Locating a Double Bond in a Name

Introduction

Once an alkene chain is long enough, the C=C bond can occupy different positions. A locant in the name tells where the double bond begins along the numbered parent chain. Numbering from the nearer end gives the lower appropriate double-bond position in simple cases.

Core explanation

Ethene has only two carbons, so its one possible C=C position needs no useful locant. Propene has three carbons and its double bond must be at an end when the chain is renumbered; “prop-1-ene” is possible in systematic notation, but “propene” is unambiguous in the simple context. Four carbons create distinct positions.

CH₂=CH–CH₂–CH₃ is but-1-ene. Number from the double-bond end: the C=C joins carbons 1 and 2, so the locant is 1. If the same structure is drawn backward as CH₃–CH₂–CH=CH₂, counting from the left would give 3, but the correct numbering begins at the right and still gives but-1-ene. A drawing's direction does not change the compound.

CH₃–CH=CH–CH₃ is but-2-ene. The double bond joins carbons 2 and 3 no matter which end is used. But-1-ene and but-2-ene share C₄H₈ but differ in double-bond position, so they are positional constitutional isomers. The numbers are not coefficients or carbon counts; they locate the first carbon of the C=C.

The parent chain for this elementary task should contain the double bond. If a branching drawing offers several possible long paths, the full systematic rules require choosing an appropriate parent that includes the principal unsaturation. Simply picking the visually longest horizontal line can omit the double bond or select the wrong chain. Trace connected carbon paths carefully.

A locant does not necessarily resolve spatial isomerism. But-2-ene can have different arrangements of substituents around its restricted C=C bond. Those require additional stereochemical descriptors beyond “but-2-ene.” At this stage, the locant distinguishes bond position in the connectivity graph.

The molecular formula cannot give the locant. Both but-1-ene and but-2-ene are C₄H₈, and cyclobutane also shares that formula. A displayed, condensed or skeletal structure must supply the position. Naming follows the structure rather than being guessed from H count.

Step-by-step reasoning

1. Identify the parent carbon chain containing C=C. 2. Count carbons to choose the parent stem. 3. Number from each end and note the first carbon of the double bond. 4. Choose the orientation giving the lower C=C locant. 5. Place the locant before -ene and check the drawn structure.

Visual explanation

Draw CH₂=CH–CH₂–CH₃ with numbers 1,2,3,4 above, then the same line reversed with numbers starting at its right end. Both diagrams point to but-1-ene. Below draw CH₃–CH=CH–CH₃ with the 2,3 double bond labeled but-2-ene.

Real-world analogy

Numbering houses from the nearer end of a street gives a smaller address for a landmark; flipping the map does not move the landmark. An alkene locant records the bond's position in the chosen numbered chain, not its left-to-right position on paper.

Real-world example

But-1-ene and but-2-ene can appear in industrial hydrocarbon mixtures. They have the same molecular formula but different double-bond positions, which can affect subsequent reaction products. The locant lets a chemist specify which structural isomer is meant.

Why?

Why name the first carbon of the two-atom double bond? A C=C connects adjacent numbered carbons. Giving the lower-numbered member identifies the pair unambiguously for a simple unbranched chain: “2” in but-2-ene means the bond is between C2 and C3.

Common misconception

“CH₃CH₂CH=CH₂ is but-3-ene because the double bond starts at the third written group.” Number from the end nearer the C=C; the correct name is but-1-ene.

Worked example

Name CH₃CH₂CH=CHCH₃. The longest shown chain containing C=C has five carbons, so the stem is pent-. Numbering left to right places the double bond between C3 and C4, locant 3. Numbering right to left places it between C2 and C3, locant 2. Choose the lower locant: pent-2-ene. The formula is C₅H₁₀ under one-double-bond acyclic assumptions.

Quick check

1. What name describes CH₃CH₂CH=CH₂ after numbering from the nearer end? Answer: But-1-ene, because the double bond joins carbon 1 and carbon 2 when numbering begins at the right.

Exam focus

Number both directions before choosing. The locant is the lower-numbered carbon of C=C, not the number of hydrogens or lines. A molecular formula does not specify bond position.

Advanced insight

When several double bonds, branches and principal functional groups coexist, nomenclature has additional priorities and tie-breaking rules. The simple lowest-double-bond-locant rule here provides a foundation but should not be extended blindly to every multifunctional molecule.

Summary

Simple alkene names identify the parent chain and locate C=C with a locant. But-1-ene and but-2-ene have the same formula but different bond positions. Number from the appropriate end and treat drawing orientation as irrelevant.

Practice questions

1. Name CH₂=CHCH₂CH₃. Answer: But-1-ene. 2. Name CH₃CH=CHCH₃. Answer: But-2-ene. 3. Are but-1-ene and but-2-ene constitutional isomers? Answer: Yes. Their formulas match but their double bonds occupy different positions. 4. Name CH₃CH₂CH=CHCH₃. Answer: Pent-2-ene after numbering from the end nearer the double bond.