draw the structure for the following IUPAC name" and "provide the IUPAC name for the following structure" are among the most common question types on CHM 255 exams. Speed and accuracy here come from practice.
⚠️ Ranking alkenes by stability is a standard exam question. Know the substitution hierarchy and remember that trans beats cis within the same substitution level.
⚠️ Cycloalkene naming with cis/trans stereochemistry is tested separately from E/Z. The recitation specifies that you do not assign E/Z to cycloalkenes, so do not overthink it on the exam.
⚠️ Stereoisomer counting (2ⁿ rule) appears regularly. The trick is correctly identifying all stereocentres, including both chiral carbons and E/Z-capable double bonds.
True or false: The parent chain for an alkene must contain the double bond, even if a longer chain exists elsewhere in the molecule.
Fill in the blank: In IUPAC naming, the double bond receives numbering priority over ______.
True or false: A trisubstituted alkene is less stable than a disubstituted alkene.
Fill in the blank: In cycloalkene naming, the double bond is always assigned carbons ______ and ______.
True or false: A molecule with 3 stereocentres has a maximum of 6 stereoisomers.
Q: Give the IUPAC name for a five-carbon chain with a double bond at C1, an ethyl group at C2, and a methyl group at C4.
A: 2-ethyl-4-methyl-1-pentene.
Q: Draw the structure of (4S,E)-4-chloro-2-hexene.
A: Draw a six-carbon chain. Place a double bond between C2 and C3 with E (trans) configuration. Place a chlorine on C4 with S stereochemistry (using CIP rules to confirm the configuration at that sp3 carbon).
Q: Rank the following in order of increasing stability: 1-butene, trans-2-butene, cis-2-butene, 2-methylpropene.
A: 1-butene (monosubstituted) < cis-2-butene (disubstituted, cis) < trans-2-butene (disubstituted, trans) < 2-methylpropene (disubstituted, but with two methyl groups on the same carbon, it is technically disubstituted and comparable to cis/trans-2-butene, though the branching pattern can slightly differ). The key ranking: mono < di (cis) < di (trans). For exam purposes, all disubstituted are more stable than mono, and trans beats cis.
Q: Name the cycloalkene: a five-membered ring with a double bond, an ethyl group at C3, and an isopropyl group at C4, both on the same face of the ring.
A: cis-3-ethyl-4-isopropyl-1-cyclopentene.
Q: A molecule has two chiral centres and one E/Z-capable double bond. What is the maximum number of stereoisomers?
A: 2³ = 8 stereoisomers.
IUPAC naming of alkenes builds directly on alkane naming (Ch. 2-3). Every rule you learnt for naming alkanes still applies; alkenes just add the double-bond locant and the "-ene" suffix.
Alkene stability connects forward to elimination reactions (E1 and E2, Ch. 9-10). Zaitsev's rule says the more substituted alkene is the major product, which is a direct consequence of the stability trend covered here.
Stereoisomer counting connects to the broader stereochemistry unit (Ch. 5). The 2ⁿ rule applies to any molecule with stereocentres, whether those are chiral carbons or geometric double bonds.
IUPAC nomenclature, alkene naming, naming alkenes, parent chain, locant, double bond position, degree of substitution, monosubstituted, disubstituted, trisubstituted, tetrasubstituted, hyperconjugation, alkene stability, heat of hydrogenation, cycloalkene, cyclopentene, cyclohexene, cyclooctadiene, cis-trans cycloalkenes, stereoisomer count, 2^n rule, Zaitsev rule, CHM 255, organic chemistry, Purdue