Constitutional isomers - quick reference

Definition

• Constitutional (or structural) isomers: same molecular formula, different connectivity, hence different names.

• Important distinction: they are not the same compound; connectivity differs even though formula is the same.

Quick counts for alkanes (example series)

• For C4H{10} (butane): 2 constitutional isomers

  • n-butane (CH3–CH2–CH2–CH3)

  • isobutane / 2-methylpropane (CH3–CH(CH3)–CH3)

• For C5H{12} (pentane): 3 constitutional isomers

  • n-pentane

  • 2-methylbutane (isopentane)

  • 2,2-dimethylpropane (neopentane)

• For C6H{14} (hexane): 5 constitutional isomers

  • n-hexane

  • 2-methylpentane

  • 3-methylpentane

  • 2,2-dimethylbutane

  • 2,3-dimethylbutane

• For C7H{16} (heptane): 9 constitutional isomers

  • Includes straight chain, various methyl- and ethyl-substituted variants, and combinations that arise from different base chains (as described in the video).

• For C8H{18} (octane): 18 constitutional isomers

  • Many combinations of chain length and substituents; total 18 as stated in the video.

Other formulae examples

• C2H6O: 2 constitutional isomers

  • ethanol (CH3CH2OH)

  • dimethyl ether (CH3OCH3)

• C4H8: typical constitutional isomers arise from alkenes or rings

  • Alkenes (one C=C): includes butenes (e.g., 1-butene, 2-butene with cis/trans as stereochemistry)

  • Rings: cyclobutane, methylcyclopropane, etc.

  • Note: cis/trans (geometric) isomers are stereoisomers, not constitutional isomers, because connectivity is the same.

Important concepts

• Distinction: constitutional isomers differ in connectivity; stereoisomers differ in spatial arrangement while connectivity remains the same.

• When counting constitutional isomers, changing the base chain length and attaching substituents (methyl, ethyl, etc.) yields new connectivity and potentially new names.

• Nomenclature must reflect different connectivity; identical connectivity yields the same name.