Difficulty: Intermediate | Prerequisites: CHEM 2301 Chapters 1–6, functional group basics, Cahn–Ingold–Prelog priority rules
This covers the IUPAC naming conventions for alkenes and alkynes tested on Midterm Exam 4 (Dr. Salmon, University of Minnesota). Nomenclature is worth 8 points on the exam and appears again implicitly in every other question, because you need to read names to draw structures and vice versa. If you can name it, you can draw it, and if you can draw it, you can react it.
IUPAC nomenclature for alkenes and alkynes follows the same parent-chain logic as alkanes, but the multiple bond gets the lowest possible locant and changes the suffix to -ene or -yne. Stereochemistry labels (E/Z for alkenes, R/S for stereocentres) are placed in parentheses before the name. If you can name it correctly, you can draw it, and vice versa.
Alkene
A hydrocarbon containing at least one carbon-carbon double bond (C=C). The suffix in IUPAC naming is -ene. In simple terms, two carbons share four electrons instead of the usual two.
Alkyne
A hydrocarbon containing at least one carbon-carbon triple bond (C≡C). The suffix is -yne. Think of it as two carbons sharing six electrons, leaving each carbon sp-hybridised with a linear geometry around the triple bond.
Parent chain
The longest continuous carbon chain that includes the multiple bond. This is the chain you base the name on. When a double or triple bond is present, the parent chain must pass through that bond, even if a longer all-single-bond chain exists elsewhere in the molecule.
Locant
The number assigned to each carbon in the parent chain, starting from the end that gives the multiple bond the lowest possible number. For example, in hex-1-yne the triple bond starts at carbon 1.
E/Z stereodescriptors
Used when an alkene has two different substituents on each carbon of the double bond. Z (zusammen, German for together) means the higher-priority groups are on the same side; E (entgegen, opposite) means they are on opposite sides. Priority is assigned using Cahn-Ingold-Prelog rules (higher atomic number wins).
R/S configuration
Used to describe the three-dimensional arrangement at a stereocentre (a carbon bonded to four different groups). Assign priorities 1-4 by atomic number, orient with priority 4 pointing away from you, then trace 1 to 2 to 3: clockwise is R (rectus), anticlockwise is S (sinister).
Degree of unsaturation (index of hydrogen deficiency)
A count of how many pairs of hydrogens are "missing" relative to the fully saturated formula. Each double bond or ring accounts for one degree; each triple bond accounts for two. The formula is (2C + 2 + N - H - X) / 2, where C, N, H, X are the counts of carbon, nitrogen, hydrogen, and halogen atoms.
Find the longest chain that contains the C=C double bond.
Number from the end nearest the double bond.
Change the -ane ending to -ene and insert the locant of the first carbon of the double bond (e.g. hex-2-ene, not hex-4-ene).
Name and number all substituents as usual (methyl, ethyl, bromo, etc.) and list them alphabetically.
If a hydroxyl (-OH) group is present, the suffix becomes -en-ol, and the -OH group gets numbering priority over the double bond.
Exam example: 2,3-dimethyl-2-butene
Parent chain: but (4 carbons), -ene suffix because of the double bond.
The double bond is between C2 and C3.
Two methyl groups sit at C2 and C3.
No stereochemistry label needed here because each double-bond carbon bears two identical substituents (two methyls and a hydrogen is not the case: both double-bond carbons have one methyl and one methyl from the chain, making them trisubstituted, but each carbon of the double bond has two different groups, not two identical ones on both sides). Actually, C2 has a methyl and a methyl-bearing chain end, C3 has a methyl and a methyl-bearing chain end. Because the two substituents on each carbon of the double bond are identical on each side, no E/Z designation is required.
Same parent-chain and numbering logic, but the suffix is -yne.
A triple bond takes numbering priority over a double bond when both are present (the -yne locant must be lowest), unless an -OH group is also present, in which case -OH wins.
When both -OH and -yne are present, the compound is named as an -yn-ol.
Exam example: (3R)-4-hexyn-3-ol
Parent chain: hex (6 carbons).
Triple bond at C4 (hex-4-yn).
Hydroxyl at C3 (-3-ol).
The stereocentre at C3 has R configuration.
Full name: (3R)-4-hexyn-3-ol. Note the numbering gives -OH the lowest locant.
The exam also asks you to look at a drawn structure and produce the IUPAC name. The process is the reverse:
Identify the longest chain containing the functional group (double bond, triple bond, or both).
Number the chain to give the principal characteristic group (e.g. -OH) the lowest locant, then the multiple bond.
Assign E/Z if the alkene has two different groups on each carbon of the double bond.
Assign R/S at any stereocentre (a carbon bearing four different substituents).
E/Z goes in parentheses at the front of the name or before the relevant locant: (E)-hex-3-ene.
R/S goes in parentheses with the locant of the stereocentre: (3R)-4-hexyn-3-ol.
When both are present, list them together in numerical order of the locant they describe.
Higher atomic number wins at the first point of difference (Br > Cl > O > N > C > H).
If two atoms are the same, move outward along each chain until a difference is found.
A double bond to atom X is treated as two single bonds to X (phantom atoms).
A triple bond to atom X is treated as three single bonds to X.
Students often number the parent chain to give substituents the lowest locants, ignoring the functional group. The multiple bond (or -OH, if present) always takes priority for lowest numbering, not the substituents.
Confusing E/Z with cis/trans. E/Z is based on CIP priority of substituents, not simply whether groups are on the same side. Two molecules can be cis by spatial arrangement but E by CIP rules if the priority assignments differ from what you would guess by looking.
Forgetting that -OH outranks a double or triple bond for numbering. In an alcohol that also has a multiple bond, the -OH carbon gets the lowest possible locant, and the name ends in -en-ol or -yn-ol.
Omitting stereodescriptors when they are required. If the double bond has two different groups on each sp2 carbon, you must specify E or Z. If there is a stereocentre, you must specify R or S. Leaving these out loses marks on Dr. Salmon's exam.
⚠️ Question 1 on this exam is pure nomenclature (8 points): two names to draw, two structures to name. It is free marks if you have the method down.
⚠️ Stereochemistry labels (E/Z, R/S) are explicitly required "where appropriate." If you forget the label, even a correct structure or name will lose points.
⚠️ Nomenclature is not isolated to Question 1. In synthesis and mechanism questions, you need to read reagent names and recognise functional groups quickly. Slow naming skills will slow you down across the whole 50-minute exam.
⚠️ Dr. Salmon's exam instructions note that unclear or illegible answers get no credit. Draw structures neatly and write names clearly.
True or false: In (E)-hex-3-ene, the higher-priority groups on each double-bond carbon are on opposite sides.
True.
Fill in the blank: The suffix for a compound containing both a triple bond and a hydroxyl group is ______.
-yn-ol.
True or false: When numbering a chain with both a double bond and an -OH group, the double bond gets numbering priority.
False. The -OH group gets numbering priority.
Fill in the blank: A clockwise 1-2-3 trace (with priority 4 pointing away) gives the ______ configuration.
R (rectus).
True or false: 2,3-dimethyl-2-butene requires an E/Z label.
False. Each doubly-bonded carbon does not have two different substituents in a way that makes E/Z meaningful (the molecule is symmetrical across the double bond).
Q: Draw the structure of (3R)-4-hexyn-3-ol. What functional groups are present?
A: A six-carbon chain with a triple bond between C4 and C5, and a hydroxyl group on C3 in the R configuration. The functional groups are an alkyne and a secondary alcohol.
Q: Give the IUPAC name for a cyclohexane ring bearing a bromine and a vinyl group (CH=CH2) at adjacent carbons, with the bromine in an axial position. How would you indicate stereochemistry?
A: You would name the parent ring as cyclohexene (if the double bond is within the ring) or as a bromocyclohexane with an ethenyl substituent (if the double bond is exocyclic). Stereochemistry at ring carbons bearing different groups is described using R/S at each stereocentre, or cis/trans for relative orientation on the ring.
Q: A student names a compound "3-hexyn-5-ol." What error have they made?
A: The -OH group should receive the lowest possible locant. Renumbering the chain from the other end gives 2-hexyn-1-ol (or an equivalent lowest locant for -OH). The hydroxyl always takes numbering priority over the triple bond.
Q: What is the degree of unsaturation for a molecular formula C6H10O? What structural features could account for it?
A: Degrees of unsaturation = (2(6) + 2 - 10) / 2 = 2. This could be one triple bond, two double bonds, one double bond plus one ring, or two rings.
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