Difficulty: Intermediate | Prerequisites: Part 1 of these notes (Alkene Structure and Stereochemistry), IUPAC naming conventions from Chapters 2–4
Big picture: This half of Chapter 5 covers how to name alkenes using IUPAC rules, including compounds with multiple double bonds and cyclic systems. It also connects alkene geometry to real-world chemistry: why butter is solid and olive oil is liquid, what trans fats are, and how your eyes detect light. If you can already assign E/Z and understand why rotation is restricted, you are ready. The naming rules build directly on the alkane naming you learned in Chapter 2, with a few new conventions for locating the double bond.
Alkenes are named like alkanes but with the suffix "-ene" instead of "-ane," and the parent chain must include the double bond. The double bond gets the lowest possible locant. In the real world, the geometry of C=C bonds in fatty acids controls melting points and health effects, and a light-triggered cis-to-trans isomerisation in retinal is the first step in vision.
Parent chain (for alkenes)
The longest continuous carbon chain that contains the C=C double bond. This chain determines the root name, even if a longer chain exists elsewhere in the molecule that does not include the double bond.
-ene suffix
The IUPAC ending for alkenes, replacing "-ane" from alkanes. The position of the double bond is indicated by a locant before or within the name (e.g. but-1-ene or 1-butene).
Diene / triene / tetraene
Compounds with two, three, or four C=C double bonds, respectively. Named with the suffixes -diene, -triene, -tetraene, with locants for each double bond (e.g. 1,3-butadiene).
Cycloalkene
A cyclic compound containing a C=C double bond within the ring. Named by adding the prefix "cyclo-" (e.g. cyclohexene). Numbering starts at the double bond.
Saturated fatty acid
A long-chain carboxylic acid with no C=C double bonds. The hydrocarbon tail is fully saturated with hydrogens and packs efficiently, resulting in higher melting points (solids at room temperature). In simple terms, these are the fats that are solid, like butter.
Monounsaturated fatty acid
A fatty acid with one C=C double bond. The kink introduced by the cis double bond disrupts packing and lowers the melting point. Think of oleic acid (18:1) in olive oil.
Polyunsaturated fatty acid
A fatty acid with two or more C=C double bonds. More kinks mean even lower melting points. Examples: linoleic acid (18:2, omega-6) and linolenic acid (18:3, omega-3).
Partially hydrogenated oil
An oil treated with hydrogen gas and a metal catalyst (Pt or Pd) to reduce some, but not all, of its C=C double bonds. This process can convert natural cis double bonds to trans double bonds as a side effect.
Trans fat
A fat containing one or more trans C=C double bonds, typically produced during partial hydrogenation. Trans fats pack more efficiently than cis fats and behave more like saturated fats, which is linked to health risks.
Retinal
A polyene aldehyde derived from vitamin A. In rod cells of the eye, 11-cis-retinal is bound to the protein opsin, forming rhodopsin. A photon of light triggers isomerisation of the 11-cis double bond to all-trans, which initiates the signal cascade for vision.
Rhodopsin
The light-sensitive protein complex in rod cells, formed from 11-cis-retinal bound to opsin. Absorption of light converts it to all-trans-retinal plus opsin ("bleaching"), which is then regenerated enzymatically.
Step-by-step procedure
Find the longest continuous carbon chain that contains the C=C double bond. This is the parent chain, even if a longer chain exists that does not include the double bond.
Number the parent chain so that the double bond receives the lowest possible locant. The locant of the double bond is the number of the first carbon of the double bond.
Change the "-ane" suffix to "-ene." Place the double-bond locant before the suffix or before the parent name (e.g. pent-1-ene or 1-pentene).
Name and number substituents as with alkanes. If the double bond has E/Z stereochemistry, place (E)- or (Z)- at the beginning of the name in parentheses.
Worked examples from the lecture
CH2=CHCH2CH3: the longest chain containing the double bond has four carbons, and the double bond starts at C1. Name: 1-butene (or but-1-ene).
4-methyl-1-pentene: a five-carbon parent chain with a double bond at C1 and a methyl branch at C4.
3-ethyl-1-hexene: a six-carbon parent chain with a double bond at C1 and an ethyl group at C3.
(E)-3-methyl-2-pentene: a five-carbon chain with the double bond at C2, a methyl group at C3, and the two highest-priority groups on opposite sides (E configuration).
Compounds with more than one double bond
When a molecule contains two or more C=C double bonds, you adjust the suffix: -diene for two, -triene for three, -tetraene for four, and so on. Each double bond gets its own locant.
1,3-butadiene: a four-carbon chain with double bonds starting at C1 and C3
2-methyl-1,4-pentadiene: a five-carbon chain with double bonds at C1 and C4, plus a methyl group at C2
Cyclic alkenes
For cycloalkenes, the double bond is assumed to be between C1 and C2, so you do not need to specify its position unless there are additional double bonds or substituents that require it. Numbering goes around the ring in the direction that gives substituents the lowest locants.
1-methyl-1-cyclopentene: a five-membered ring with a double bond and a methyl group at C1
3-methyl-1,4-cyclohexadiene: a six-membered ring with double bonds at C1 and C4, and a methyl group at C3
Fatty acids and melting points
Fatty acids are long-chain carboxylic acids, and their physical properties depend heavily on the number and geometry of C=C double bonds.
Stearic acid (18:0): fully saturated, no double bonds. Melting point 70°C. The straight chain packs tightly, so it is a solid at room temperature.
Oleic acid (18:1): one cis double bond. Melting point 16°C. The cis kink disrupts packing.
Linoleic acid (18:2): two cis double bonds. Melting point -5°C. More kinks, even poorer packing.
Linolenic acid (18:3): three cis double bonds. Melting point -11°C. This is an omega-3 fatty acid.
The pattern: more cis double bonds mean more kinks, worse molecular packing, and lower melting points. This is why animal fats (high in saturated fatty acids) are solid and vegetable oils (high in unsaturated fatty acids) are liquid.
Partially hydrogenated oils and trans fats
Industrial hydrogenation adds H2 across some C=C double bonds in vegetable oils, using a metal catalyst (Pt or Pd). The goal is to make the oil more solid (for margarine, shortening). A side effect is that some remaining cis double bonds isomerise to trans during the process. Trans double bonds remove the kink, allowing the chain to pack more like a saturated fat. Trans fats have been linked to increased cardiovascular risk, which is why they are now regulated or banned in many countries.
Retinal and the process of vision
Your ability to see in dim light depends on a single cis-to-trans isomerisation. In rod cells, 11-cis-retinal is bound to the protein opsin, forming rhodopsin. When a photon of light hits rhodopsin, the 11-cis double bond isomerises to all-trans-retinal. This shape change triggers a signal cascade that the brain interprets as vision ("bleaching"). An enzyme then uses ATP to convert all-trans-retinal back to 11-cis-retinal, and it recombines with opsin to regenerate rhodopsin ("regeneration"). The entire cycle is a beautiful example of how alkene geometry has biological consequences.
Students often pick the longest carbon chain in the molecule as the parent chain, forgetting that the parent chain must contain the double bond, even if that chain is shorter.
When numbering, students sometimes give substituents the lowest locants instead of the double bond. The double bond takes priority over substituents in determining numbering direction.
Students sometimes think "unsaturated" means unhealthy. Unsaturation in fatty acids simply refers to the presence of C=C double bonds. Mono- and polyunsaturated fats (cis geometry) are generally considered beneficial. Trans unsaturated fats are the problematic ones.
Students occasionally confuse partial hydrogenation with full hydrogenation. Full hydrogenation removes all double bonds and produces a fully saturated fat (no trans fat issue). Partial hydrogenation leaves some double bonds and can create trans isomers as a side reaction.
⚠️ Naming alkenes is a guaranteed exam topic. Practise finding the parent chain containing the double bond and assigning the correct locant.
⚠️ Questions linking unsaturation to physical properties (melting point trends in fatty acids) appear regularly. Know the pattern: more cis double bonds = more kinks = lower melting point.
⚠️ The retinal/vision example is a favourite for illustrating the biological significance of alkene geometry. Be ready to explain the cis-to-trans isomerisation and the visual cycle.
⚠️ Know the difference between partial and full hydrogenation, and why partial hydrogenation produces trans fats.
True or False: The parent chain for naming an alkene must be the longest chain in the molecule.
Fill in the blank: A compound with two C=C double bonds is called a ______.
True or False: Oleic acid has a higher melting point than stearic acid.
Fill in the blank: In the visual cycle, light converts 11--retinal to all--retinal.
True or False: Full hydrogenation of a vegetable oil produces trans fats.
Answers: 1. False (it must be the longest chain that contains the double bond). 2. diene. 3. False (oleic acid, mp 16°C, is lower than stearic acid, mp 70°C). 4. cis, trans. 5. False (full hydrogenation removes all double bonds; partial hydrogenation can produce trans fats).
Q: Name the compound CH2=CHCH(CH3)CH2CH3 using IUPAC rules.
A: The longest chain containing the double bond has five carbons. The double bond is at C1. There is a methyl group at C3. Name: 3-methyl-1-pentene.
Q: Why does cis-2-butene have a higher boiling point than trans-2-butene, even though trans is more thermodynamically stable?
A: The cis isomer has a small net dipole moment (the two C–CH3 bond dipoles do not cancel), giving it slightly stronger intermolecular forces. The trans isomer is more symmetrical and its dipoles cancel, so its intermolecular forces are weaker. Stability (thermodynamic) and boiling point (intermolecular forces) are separate properties.
Q: Explain why increasing the number of cis C=C double bonds in a fatty acid lowers its melting point.
A: Each cis double bond introduces a kink in the hydrocarbon chain. Kinks prevent the chains from packing closely together in a regular array, which weakens the London dispersion forces between molecules. Weaker intermolecular forces mean less energy is needed to disrupt the solid, so the melting point drops.
Q: What is the IUPAC name of a six-membered ring with double bonds at positions 1 and 3, and a methyl group at position 5?
A: 5-methyl-1,3-cyclohexadiene.
Q: Describe the role of light in the visual cycle, in terms of alkene geometry.
A: A photon of light provides the energy to isomerise the C11=C12 double bond in 11-cis-retinal from the Z (cis) to the E (trans) configuration. This shape change triggers the protein opsin to change conformation and begin the signal cascade that the brain reads as vision.
Alkene naming conventions carry forward to every functional group you will encounter. Alcohols use "-ol," ketones use "-one," and so on, but the procedure for finding the parent chain, numbering, and placing locants is the same.
The reactivity of the pi bond is the subject of Chapters 6 and 7 (addition reactions). Understanding that the pi electrons are exposed above and below the plane of the molecule explains why electrophiles are attracted to alkenes.
The fatty acid and trans fat material connects organic chemistry to nutrition and public health. If your course covers lipids or membranes later (in a biochemistry context), the melting-point trend from this chapter is the foundation.
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