Organic Chemistry · Alkenes: Structure and Reactivity
Naming Alkenes
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An alkene A hydrocarbon containing a carbon–carbon double bond; general formula CnH2n for a single double bond. Full entry → is a hydrocarbon that contains at least one carbon–carbon double bond. The double bond changes everything about naming: it becomes the single most important feature of the molecule, so it controls which chain is the parent, how the chain is numbered, and what suffix the name carries. In this topic you will apply the IUPAC system you already used for alkanes (Chapter 3) with one critical new rule: the double bond outranks every substituent A branch attached to the parent chain, named by replacing -ane with -yl (methyl, ethyl). Full entry →.
The goal is two-way fluency: given a structure, produce a correct name; given a name, draw the structure. The system also extends to compounds with two or three double bonds (dienes, trienes) and to cyclic alkenes, and it sets up the E/Z stereochemical prefixes you will meet in the next topic.
Why this matters
A name is a contract: it must describe exactly one compound. 1-Butene (CH2=CH–CH2–CH3), cis-2-butene, and trans-2-butene all have the formula C4H8, yet they differ in boiling point, stability, and reactivity — a misplaced locant The number giving a double bond’s or substituent’s position in the parent chain. Full entry → in a lab order, patent, or prescription can mean a completely different chemical. Nomenclature is also the language of the rest of the course: every reaction in Chapters 8–11 is discussed by named starting materials and products. And on exams, nomenclature is one of the most reliably testable skills: few rules, consistent logic, pure practice.
The college version
Core Concepts
The double bond determines the parent chain
Find the longest continuous chain of carbon atoms that contains the double bond — this chain, not necessarily the longest in the molecule, is the parent. The parent takes the alkane stem (pent-, hex-, …) with the suffix -ene instead of -ane. In CH2=CH–CH2–CH(CH3)–CH3 the 5-carbon chain through the double bond makes the parent pentene; the methyl is a substituent, so the compound is a methylpentene, not a branched hexane.
Numbering to give the double bond the lowest locant
Number the chain from whichever end gives the double bond the lowest locant — the position number of the first double-bond carbon — placed immediately before the suffix: pent-2-ene, never pent-3-ene. If both ends give the same double-bond locant, break the tie by giving substituents the lowest locants. In CH3–CH=C(CH3)–CH2–CH3, numbering from the left puts the double bond at C2 and the methyl at C3; numbering from the right would put the double bond at C3. The left numbering wins: 3-methyl-2-pentene.
Substituents: alphabetical order and locants
Substituent names are listed alphabetically, each preceded by its locant (3-ethyl-2-methylpent-1-ene: ethyl before methyl). Multiplicative prefixes such as di-, tri-, and tetra- are ignored when alphabetizing — 2,2-dimethyl sorts under "m," not "d." Repeated substituents get a prefix and every locant: 2,2-dimethylbut-1-ene means two methyls, both on C2.
More than one double bond: dienes and trienes
Compounds with two double bonds use the suffix -diene, three use -triene, and every double bond gets a locant: CH2=CH–CH2–CH=CH2 is penta-1,4-diene (numbering from the other end would give 2,5, so 1,4 is correct). Cumulated (CH2=C=CH2, allene) and conjugated (CH2=CH–CH=CH2, buta-1,3-diene) dienes reappear in later chapters, but the naming logic is the same.
Cycloalkenes
In a cyclic alkene the ring is the parent and the double bond is assumed to be C1=C2; the name begins with cyclo- (cyclohexene, not hexene). Numbering starts at a double-bond carbon and proceeds through the double bond so substituents get the lowest locants: a methyl on the double-bond carbon gives 1-methylcyclohexene; two carbons away, 3-methylcyclohexene. Small-ring cycloalkenes exist only in the cis form (Topic 4), so no stereochemical prefix is normally needed.
Stereochemistry in the name
If an alkene can exist as E or Z isomers (Topics 4–5), the configuration is written as a prefix with the locant of the first double-bond carbon: (E)-2-pentene or (2E)-pent-2-ene, placed at the front of the name. You will assign these prefixes in Topic 5; the key point here is that a complete alkene name carries three kinds of information — parent + locants, substituents, and stereochemistry.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| 1-pentene | 2-pentene | Different double-bond positions, different compounds: CH2=CH–CH2–CH2–CH3 vs CH3–CH=CH–CH2–CH3. |
| -ene suffix | -yne suffix | -ene = double bond (CnH2n); -yne = triple bond (CnH2n−2). A molecule with both is an en-yne (e.g., pent-1-en-4-yne). |
| Longest chain overall | Longest chain containing the double bond | The double bond decides the parent even if a longer branch-free chain exists elsewhere. |
| Numbering for the double bond | Numbering for substituents | The double bond always gets the lowest locant first; substituents only break ties. |
| 2,2-dimethyl (alphabetizing) | 3-ethyl before 2,2-dimethyl | Ignore di-/tri-/tetra- prefixes when alphabetizing: ethyl (e) precedes methyl (m). |
| 1-methylcyclohexene | 3-methylcyclohexene | The double bond is fixed at C1=C2 in rings; the methyl locant follows the shortest path through the double bond. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Naming an alkene is like giving someone a street address. The double bond is the most important landmark, so you first find the longest street (chain) that has it. Then you count from the end that reaches the landmark soonest, and finally you list any side streets (substituents) in alphabetical order. If you follow the same steps every time, two different people will always end up at the same house.
Worked example
Example 1 — Name CH3–CH=CH–CH2–CH3. The longest chain containing the double bond has five carbons: pent-. The double bond sits between C2 and C3. Numbering from the left gives locant 2; from the right gives 3 — so the correct name is pent-2-ene (2-pentene). There are no substituents, so the name is complete.
Example 2 — Name CH3–CH=C(CH3)–CH2–CH3. Parent chain: five carbons through the double bond (pent-). The double bond must be C2=C3 (numbering from the left; the other direction would give 3). The methyl branch sits on C3. Because the double bond locant is fixed at 2, the methyl locant is 3: 3-methylpent-2-ene (3-methyl-2-pentene).
Example 3 — Draw 4-methylpent-1-ene. The name says: five-carbon chain, double bond between C1 and C2, methyl on C4. Working from the double-bond end: CH2=CH–CH2–CH(CH3)–CH3. Check: numbering from the double bond puts the methyl at C4; numbering from the other end would put the double bond at C4 — wrong. The drawing is correct.
Example 4 — Name CH2=CH–CH2–CH=CH2. Two double bonds → penta-…-diene. Numbering from either end is equivalent; choose the end that gives the lower set of locants: double bonds at C1 and C4 (1,4-diene; the alternative 2,5 is larger). Name: penta-1,4-diene.
Example 5 — Name the cycloalkene with a methyl group on a double-bond carbon of a six-membered ring. Ring parent: cyclohexene. Double bond is C1=C2, so the methyl on a double-bond carbon is at C1: 1-methylcyclohexene.
Key takeaways
- The parent chain is the longest chain containing the double bond, not the longest chain overall.
- Number to give the double bond the lowest locant; use substituents only to break ties.
- The suffix -ene marks one double bond; -diene and -triene mark two and three.
- Substituents go in alphabetical order, ignoring di-, tri-, tetra- prefixes.
- In cycloalkenes the double bond is C1=C2 and numbering proceeds through it.
- A complete name may include an (E)- or (Z)- stereochemical prefix — the configurational locant always refers to the double bond.
- Same formula, different name = different compound: C4H8 includes 1-butene, cis-2-butene, and trans-2-butene.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
What is the parent chain The longest continuous carbon chain that includes the double bond. Full entry → for naming an alkene, and why does it differ from the longest chain in the molecule?
Show answer
The parent chain is the longest continuous chain that contains the double bond, because the double bond is the highest-priority feature of an alkene and must lie inside the parent. A longer chain that excludes the double bond would misplace the functional group.
Name the compound CH3–CH2–CH=CH–CH2–CH3.
Show answer
Hex-3-ene (3-hexene): the six-carbon chain carries the double bond between C3 and C4; numbering from the nearer end gives locant 3.
Name the compound CH2=C(CH3)–CH2–CH3 and explain why the methyl is not listed first in the name.
Show answer
2-methylbut-1-ene (2-methyl-1-butene). The double bond is between C1 and C2 (lowest locant), and the methyl sits on C2. Only one substituent exists, so alphabetical order is not the issue — the methyl gets locant 2 because the double bond already fixed the numbering direction.
Draw the structure of penta-1,4-diene and explain why the locants are 1,4 rather than 2,5.
Show answer
CH2=CH–CH2–CH=CH2. Numbering from either end is symmetric; the set 1,4 is smaller than the alternative 2,5, so 1,4 is correct.
A student names CH3–CH=C(CH3)–CH2–CH3 as "2-ethylpent-2-ene." What is wrong with that name?
Show answer
Two errors: (a) the parent must contain the double bond and be numbered to give it the lowest locant, which makes the "ethyl" group actually part of the parent chain — the correct parent is pent-2-ene; (b) the branch is therefore a methyl on C3, giving 3-methylpent-2-ene, not "2-ethylpent-2-ene."
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- alkene
- A hydrocarbon containing a carbon–carbon double bond; general formula CnH2n for a single double bond.
- parent chain
- The longest continuous carbon chain that includes the double bond.
- locant
- The number giving a double bond’s or substituent’s position in the parent chain.
- suffix (-ene)
- The ending that signals a double bond in the parent name.
- substituent
- A branch attached to the parent chain, named by replacing -ane with -yl (methyl, ethyl).
- diene / triene
- Alkenes with two / three double bonds.
- cycloalkene
- A ring alkene; the double bond is always between C1 and C2.
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