Key Insight: Stability is influenced by aromaticity, ring strain, and conjugation. Cyclobutadiene is antiaromatic and unstable, while methylenecyclopropene is surprisingly stable despite ring strain.
Chemical Reactivity Differences between the isomers of C4H4
Isomer
Reactivity Type
Example Reactions
Cyclobutadiene
Highly reactive, antiaromatic
Undergoes [4+2] cycloadditions; unstable in isolation
Vinylacetylene
Electrophilic addition, polymerization
Reacts with HCl to form chloroprene (synthetic rubber precursor)
Butatriene
Diels-Alder reactions
Can act as diene in cycloadditions
Methylenecyclopropene
Radical and ionic reactions
Forms stable radical cations; used in mass spectrometry studies
Exam Tip: Link reactivity to structure – conjugated systems favour pericyclic reactions; strained rings favour ring-opening or rearrangement.
Uses and Applications of the isomers of C4H4
Isomer
Application
Notes
Vinylacetylene
Intermediate in chloroprene synthesis
Used in synthetic rubber production
Cyclobutadiene
Theoretical interest, organometallic complexes
Ligand in metal complexes; not used industrially due to instability
Butatriene
Spectroscopic studies, astrochemistry
Detected in interstellar medium derivatives
Methylenecyclopropene
Mass spectrometry calibration, theoretical chemistry
Stable radical cation; benchmark for computational studies
Common Misconceptions about the isomers of C4H4
-
“All isomers have similar properties” – False. Even small structural changes (e.g. ring versus chain) drastically affect boiling point, reactivity, and stability.
-
“Cyclobutadiene is aromatic” – Incorrect. It’s antiaromatic, violating Hückel’s rule (4π electrons).
-
“Isomers differ only by rotation” – Misleading. True isomers require bond breaking/reforming, not just rotation.
Exam Revision Tips for questions that may involve the isomers of C4H4
-
Draw all possible isomers – Use skeletal formulae and identify ring versus chain structures.
-
Classify isomerism types – Structural (chain, functional group), not stereoisomerism for C4H4
-
Link structure to reactivity – Use examples like vinylacetylene’s industrial role.
-
Use mnemonic for aromaticity – Hückel’s rule: ( 4n + 2 ) π electrons = aromatic; ( 4n ) = antiaromatic.
-
Spectra – C4H4 isomers show distinct mass spectra fragmentation patterns and differences in NMR spectra.
Learning objectives – questions to be answered?
How do you work out the structure of the isomers of molecular formula C4H4?
How do you draw the structural formula and skeletal formula of the isomers of molecular formula C4H4?
How do you name the isomers of molecular formula C4H4?
How many aliphatic structural isomers are there of molecular formula C4H4?
How many aliphatic carbon chain isomers are there of molecular formula C4H4?
How many positional isomers are there of molecular formula C4H4?
Are there any aliphatic open chain alkene isomers of molecular formula C4H4?
Are there any carboxylic acid isomers of molecular formula C4H4?
Are there any ester isomers of molecular formula C4H4?
Are there any enol isomers of molecular formula C4H4?
Are there any alkene isomers of molecular formula C4H4?
Are there any alcohol isomers of molecular formula C4H4?
Are there any aldehyde isomers of molecular formula C4H4?
Are there any ketone isomers of molecular formula C4H4?
Are there any functional group isomers with a molecular formula C4H4?
Does C4H4 have any stereoisomers?
Are there any E/Z (geometrical) isomers with a molecular formula C4H4?
Are there any R/S (optical) isomers (enantiomers) with a molecular formula C4H4?
How many E/Z (geometrical) isomers are there of molecular formula C4H4?
How many R/S (optical) isomers (enantiomers) of molecular formula C4H4?
This page will answer these questions for molecular formula C4H4
Associated organic chemistry links
Index of sets of isomers for a given molecular formula
All my advanced Level pre-university organic chemistry notes
IR, mass and H-1 and C-13 NMR spectra of organic compounds
The molecular structure and naming of ALKANES (how to name and draw alkane structures)
The molecular structure and naming of ALKENES
(how to name and draw alkene structures)
Index of revision notes on the chemistry of ALKANES and the petrochemical industry
INDEX of ALL revision notes on the chemistry ALKENES including reactions and polymers
Isomerism: introduction, structural isomerism – chain, positional, functional group, tautomerism
Stereoisomerism: introduction, definition, priority rules, E/Z isomerism (cis/trans isomerism)
Stereoisomerism – R/S isomerism (optical isomerism) – definition – examples explained
This is a big chemistry website, please allow time to explore it
Summary diagram of isomerism which links to details of the types of isomerism